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Technical Report #1: Estimating ADU Potential in Unincorporated San Mateo County
Center for Community Innovation – University of California, Berkeley
April 30, 2017
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TABLE OF CONTENTS
EXECUTIVE SUMMARY ................................................................................................................................... 7
PART 1: PROPERTY DATABASE FOR UNINCORPORATED SAN MATEO COUNTY ........................................... 13
PART 2: SELECTED COMMUNITIES .............................................................................................................. 15
Bayside Flat Communities ....................................................................................................................... 16
Bayside Hill Communities ........................................................................................................................ 17
Coastal Communities ............................................................................................................................... 17
Rural Open Communities ........................................................................................................................ 17
Rural Small Towns ................................................................................................................................... 18
San Francisco Urban Communities .......................................................................................................... 18
PART 3: ESTIMATING EXISTING ADUS .......................................................................................................... 20
Data Review of Existing ADUs in San Mateo County ............................................................................... 20
Permitted ADUs: San Mateo County ADU Database ........................................................................... 20
Non-Permitted ADUs: Unit Count Above Zoning and Use Restriction ................................................. 21
Non-Permitted ADUs: Duplicate Address ............................................................................................ 23
Field Review of Existing ADUs in San Mateo County ............................................................................... 23
Methods .............................................................................................................................................. 23
Results ................................................................................................................................................. 28
PART 4: ESTIMATING ADU DEVELOPMENT POTENTIAL ............................................................................... 29
Assessor Parcel Data Analysis .................................................................................................................. 29
Building Constraint Layer 1: Zoning Designation ................................................................................. 32
Building Constraint Layer 2: Current Land Use .................................................................................... 33
Building Constraint Layer 3: Floor Area Requirements ........................................................................ 33
Building Constraint Layer 4: Building Coverage Requirements ........................................................... 35
Building Constraint Layer 5: Slope ....................................................................................................... 36
Building Constraint Layer 6: Parking Requirements ............................................................................ 37
Google Earth Parcel Measurements Analysis .......................................................................................... 39
Set-Back Requirements ....................................................................................................................... 39
Parking Requirements ......................................................................................................................... 40
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TABLES
Table 1: Assessor Parcel Database - Study Eligibility Summary ................................................................... 14
Table 2: Eliminated Unincorporated Communities ..................................................................................... 15
Table 3: Bayside Flat Communities – ADU Zoning Eligibility and Construction Accommodation ................ 16
Table 4: Bayside Hill Communities - ADU Zoning Eligibility and Construction Accommodation .................. 17
Table 5: Coastal Communities - ADU Zoning Eligibility and Construction Accommodation ........................ 17
Table 6: Rural Open Communities - ADU Zoning Eligibility and Construction Accommodation .................. 18
Table 7: Rural Small Towns - ADU Zoning Eligibility and Construction Accommodation ............................. 18
Table 8: San Francisco Urban Communities - ADU Zoning Eligibility and Construction Accommodation ... 19
Table 9: Summary of Community Types, Selected Communities, and Additional Case Study Possibilities . 19
Table 10: Division of Un-Matched ADU Properties to Study Unincorporated Naming Conventions ........... 21
Table 11: Summary of Permitted ADUs in San Mateo County..................................................................... 21
Table 12: Unit Count Restriction by Zone in Unincorporated San Mateo County ....................................... 22
Table 13: Use Codes and Definitions ........................................................................................................... 22
Table 14: Summary of Parcels with Unit Counts Greater than Zoning and Use Restrictions ....................... 22
Table 15: Number of Duplicate Addresses .................................................................................................. 23
Table 16: Removal Steps of Existing ADUs from Eligible Pool ...................................................................... 25
Table 17: Pool of Eligible Parcels for Online and Field Review .................................................................... 25
Table 18: Secondary Review Combinations ................................................................................................. 26
Table 19: Final ADU Determination in Online vs. Field Review .................................................................... 27
Table 20: Online and Field Review Results .................................................................................................. 28
Table 21: ADU Development Potential Analysis Dataset Summary ............................................................. 29
Table 22: Assessor Parcel Data Analysis Summary of Results ...................................................................... 30
Table 23: Parcels Eligible for ADU Construction by Zoning Designation ...................................................... 32
Table 24: Parcels Capable of ADU Construction based on Floor Area Availability, by ADU Size .................. 34
Table 25: Attached ADU vs. Detached ADU Feasibility based on Building Coverage, by Community Type . 35
Table 26: Attached ADU vs. Detached ADU Feasibility based on Slope, by Community Type .................... 36
Table 27: Randomized Set-Back Review for One-Story Backyard ADU ........................................................ 40
Table 28: Randomized Set-Back Review for Two-Story Backyard ADU ........................................................ 40
Table 29: Randomized Parking Review for One Front or Side Yard ADU Parking Space .............................. 41
Table 30: Randomized Parking Review for Two Front or Side Yard ADU Parking Space .............................. 41
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FIGURES
Figure 1: Parcels with ADU Construction Feasibility in Unincorporated San Mateo County ......................... 9
Figure 2: Parcels with ADU Construction Feasibility in Bayside Flat Communities ...................................... 10
Figure 3: Parcels with ADU Construction Feasibility in Bayside Hill Communities ....................................... 10
Figure 4: Parcels with ADU Construction Feasibility in Coastal Communities ............................................. 11
Figure 5: Parcels with ADU Construction Feasibility in Rural Open Area Communities and Small Towns ... 11
Figure 6: Parcels with ADU Construction Feasibility in San Francisco Urban Communities ........................ 12
Figure 7: Assessor Parcel Data Analysis Process Diagram ........................................................................... 31
Figure 8: Parcels Eligible for ADU Construction under Zoning Designation, by Community Type ............... 32
Figure 9: Parcels Capable of ADU Construction based on Floor Area Availability, by Community Type ...... 34
Figure 10: ADU Parking Accommodation .................................................................................................... 37
Figure 11: ADU Eligible Properties with Parking Exemption ........................................................................ 38
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APPENDICES
Appendix 1 – Summary of Assessor Parcel Database and Community Types
Appendix 2 – Permitted ADU Database
Appendix 3 – Unit Count Above Zoning and Use Restrictions
Appendix 4 – Duplicate Addresses
Appendix 5 – Select Properties Online and Field Review Methodology
Appendix 6 – ADU Field Study Letter from Karen Chapple
Appendix 7 – ADU Field Study Letter from San Mateo County Department of Housing
Appendix 8 – Online and Field Review Notes Template
Appendix 9 – Online and Field Review ADU Identification Results 10Mar2017
Appendix 10 – Methodology to Assess ADU Development Feasibility
Appendix 11 – Google Earth Set-Back and Parking Analysis Methodology
Appendix 12 – Google Earth Set-Back and Parking Analysis Template
Appendix 13 – ADU Development Potential Summary
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KEY TERMS AND ACRONYMNS
ADU Accessory Dwelling Unit
Assessor Parcel Database The primary database used for this analysis, containing detailed financial and physical characteristics of all parcels; this database was joined with the San Mateo County GIS Database to add zoning and unincorporated community names to each parcel
CCI Center for Community Innovation
Community Types One of the six community types assigned to organize data in this analysis: Bayside Flat Communities, Bayside Hill Communities, Coastal Communities, Rural Open Area Communities, Rural Small Towns, San Francisco Urban Communities
Permitted ADU Database An internal database of ADUs maintained by San Mateo County and provided to the Center for Community Innovation in July 2016
San Mateo County GIS Database GIS Database created from combining parcel, zoning, and unincorporated community name shapefiles
Second Unit Ordinance Ordinance approved by the San Mateo County Board of Supervisors in January 2017 to expand opportunities for ADU construction in unincorporated San Mateo County
Zoning Ordinance County-wide zoning ordinance released by the San Mateo County Planning and Building Department in January 2016
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EXECUTIVE SUMMARY This technical report analyzes the physical feasibility of building Accessory Dwelling Units (ADUs) in
unincorporated San Mateo County. After estimating the prevalence of existing ADUs, the report
estimates the potential market for ADUs by reviewing all parcels in terms of ADU ordinance eligibility
restrictions and parcel characteristics.
In order to analyze these markets, we combined multiple sources to create a new Assessor Parcel
Database of 23,383 parcels in unincorporated San Mateo County. In addition, we developed a system of
neighborhood typologies in order to best demonstrate the varied housing situations throughout the
county; within each of these typologies, we selected one to three communities to use as a representative
case study for specific analyses.
Community Type All Unincorporated Communities* Selected Case Study
Communities Total Properties
Analyzed
Bayside Flat Communities
Kensington Square, Menlo Oaks, North Fair Oaks, Sequoia Tract,
Weekend Acres, West Menlo Park
North Fair Oaks, Menlo Oaks, West
Menlo Park 6,391
Bayside Hill Communities
Burlingame Hills, Devonshire, Emerald Lake Hills, Ladera, Los Trancos Woods,
Palomar Park, San Mateo Highlands San Mateo Highlands 5,615
Coastal Communities El Granada, Miramar, Montara, Moss
Beach, Princeton El Granada 5,467
Rural Open Communities
North Skyline, Pescadero East, Pescadero West, Rural Midcoast, San
Gregorio, South Skyline North Skyline 3,546
Rural Small Communities
Dearborn Park, La Honda, Loma Mar La Honda 763
San Francisco Urban Communities
Broadmoor, Country Club Park, Unincorporated Colma
Broadmoor 1,601
TOTAL 23,383
*13 unincorporated communities were eliminated from the analysis due to a limited number of eligible
parcels given the ADU ordinance restrictions.
Key Findings:
Existing ADUs
A total of 726 parcels in unincorporated San Mateo County have a permitted ADU, representing
3.0% of all such parcels. Bayside Hill communities have the highest percent of all parcels with a
permitted ADU (3.9%) while Rural Small communities have the lowest percent of all parcels with
a permitted ADU (1.7%).
Based on fieldwork on a random sample of parcels, we estimate that an additional 11.9% of all
parcels in unincorporated San Mateo County have an unpermitted ADU. The highest percent was
in Menlo Oaks (15.2%) and the lowest percent was in San Mateo Highlands (1.5%).
Another method to determine unpermitted ADUs is via tax assessor data. However, this method
captures only a small share of unpermitted ADUs. Based on this method, we found that 0.5% of
parcels in the unincorporated county with potential unpermitted ADUs.
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ADU Development Potential
An analysis of the assessor database tested seven layers of building constraints on ADU
development potential, including zoning designation, current land use, zoning designation, floor
area requirements, building coverage requirements, slope, and parking requirements.
o A total of 15,187 parcels can feasibly accommodate an ADU between 150 and 1,200
square feet based on their zoning designation, current land use, and allowable floor area.
o Of these, 14,825 parcels have the allowable building coverage and slope to
accommodate a detached ADU adjacent to the primary residence, while 362 parcels are
limited to the construction of an attached ADU that does not expand the current building
footprint on the parcel.
o All 15,187 parcels feasible for ADU development can accommodate at least one ADU
parking space. 14,037 can provide up two ADU parking spaces and 1,150 can only provide
one parking space.
o While all parcels have the sufficient square footage to provide ADU parking, 12,671
parcels are located within one half mile of a transit stop and are exempt from the ADU
parking requirement.
After developing a pool of feasible parcels in the above analysis, a randomized sample of parcels
from this pool were analyzed using Google Earth lot dimension measurements to demonstrate
the impact of setback and parking requirements on ADU development potential given the existing
building footprint. This analysis concluded that 98.2% of parcels can accommodate a one-story
detached ADU and 95.7% can accommodate a two-story detached ADU in the backyard.
Additionally, 98.0% can accommodate one ADU parking space and 97.0% can accommodate two
ADU parking spaces in the front and/or side yards.
The following maps illustrate the total 15,187 parcels that can feasibly accommodate the
construction of an ADU for the whole county and for each of community type.
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Figure 1: Parcels with ADU Construction Feasibility in Unincorporated San Mateo County
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Figure 2: Parcels with ADU Construction Feasibility in Bayside Flat Communities
Figure 3: Parcels with ADU Construction Feasibility in Bayside Hill Communities
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Figure 4: Parcels with ADU Construction Feasibility in Coastal Communities
Figure 5: Parcels with ADU Construction Feasibility in Rural Open Area Communities and Small Towns
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Figure 6: Parcels with ADU Construction Feasibility in San Francisco Urban Communities
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PART 1: PROPERTY DATABASE FOR UNINCORPORATED SAN MATEO COUNTY In order to create a property database for this analysis, the following steps were taken
1. Three shapefiles from San Mateo County’s Information Service’s open data portal were
downloaded in June 2016:
a. “Active Parcels with APN,” which contained a total of 223,444 parcels in the entire
county;
b. “Zoning,” which contained zoning attributes for all parcels in unincorporated areas; and
c. “Unincorporated Areas,” which identified an unincorporated community name for all
parcels in unincorporated areas.
2. These three shapefiles were spatially joined in ArcMap in order to assign both a 1)
unincorporated community name and 2) a zone for all parcels in unincorporated communities.
This spatial join process identified a total of 24,031 parcels in unincorporated San Mateo County.
3. The San Mateo County GIS database was then joined to an assessor parcel database; this assessor
parcel database contained a total of 221,792 parcels in the entire county. The two files were
joined with the “APN” number in the San Mateo County GIS database and the
“PARCEL_NBR_PRIMARY” in the assessor parcel database.
4. The Assessor Parcel Database—which contains more descriptive information on the physical and
financial characteristics of each property—was considered the primary database. The San Mateo
County GIS Database was primarily used to identify properties in the Assessor Parcel Database
that are in an unincorporated communities and then, for these properties, to assign both 1) an
unincorporated community name and 2) a zone.
5. This join process between these two files resulted in identifying a total of 23,823 parcels in the
Assessor Parcel Database that were in unincorporated communities. With this result, a total of
208 properties in the San Mateo County GIS database, or 0.9% of all properties in this database,
could not be matched to the assessor parcel database.
6. Finally, the Assessor Parcel Database was further restricted based on the basic zoning restrictions
in the ADU ordinance Section 6427, which restricts second units to the following locations: “R-1,
R-2, R-E, RH, RM and TPZ Districts outside the Coastal Zone and the R-1 District within the Coastal
Zone.” Please see Table 1 below and Appendix 1.
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Table 1: Assessor Parcel Database - Study Eligibility Summary
Unincorporated Community Name
Coastal vs. Non-Coastal
Zone
Total Number of
Parcels
Total Number of Parcels Eligible per ADU Ordinance
Zoning Restrictions
Percent of all Properties Eligible per ADU Ordinance
Zoning Restrictions
BRISBANE QUARRY Non-Coastal 8 1 12.5%
BROADMOOR Non-Coastal 1,352 1,335 98.7%
BURLINGAME HILLS Non-Coastal 429 428 99.8%
BUTANO FALLS TRACT Non-Coastal 1 0 0.0%
CALIFORNIA GOLF CLUB Non-Coastal 6 6 100.0%
COUNTRY CLUB PARK Non-Coastal 62 62 100.0%
DEARBORN PARK Non-Coastal 83 81 97.6%
DEVONSHIRE Non-Coastal 394 389 98.7%
EL GRANADA Coastal 2,110 1,940 91.9%
EMERALD LAKE HILLS Non-Coastal 2,128 2,123 99.8%
HARBOR/INDUSTRIAL Non-Coastal 60 0 0.0%
KENSINGTON SQUARE Non-Coastal 74 74 100.0%
LA HONDA Non-Coastal 555 542 97.7%
LADERA Non-Coastal 554 544 98.2%
LOMA MAR Non-Coastal 125 92 73.6%
LOS TRANCOS WOODS Non-Coastal 299 287 96.0%
MENLO OAKS Non-Coastal 277 277 100.0%
MIRAMAR Coastal 440 416 94.5%
MOBILE HOME PARKS Non-Coastal 29 0 0.0%
MONTARA Coastal 1,328 1,124 84.6%
MOSS BEACH Coastal 1,083 1,001 92.4%
NORTH FAIR OAKS Non-Coastal 3,185 1,991 62.5%
NORTH SAN GREGORIO Coastal 42 0 0.0%
NORTH SKYLINE Non-Coastal 1,412 1,364 96.6%
OLYMPIC COUNTRY CLUB Non-Coastal 7 5 71.4%
PALOMAR PARK Non-Coastal 358 340 95.0%
PENINSULA GOLF AND COUNTRY CLUB Non-Coastal 3 3 100.0%
PESCADERO WEST Coastal 409 78 19.1%
PESCDERO EAST Coastal 328 131 39.9%
PRINCETON Coastal 506 12 2.4%
RURAL MIDCOAST Coastal 427 2 0.5%
SAN BRUNO MTN PARK Non-Coastal 62 40 64.5%
SAN FRANCISCO INTERNATIONAL AIRPORT Non-Coastal 16 0 0.0%
SAN FRANCISCO JAIL Non-Coastal 4 4 100.0%
SAN FRANCISCO WATERSHED LANDS Non-Coastal 154 151 98.1%
SAN GREGORIO Coastal 265 3 1.1%
SAN MATEO HIGHLANDS Non-Coastal 1,453 1,433 98.6%
SEQUOIA TRACT Non-Coastal 1,331 1,293 97.1%
SOUTH SKYLINE Non-Coastal 705 691 98.0%
STANFORD LANDS Non-Coastal 48 48 100.0%
UNINCORPORATED COLMA Non-Coastal 187 40 21.4%
WEEKEND ACRES Non-Coastal 136 135 99.3%
WEST MENLO PARK Non-Coastal 1,388 1338 96.4%
TOTAL 23,823 19,824 83.2%
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PART 2: SELECTED COMMUNITIES Following the review of all unincorporated parcels, the communities were further reviewed to develop
community typologies and selected communities to guide subsequent research and report writing.
As a preliminary step, certain unincorporated communities were eliminated from the dataset due to
either primarily non-residential use or lack of properties eligible under the ADU ordinance (see Table 2).
Table 2: Eliminated Unincorporated Communities
Eliminated Unincorporated Commodities Reason
Brisbane Quarry Properties primarily intended for heavy industrial use
Butano Falls Tract No properties eligible under ADU ordinance
California Golf Club Properties primarily composed of a golf course
Harbor/Industrial No properties eligible under ADU ordinance
Mobile Home Parks No properties eligible under ADU ordinance
North San Gregorio No properties eligible under ADU ordinance
Olympic Country Club Properties primarily composed of a golf course
Peninsula Golf and Country Club Properties primarily composed of a golf course
San Bruno Mountain Park Properties primarily intended for municipal use
San Francisco International Airport No properties eligible under ADU ordinance
San Francisco Jail Properties primarily intended for municipal use
San Francisco Watershed Lands Properties primarily intended for municipal use
Stanford Lands Properties primarily intended for institutional use
This elimination shifted the assessor parcel database as follows (compare to summary figures in Table 1):
Total Number of Parcels: 23,383
Total Number of Parcels Eligible Under ADU Ordinance Zoning Restrictions: 19,566
% of all Properties Eligible Under ADU Ordinance Restrictions: 83.7%
These communities were then further reviewed on the basis of location, property values, and physical
property characteristics in order to develop a series of seven community typologies and select a
representative community from each typology to guide research. In particular, a preliminary analysis of
ADU construction accommodation feasibility was conducted in order to determine the percentage of
parcels that could support an ADU. A more thorough analysis of construction feasibility can be found in
the “Estimating ADU Development Potential” section of this report. Please see Appendix 1 for detailed
summary tables of community types and selected communities.
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Bayside Flat Communities These communities are located in generally flat areas close to San Francisco Bay; the location of these
communities likely allow for easier access to transportation as well as more potential for ADU
construction on flatter parcels.
Table 3: Bayside Flat Communities – ADU Zoning Eligibility and Construction Accommodation Total Number of Parcels Eligible Under ADU
Ordinance Zoning Restrictions Percent of Parcels with Floor Area to
Accommodate an ADU
Kensington Square 74 100%
Menlo Oaks 277 92.4%
North Fair Oaks 1,991 95.4%
Sequoia Tract 1,293 92.0%
Weekend Acres 135 78.5%
West Menlo Park 1,338 96.1%
TOTAL 5,108 94.2%
We selected North Fair Oaks, Menlo Oaks, and West Menlo Park as our representative communities for this area.
North Fair Oaks is selected as a representative community due to its generally large number of parcels, with nearly all capable of accommodating an ADU. In addition, the neighborhood has unique attributes that provide additional case study information. For example, it is the only community to have its own community plan among all of the unincorporated communities. In addition, North Fair Oaks also has a large Hispanic population, at 73.1%1 of the total population, and relatively low value properties compared to other unincorporated communities in the flat bayside area. Although Menlo Oaks has a generally lower number of eligible parcels compared to other communities, it serves as an important case study for understanding ADU construction in communities with a high property values and home size. The properties in Menlo Oaks have among the highest average assessed value (approximately $1.45 million) and the second-largest average home sizes (approximately 2,600 square feet) in the entire data set. In addition, it has one of the largest average parcel sizes (approximately 21,000 square feet) of all urban-based unincorporated communities in the data set.
Finally, West Menlo Park represents a more stereotypical bayside flats community, especially those found
adjacent to the El Camino Real and Highway 101 Corridor. The lot sizes in West Menlo Park are highly
consistent, located on a gridded street system, and are generally eligible for an ADU. The homes here are
nearly twice the value of those located in North Fair Oaks and can thus be more representative of higher-
income communities.
1 American Community Survey, 5YR 2010-2015: North Fair Oaks CDP, California. Accessed March 25, 2017, https://www.census.gov/quickfacts/table/PST045215/0651840
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Bayside Hill Communities These communities are located on the bayside of the central ridge running the length of San Mateo
county; they are defined by generally steep topography and more expensive assessed values.
Table 4: Bayside Hill Communities - ADU Zoning Eligibility and Construction Accommodation
Total Number of Parcels Eligible Under ADU Ordinance Zoning Restrictions
Percent of Parcels with Floor Area to Accommodate an ADU
Burlingame Hills 428 96.5%
Devonshire 389 55.0%
Emerald Lake Hills 2,123 71.7%
Ladera 544 90.4%
Los Trancos Woods 287 80.8%
Palomar Park 340 75.6%
San Mateo Highlands 1,433 97.5%
TOTAL 5,544 81.7%
We selected San Mateo Highlands as the representative community for this area due to its large number of parcels in eligible zones and with floor area to accommodate an ADU (approximately 97.5% or 1,397). In addition, San Mateo Highlands contains a large community of homes developed by Joseph Eichler, a well-known mid-century modern architect. San Mateo Highlands could provide an interesting case study as to how ADUs can be incorporated into a community with both consistent parcel and building layout as well as strongly defined architectural standards.
Coastal Communities The communities are situated adjacent to the oceanfront west coast of San Mateo County. In addition, they are all within the Coastal Zone.
Table 5: Coastal Communities - ADU Zoning Eligibility and Construction Accommodation
Total Number of Parcels Eligible Under ADU Ordinance Zoning Restrictions
Percent of Parcels with Floor Area to Accommodate an ADU
El Granada 1,940 76.2%
Miramar 416 48.8%
Montara 1124 80.2%
Moss Beach 1,001 68.7%
Princeton 12 16.7%
TOTAL 4,493 72.8%
El Granada was the selected community for coastal communities since it had the largest number of
eligible properties to review.
Rural Open Communities These communities are primarily located to the west of San Mateo County’s central ridge; they are defined by generally large parcels sizes and smaller homes.
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Table 6: Rural Open Communities - ADU Zoning Eligibility and Construction Accommodation
Total Number of Parcels Eligible Under ADU
Ordinance Zoning Restrictions Percent of Parcels with Floor Area to
Accommodate an ADU
North Skyline 1,364 37.4%
Pescadero East 131 65.6%
Pescadero West 78 65.4%
Rural Midcoast 2 0.0%
San Gregorio 3 66.7%
South Skyline 691 21.0%
TOTAL 2,269 35.0%
North Skyline was selected since it had the largest number of eligible properties to review.
Rural Small Towns These communities are located to the west of San Mateo County’s central ridge but also inland from the
county’s coastal west coast. They are defined by generally smaller parcel sizes (as compared to the rural
open areas) and smaller homes.
Table 7: Rural Small Towns - ADU Zoning Eligibility and Construction Accommodation
Total Number of Parcels Eligible Under ADU Ordinance Zoning Restrictions
Percent of Parcels with Floor Area to Accommodate an ADU
Dearborn Park 81 35.8%
La Honda 542 57.9%
Loma Mar 92 46.7%
TOTAL 715 54.0%
La Honda was selected for rural small towns since it had the largest number of eligible properties to
review.
As an important side note, we decided to separate the analysis of each of these small towns from the
larger surrounding rural open areas; in general, these small towns seem to present different
characteristics (especially in terms of house and property size) than the surrounding areas and therefore
should be analyzed separately.
San Francisco Urban Communities These communities are located on the northernmost end of San Mateo county and are essentially an extension of the San Francisco urban area.
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Table 8: San Francisco Urban Communities - ADU Zoning Eligibility and Construction Accommodation
Total Number of Parcels Eligible Under ADU Ordinance Zoning Restrictions
Percent of Parcels with Floor Area to Accommodate an ADU
Broadmoor 1,335 97.8%
Country Club Park 62 85.5%
Unincorporated Colma 40 95.0%
TOTAL 1,437 97.1%
We selected Broadmoor as the representative community of San Francisco Urban Communities because it
has a large number of eligible parcels and consistent parcel sizes, which improves the applicability of a
randomized sample to the larger community.
Table 9: Summary of Community Types, Selected Communities, and Additional Case Study Possibilities Community Type Selected Representative Community Additional Case Study Possibilities
Bayside Flat Communities North Fair Oaks Community with existing community plan; large Hispanic population; low-valued properties
Menlo Oaks Community with high-valued properties, large home sizes, and large parcel sizes (for urban properties)
West Menlo Park X
Bayside Hill Communities San Mateo Highlands Community with consistent parcel and building sizes; high architectural standards
Coastal Communities El Granada X
Rural Open Area Communities
North Skyline X
Rural Small Towns La Honda X
San Francisco Urban Communities
Broadmoor X
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PART 3: ESTIMATING EXISTING ADUS To create an effective set of programs to encourage ADU construction, it is important to first understand
the existing prevalence of these types of units. ADUs are often a more common form of housing than is
officially indicated in housing data. ADUs not listed in official housing data are often hidden, not
permitted, and unregulated by local housing codes.
In this section, we aim to understand the current extent of existing ADUs in San Mateo County. This
information may assist government departments in two regards. First, understanding the general
prevalence of existing ADUs can help with the implementation of both ADU marketing and
implementation strategies. Second, an understanding of the general number of unpermitted units can
assist with the targeting of ADU amnesty programs.
This review relied upon both a variety of data sources and fieldwork in order to develop estimates.
Data Review of Existing ADUs in San Mateo County Three data reviews were conducted in order to better understand existing ADUs:
A review of San Mateo County’s Permitted ADU Database
A review of the tax assessor database to identify parcels with a unit count above zoning and use restrictions; review conducted to identify potential unpermitted ADUs
A review of the assessor database to identify parcels with duplicate addresses; review conducted to identify potential unpermitted ADUs
Permitted ADUs: San Mateo County ADU Database San Mateo County tracks permitted ADUs through an internal database (“Permitted ADU Database”). We received this word-based database in July 2016 and transferred into an excel format through the following steps: 1. Create Single Row Data: The July 2016 database provided multiple pieces of information, on multiple
lines, for each ADU. This information was transferred into a single-row excel database in order to better accommodate data and mapping analysis. The key found in Appendix 2 provides a full description of the how data was transferred into each category.
2. Match to Unincorporated Community Database Name: In particular, the naming conventions used in the July 2016 database did not align with the naming conventions identified for the study. The following steps were taken to align unincorporated community names:
a. Organize APN Numbers: Up to three APNs were associated with each ADU. These APNs were divided into individual cells, with up to three APNs associated with each of the 726 ADU properties found within the database. Then, these APN numbers were matched to the Assessor Parcel Database.
b. Align APN Numbers to Unincorporated Community Names: If a match was found between the APN number in the Permitted ADU Database and the Assessor Parcel Database, the ADU property was re-assigned with the community name based in the Assessor Parcel Database. In the limited number of cases with an ADU property that had multiple APNs, we found that the matched community name remained consistent across all APNs.
c. Divide Remaining APNs: Of the 726 ADU properties analyzed, approximately 8% or 58 properties had APN numbers that did not align to the Permitted ADU Database. Of these 58 properties, the following decisions (Table 10) were made to allocate these properties under this study’s naming conventions based on the original community name:
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Table 10: Division of Un-Matched ADU Properties to Study Unincorporated Naming Conventions Original Community Name Count Division Strategy
El Granada, Miramar (R-1 Coastal Areas Only) 7 1/2 into El Granada, 1/2 into Mirmar
Montara, Moss Beach, Princeton (R-1 Coastal Areas Only)
12 1/3 into Montara, 1/3 into Moss Beach, 1/3 into Princeton
Emerald Lake Hills - RH & R-1 Areas 31 All into Emerald Lake Hills
West Menlo Park/ Weekend Acres 5 1/2 into West Menlo Park, 1/2 into Weekend Acres
Los Trancos Woods-Vista Verde (R-1 & R-E Areas)
2 All into Los Trancos Woods
San Mateo 1 All into San Mateo Highlands
d. Unknown and Not Analyzed: Finally, a total of 24 ADU properties from the Permitted ADU
Database were not originally assigned a community name and had APN numbers that did not align to properties within the Assessor Parcel Database; these properties were labelled with an “Unknown” community name. In addition, two properties were labelled as “Not Analyzed” for being matched to properties in the Assessor Parcel Database located in the “San Francisco Watershed Lands,” which was a community eliminated from the study.
Table 11 presents the range of ADUs currently present in each type of unincorporated community. As the data demonstrates, ADUs are most commonly present in both the Bayside Flat and Bayside Hill Communities, with 3.5% and 3.9% of all properties respectively supporting an ADU; ADUs are least common in Rural Small Towns, with 1.7% of the properties supporting an ADU.
Table 11: Summary of Permitted ADUs in San Mateo County
Community Type Number of
Permitted ADUs Total # of Properties in
Community Type Percent of Properties in Community Type Supporting a Permitted ADU
Bayside Flat 221 6,391 3.5%
Bayside Hill 217 5,615 3.9%
Coastal 114 5,467 2.1%
Rural Open 103 3,546 2.9%
Rural Small Town 13 763 1.7%
SF Urban 32 1,601 2.0%
Not Analyzed 2 433 0.5%
Unknown 24 x x
TOTAL 726 23,816 3.0%
Please see Appendix 2 for a full review of the Permitted ADU Database.
Non-Permitted ADUs: Unit Count Above Zoning and Use Restriction In addition to the Permitted ADU Database, the Assessor Parcel Database was test to identify potential
counts of non-permitted ADUs. The first test was to identify parcels with a unit count and greater than
expected for the zoning restriction and use category.
To run this test, we first identified parcels with a unit count greater than allowed for in the zone assigned
to the parcels. The zones reviewed are listed in Table 12.
22
Table 12: Unit Count Restriction by Zone in Unincorporated San Mateo County2 Zone Unit Restriction
R-1: One-Family Residential District “One-family dwellings”
R-2: Two-Family Residential District “One (1) two-family dwelling or two (2) one-family dwellings”
R-E: Residential Estates District “One-family dwellings”
RH: Residential Hillside District “Provide residential areas intended primarily for the location of one-family dwellings, accessory structures and uses on hillside parcels”
If a parcel had a unit count greater than allowed in the assigned zone, the use for the parcel was then
identified. If the assigned use code, as listed in Table 13 below, allowed for a unit count greater than the
assigned zoning code, then the property was eliminated. However, if the assigned use code aligned with
the assigned zoning, and indicated that the unit count was greater than expected (i.e. a parcel with 2
units but within an assigned R-1 zone and with an RSFR use code), the property was included in Table 13
below.
Table 13: Use Codes and Definitions in Tax Assessor Database Use Code Definition
RDUP Duplex
RMFD Multi-Family Dwelling (2-4)
RQUA Quadraplex
RSFR Single-Family Residence
RTRI Triplex
Table 14 below shows that a fairly low number of parcels have unit counts greater than zoning or use
restrictions, particularly as percentage of all properties in each community type. In addition, it should be
noted that these numbers could be demonstrating either erroneous entries or perhaps even parcels with
an ADU.
Table 14: Summary of Parcels with Unit Counts Greater than Zoning and Use Restrictions
Community Type Number of Parcels with Unit Counts
Greater than Zoning and Use Restrictions
Percent of all Properties with a Unit Count Above Zone and Use Restriction
Bayside Flat 17 0.3%
Bayside Hill 8 0.1%
Coastal 8 0.1%
Rural Open Area 1 0.0%
Rural Small Towns 4 0.5%
San Francisco Urban 2 0.1%
TOTAL 40 0.2%
Please see Appendix 3 – Unit Count Above Zoning and Use Restrictions for a full review of this data
source.
2 County of San Mateo: Department of Planning and Building, Zoning Regulations, January 2016. (San Mateo County, CA), http://planning.smcgov.org/sites/planning.smcgov.org/files/ZoneRegs-Nov2016.pdf
23
Non-Permitted ADUs: Duplicate Address The Assessor Parcel Database was also tested to identify potentially non-permitted ADUs through
duplicate address listings. A full address descriptor—combining the number, street, and city—was
created for each of the parcels. A duplication test then listed each time the address was listed more than
once. The results of this review are listed below in Table 15 and can additionally be viewed in Appendix 4.
Similar to the unit count test above, this review resulted in that a fairly low number of parcels with
duplicate addresses, particularly as percentage of all properties in each community type. It should again
be noted that these numbers could be demonstrating either erroneous entries or perhaps even parcels
with an ADU.
Table 15: Number of Duplicate Addresses
Community Type Number of Duplicate Addresses
Total Number of Properties in Community
Percent of Properties in Community with a Duplicate Address
Bayside Flat 6 6,391 0.1%
Bayside Hill 24 5,615 0.4%
Coastal 10 5,467 0.2%
Rural Open Area 22 3,546 0.6%
Rural Small Towns 5 763 0.7%
San Francisco Urban 1 1,601 0.1%
TOTAL 68 23,383 0.3%
Field Review of Existing ADUs in San Mateo County Although the data sets analyzed above provide some indication as to the extent of ADUs in San Mateo
County, in-person field review of actual parcels can provide more thorough evidence of ADUs. In
particular, certain unpermitted units may not be captured in any existing data sets, but only identified
through observation. We surveyed randomly selected properties in the eight selected neighborhoods in
order to count existing ADUs.
Methods
Property Selection Process
The following steps were taken to choose a random sample of properties from the eight selected
neighborhoods:
1. Limit Parcel Selection Pool to Selected Eligible Zones: San Mateo County’s new ADU ordinance sets specific restrictions on the zones that are eligible to construct an ADU; the pool of eligible parcels was first limited to these zones. In addition, although the ADU ordinance allows for the construction of ADUs in zones that are primarily intended for multi-family units, we eliminated properties from the eligible pool. Only parcels in single-family or primarily single-family zones were included in the eligible pool; this limitation was to reduce erroneous observations of suspected ADUs that are actually multi-family units. Therefore, the following specific decisions were made regarding the pool of eligible parcels:
1. INCLUDED - R-1 (in both the coastal and non-coastal zones): “One Family Dwellings” as labelled in section 6161 (a) on page 6.1;
24
2. INCLUDED - R-E (in the non-coastal zone): “One Family Dwellings” as labelled in section 6151 (a) on page 5.1.;
3. INCLUDED - RH (in the non-coastal zone): “Provide residential areas intended primarily for the location of one-family dwellings, accessory structures and uses on hillside parcels” as labelled in section 6800.1 on page 35.1.3;
4. NOT INCLUDED – R-2, RM, and TPZ: Although R-2, RM, and TPZ outside of the Coastal Zone are allowed to construct an ADU, they were not included in order to reduce erroneous observations (all of these zones allow for multi-family housing units); and
5. NOT INCLUDED - Blank Zones: Properties without a labelled zone were not included. 2. Limit Parcel Selection Pool to Single-Family Residential Use: Next, we limited the pool to parcels
with a single-family residential use (i.e. RSFR use code in Column: USE_CODE_STD); parcels without an assigned Use Code were removed.
3. Remove Parcels with a Known ADU: Next, we removed all parcels with a permitted ADU as listed in San Mateo County’s Permitted ADU Database. However, of the 726 ADUs listed in this database, 127 had APNs that did not match to the Assessor Parcel Database. Of these 127 ADUs, 36 were located in selected communities (known due to the original “Area” identifier in the Permitted ADU Database). Of these 36 ADUs, were able to remove eight via an address match with the Assessor Parcel Database. Therefore, up to 28 parcels with an ADU may have remained in the pool of eligible parcels (see Table 16 for a summary of parcels with ADUs that may have potentially remained within the selection pool); however, these 28 ADU parcels could also be located in non-selected communities given the original naming conventions (see Table 16) and additionally represent only a negligible 0.35% of the total number of parcels eligible for review (see Table 17).
4. Randomly Select Properties from Each Neighborhood: A final database of eligible properties was
created for each of the eight selected neighborhoods; we then randomly selected 66 properties for the final review. The final number of parcels eligible for review is listed in Table 17.
3 County of San Mateo: Department of Planning and Building, Zoning Regulations, January 2016. (San Mateo County, CA), http://planning.smcgov.org/sites/planning.smcgov.org/files/ZoneRegs-Nov2016.pdf
25
Table 16: Removal Steps of Existing ADUs from Eligible Pool
Community Type
Selected Community
Community Name with
Original Permitted ADU
Database
Number of APN Unmatched Parcels between Permitted ADU Database and
Assessor Parcel Database via APN
Number of Parcels Matched
between Permitted ADU Database and
Assessor Parcel Database via
Address Match
Remaining Number of Unmatched
Parcels
Bayside Flat
North Fair Oaks North Fair Oaks 15 4 11
Menlo Oaks Menlo Oaks 7 1 6
West Menlo Park
West Menlo Park/Weekend
Acres 5 2 3
Bayside Hill San Mateo Highlands
San Mateo 1 0 1
Coastal El Granada
El Granada, Miramar (R-1 Coastal Areas
Only)
7 1 6
Rural Open North skyline [Not Listed] 0 0 0
Rural Small Town
La Honda [Not Listed] 0 0 0
Sand Francisco
Urban Broadmoor Broadmoor 2 0 2
TOTAL 36 8 28
Table 17: Pool of Eligible Parcels for Online and Field Review
Community Type
Selected Community Total Number of Parcels in Community
Total Number of Parcels Eligible for Review Given
Zone, Use, and Removal of Existing ADUs
Percent of All Eligible Parcels
Selected for Review
Bayside Flat North Fair Oaks 3,185 1,749 3.77%
Menlo Oaks 277 231 28.57%
West Menlo Park 1,388 1,265 5.22%
Bayside Hill San Mateo Highlands 1,453 1,380 4.78%
Coastal El Granada 2,110 1,547 4.27%
Rural Open North skyline 1,412 329 20.06%
Rural Small Town
La Honda 555 313 21.09%
Sand Francisco
Urban
Broadmoor 1,352 1,279 5.16%
TOTAL 11,732 8,093 6.52%
26
Research Assistant Training
After these properties were selected, a group of research assistants were trained to review each property
to determine if there was an ADU present on the property. These assistants were trained to understand
the various characteristics that may indicate an ADU is present, such as having multiple mailboxes,
electric meters, or address signs. They were additionally provided a full training on how to use Google
Earth Pro to conduct online data collection and the various methods to utilize during field data collection.
Please see Appendix 5 for a full review of this research assistant training.
Online Review
The research assistants first conducted an online review of each property to determine if an ADU was
present. They were asked to make a final determination if the ADU was present with these levels of
response:
YES – A definitive indication that an ADU is situated on the property
NO – No indication that an ADU is situated on the property
MAYBE – Some indication that an ADU may be situated on the property, but difficult to make a
final determination
UNCLEAR – Unable to make a determination if an ADU is present on the property due to tree
cover, obstructions, or unclear google imagery.
After the first online review data was finalized, certain research assistant reviewers appeared to either
over- or under- estimated the presence of ADUs in their assigned communities. Of the eight communities
reviewed, three seemed to have skewed results, with the reviewer confirming either “YES” or “NO” to the
presence of an ADU more often than expected. The following decisions were made to conduct secondary,
follow-up reviews and then to combine results from multiple reviewers:
Table 18: Secondary Review Combinations
Community Concerns from First
Review Secondary Review
Process Secondary Review
Results Combination Process from First and
Second Review
El Granada
The original reviewer was answering “YES”
more often the expected; a total of 20 of 66 properties reviewed had a final “YES” determination
by the original reviewer
A second, more conservative
reviewer was sent these “YES”
responses for an additional review
Of the 20 re-reviewed properties,
the secondary reviewer determined
0 “YES” results, 12 “NO” results, 8
“MAYBE” results, and 0 “UNCLEAR” results
After this second review, (1) all “NO” answers from the second reviewer were accepted as the final answer; (2) ½ of all “MAYBE” answers from
the second reviewer were accepted as the final answer and sent for field
review; and (3) ½ of all of the “MAYBE” answers from the second
reviewer were not accepted and the original “YES” answer was accepted
as the final answer
North Fair Oaks
The original reviewer was answering “NO”
more often the expected; a total of
57 of the 66 properties reviewed
had a final “NO” determination by the
original reviewer
A second, less conservative
reviewer was sent these “NO”
responses for an additional review
Of the 57 re-reviewed properties,
the secondary reviewer determined
4 “YES” results, 35 “NO” results, 16
“MAYBE” results, and 2 “UNCLEAR” results
After this second review, (1) All mutual “NO” answers from the first
and second review were accepted as the final answer; (2) All of the new
“YES” answers from the second review were accepted as the final
result; (3) of the 18 combined “MAYBE” and “UNCLEAR” answers from the second review, ½ or 9 of
these new answers were not
27
accepted and the original “NO” answer was accepted as the final
result; and (4) of the 18 combined “MAYBE” and “UNCLEAR” answers from the second review, ½ or 9 of these new answers were accepted
and sent for further fieldwork review
North Skyline
The original reviewer was answering “YES”
more often the expected; a total of 8
of 66 properties reviewed had a final “YES” determination
by the original reviewer
A second, more conservative
reviewer was sent these “YES”
responses for an additional review
Of the 8 re-reviewed properties, the
secondary reviewer determined 0 “YES”
results, 1 “NO” result, 3 “MAYBE” results, and 4 “UNCLEAR”
results
After this second review, (1) all “NO” answers from the second reviewer were accepted as the final answer;
(2) approximately ½ or 3 of all “MAYBE”/”UNCLEAR” answers from the second reviewer were accepted as the final answer and sent for field review; and (3) approximately ½ or 4 of the “MAYBE”/”UNCLEAR” answers from the second reviewer were not
accepted and the original “YES” answer was accepted as the final
answer
After this process of combining online review responses, all “YES” or “NO” answers from the online
review were accepted as the final answer for the property. However, all “MAYBE” or “UNCLEAR” results
were determined to need a field review to verify the presence of an ADU.
Field Review
After the online review results were finalized, the research assistants were sent to conduct a field review
of all properties with either an “UNCLEAR” or “MAYBE” result during the online review. The researchers
followed the ADU identification methodology (see Appendix 5) and were additionally aided with field
research aided with field research letters from both CCI and San Mateo County Department of Housing
(see Appendix 6 and 7). A template of the notes page used to collect both online and field review notes
can be viewed in Appendix 8.
Table 19 below summarizes the number of properties with a final ADU determination made either online
or in the field.
Table 19: Final ADU Determination in Online vs. Field Review
Community Type Community Name Final ADU Determination
in Online Review
Final ADU Determination in Field
Review
Bayside Flat
North Fair Oaks 48 18
Menlo Oaks 37 29
West Menlo Park 39 27
Bayside Hill San Mateo Highlands 46 20
Coastal El Granada 48 18
Rural Open North Skyline 8 58
Rural Small Town La Honda 27 39
San Francisco Urban Broadmoor 50 16
TOTAL 303 225
28
Results The cumulative results from the online and field review can be viewed in Table 20 below. In addition,
please view Appendix 9 for the full data set resulting from the online and field review.
Table 20: Online and Field Review Results
Community Type Community Name ADU
Identified NO ADU
Identified Unclear Result
Estimated Percent of All Single-Family Zone and
Use Parcels with an ADU
Bayside Flat
North Fair Oaks 6 60 0 9.1%
Menlo Oaks 10 56 0 15.2%
West Menlo Park 6 60 0 9.1%
Bayside Hill San Mateo Highlands
1 65 0 1.5%
Coastal El Granada 9 54 3 13.6%
Rural Open North Skyline4 N/A N/A N/A N/A
Rural Small Town La Honda 5 53 8 7.6%
San Francisco Urban Broadmoor 7 55 4 10.6%
TOTAL 63 427 38 11.9%
4 Data for North Skyline is not included due to a generally high amount of tree cover for both the online and field review.
29
PART 4: ESTIMATING ADU DEVELOPMENT POTENTIAL This data analysis explores the effect of regulatory and physical constraints on ADU development potential for properties located in Unincorporated San Mateo County. Estimates for the number of parcels that can feasibly accommodate the construction of an ADU were developed by analyzing two datasets:
Assessor Parcel Database for Unincorporated San Mateo County, the creation of which is described in Part 1 of this technical report
Google Earth Parcel Measurements, gathered by our research team between October 2016 and December 2016
Each dataset was comprised of different property samples and provided different data variables for analysis. As such, each dataset allowed for the testing of distinct building constraints on ADU construction, which are summarized in Table 21 and described in detail in subsequent sections. All regulatory constraints tested in this report were based upon the county-wide Zoning Ordinance from January 2016 and the Second Unit Ordinance adopted in January 2017. Because the county has numerous hillside properties that can be difficult and expensive to build on, we have also included slope as an additional layer of physical constraint.
Table 21: ADU Development Potential Analysis Dataset Summary
Dataset Data Sample Data Variables Provided Building Constraints Tested
Assessor Parcel Database for Unincorporated San Mateo County
All active parcels in Unincorporated San Mateo County
-Total lot size -Current land use/vacancy -Square footage for all building floors -Square footage for first building floor -Existing garage and carport parking spaces
-Zoning Designation -Current Land Use -Floor Area Requirements -Building Coverage Requirements -Slope -Parking Requirements
Google Earth Parcel Measurements
Random sample of 66 properties from each of the 8 selected communities
-Front and side dimensions -Backyard dimensions
-Setback Requirements -Parking Requirements
Assessor Parcel Data Analysis Our overall approach to the Assessor Parcel Data Analysis applies multiple layers of building constraints to all parcels located in Unincorporated San Mateo County to incrementally filter out properties on which ADU construction would be unpermitted or physically infeasible. Each building constraint test was more restrictive than the one before it, decreasing the pool of potentially feasible parcels with each step of analysis. Building constraint layers were applied in the following order:
30
1. Zoning Designation
2. Current Land Use
3. Floor Area Requirements
4. Building Coverage Requirements
5. Slope
6. Parking Requirements
The first three building constraints, zoning designation, current land use, and floor area requirements, were used to develop an estimate for the number of properties that could accommodate some type of ADU. Within that pool of feasible properties, the fourth and fifth building constraints, building coverage requirements and slope, were used to determine whether a property could accommodate a detached ADU or an attached ADU. Finally, a parking analysis was conducted to determine how ADU parking regulations affected the feasible pool of properties and whether they could accommodate one or two ADU parking spaces. The results for each building constraint layer are summarized in the subsequent sections and Appendix 10 provides a detailed description the methodology for each step of the analysis. Overall, our analysis concluded that 15,187 parcels in Unincorporated San Mateo County can accommodate the construction of either an attached or detached ADU and can provide at least one parking space for that ADU. The following graphic and map illustrate the final feasibility analysis results and the process by which these estimates were obtained:
Table 22: Assessor Parcel Data Analysis Summary of Results
Universe of properties 23,383 parcels located in Unincorporated San Mateo County
ADU Accommodation 15,187 parcels can accommodate an ADU 8,196 parcels cannot
accommodate an ADU
Attached vs. Detached 14,825 can accommodate a
detached ADU 362 can only accommodate
an attached ADU N/A
Parking Accommodation
14,037 can accommodate up to two ADU parking spaces
1,150 can only accommodate one ADU parking space
N/A
31
Figure 7: Assessor Parcel Data Analysis Process Diagram
Parking
Slope
Building Coverage Requirements
Floor Area Requirements
Current Land Use
Zoning Designation
Universe of Properties23,383 parcels located in Unincorporated San
Mateo County
19,566 parcels in zone where ADUs
are permitted
16,262 parcels with current
residential use
15,187 parcels with sufficient floor area
to build ADU
15,075 parcels with sufficient building
coverage for detached ADU
14,825 parcels with slope that can accomodate
detached ADU
250 parcels with slope that can accomodate
attached ADU
112 parcels with sufficient building
coverage for attached ADU
1,075 parcels with insufficient floor
area to build ADU
3,304 parcels with current non-residential use or
vacant
3,817 parcels in zone where ADUs are unpermitted
14,037 parcels can
provide two ADU
parking spaces
1,150 parcels can
only provide one
ADU parking
space
32
Building Constraint Layer 1: Zoning Designation According to the Second Unit Ordinance, second units will be allowed in the R-1, R-2, R-E, RH, RM and TPZ Districts outside the Coastal Zone and the R-1 District within the Coastal Zone. These zones each allow for different residential uses at the parcel level but are generally targeted for low-density residential housing. There are a total of 19,566 properties located in these zones that are eligible for the construction of an ADU as well as a primary residential structure.
Table 23: Parcels Eligible for ADU Construction by Zoning Designation5 Zone Description Permitted Residential Uses Parcels
R-1 One-Family Residential District Single Family Dwellings 15,706
R-2 Two-Family Residential District Single Family Dwellings & Duplexes 142
R-E Residential Estates District Single-Family Dwellings 209
RH Residential Hillside District Single-Family Dwellings 2,047
RM Research Management District Single Family & Multi-Family Dwellings 1,289
TPZ Timberland Preserve Zone Generally allows “residential housing” 173
Total number of parcels with zoning that allows for ADUs 19,566
Figure 8: Parcels Eligible for ADU Construction under Zoning Designation, by Community Type
While the construction of ADUs is permitted on all of these parcels per the ordinance’s zoning restrictions, there are a number of other development standards and physical site characteristics that affect the feasibility of actually building an ADU.
5 County of San Mateo: Department of Planning and Building, Zoning Regulations, January 2016. (San Mateo County, CA), http://planning.smcgov.org/sites/planning.smcgov.org/files/ZoneRegs-Nov2016.pdf
5,108 5,544
4,493
2,269
715
1,437
-
1,000
2,000
3,000
4,000
5,000
6,000
Bayside Flat Bayside Hill Coastal Rural Open Rural SmallTown
San FranciscoUrban
Par
cels
loca
ted
in e
legi
ble
zo
nes
Community Types
33
Building Constraint Layer 2: Current Land Use Not all of the parcels located in the zones described above are currently used for residential activities. We assume that in order to build an ADU, a primary residential structure must already exist on the parcel such that a property owner can build a second unit within or adjacent to that structure in the near future. For this reason, vacant parcels and parcels with non-residential land uses, such as commercial and institutional buildings, were excluded from the pool of feasible parcels. In our county-wide pool of eligible parcels, there are 3,304 vacant and non-residential parcels, bringing the number of parcels feasible for ADU construction down to a total of 16,262 with this constraint.
Building Constraint Layer 3: Floor Area Requirements According to the Second Unit Ordinance, the floor area of an ADU cannot exceed 750 square feet or 35%
the primary residence floor area for detached ADUs and 50% for attached ADUs, whichever is larger. The
maximum floor area cap for all ADU types is 1,200 square feet. While the ordinance does not provide
minimum floor area requirements, the statewide minimum floor area for a residential dwelling unit is 150
19,566 parcels located in zone that permits ADU construction
23,383 parcels
located in Unincorporated San Mateo
County
16,262 parcels with current residential use
19,566 parcels
located in zone that
permits ADU construction
34
square feet6. Finally, the floor area of a second unit will count against the total floor area allowed on the
parcel as defined by the parcel’s zoning designation.
Given these parameters, we generated a preliminary list of parcels feasible for ADU construction by identifying those with sufficient floor area to accommodate an ADU between 150 square feet and 1,200 square feet. We did this by determining the maximum floor area ratio allowed on each property given its zoning designation from the Zoning Ordinance and subtracting the existing built floor area on the property from the Assessor Parcel Database. The residual square footage is the maximum floor area that can used for the construction of a new ADU on the parcel. If a parcel has already maxed out on its allowable floor area, we assume it will not be feasible to build a new ADU on that property. This calculation yielded 15,187 properties with sufficient remaining floor area to build an ADU. The following table and figure below detail the maximum ADU size that can be accommodated by the ADU size and community type.
Table 24: Parcels Capable of ADU Construction based on Floor Area Availability, by ADU Size ADU Size Square footage Parcels with floor area to accommodate ADU
Tiny Home 150-250 15,187
Studio 250-500 14,995
One-Bedroom 500-750 14,202
Two-Bedroom 750-1,000 9,896
Three-Bedroom 1,000-1,200 8,407
Figure 9: Parcels Capable of ADU Construction based on Floor Area Availability, by Community Type
6 California Department of Housing and Community Development, “Accessory Dwelling Unit Memorandum,” December 2016. http://www.hcd.ca.gov/policy-research/docs/2016-12-12-ADU-TA-Memo.docx.pdf
4,810 4,528
3,273
794 386
1,396
-
1,000
2,000
3,000
4,000
5,000
6,000
Bayside Flat Bayside Hill Coastal Rural Open Rural SmallTown
San FranciscoUrban
Pro
per
ties
wit
h fe
asib
le f
loo
r ar
ea
35
Building Constraint Layer 4: Building Coverage Requirements The Second Unit Ordinance states that an ADU can either be a detached unit independent from the primary residence or an attached unit built as an addition or extension of the primary residence. Starting with the pool of properties with a feasible allowable floor area for ADU construction, we used the maximum building coverage assigned to each zone in the Zoning Ordinance in order to determine (on a preliminary basis) if each parcel within the pool could accommodate an attached ADU or a detached ADU. Attached: If the parcel has already reached its maximum allowable building coverage, this means that a new ADU would need to be incorporated into the existing primary residence as an attached ADU without expanding the current ground floor building footprint. In order to construct an ADU, owners of such properties would need to add an additional story or renovate the existing space within their homes. Our analysis found that 112 properties have maxed out on their allowable building coverage and are limited to building an attached ADU. Detached: If a property has not already reached its maximum allowable building coverage, this means that there is room to expand the ground floor building footprint. These property owners are not limited to building attached ADUs within the structure of their homes. They have the additional options of building a separate ADU structure in their backyard or building an addition abutting the primary residence on the ground floor. We found that the majority of properties, or 15,075, have enough remaining allowable building coverage to accommodate a detached ADU.
Table 25: Attached ADU vs. Detached ADU Feasibility based on Building Coverage, by Community Type
Community Type Building coverage can only
accommodate attached ADU Building coverage can
accommodate detached ADU
Bayside Flat 3 4,807
Bayside Hill 52 4476
Coastal 43 3,230
Rural Open Area 6 788
Rural Small Towns 6 380
San Francisco Urban 2 1,394
Unincorporated County Total 112 15,075
15,187 properties with sufficient allowable floor area to build an ADU
16,262 parcels with
current residential
use
36
Building Constraint Layer 5: Slope Unincorporated San Mateo County has a number of hill communities where the steep topography will likely inhibit the construction of a detached ADU adjacent to the primary residence. As a general rule of thumb, any property with a slope that is greater than 25% requires extra structural supports for newly constructed buildings, which can significantly increase project costs. Thus, while building an ADU on such properties may be technically feasible given building coverage regulations, it may not be realistic in terms of the overall cost. Properties that were preliminarily identified as feasible for detached ADU construction given building coverage availability are broken down by slope below. One can see that 14,825 parcels can accommodate a detached ADU because they have both sufficient building coverage and a slope that is less than 25%, while 250 parcels have a slope greater than 25% and thus can only accommodate an attached ADU despite the fact that they have sufficient building coverage for a detached ADU.
Table 26: Attached ADU vs. Detached ADU Feasibility based on Slope, by Community Type7 Community Type Slope that can only accommodate
attached ADU (>25%) Slope that can accommodate
detached ADU (<25%)
Bayside Flat 59 4,749
Bayside Hill 150 4,326
Coastal 7 3,223
Rural Open 28 759
Rural Small Town 1 379
San Francisco Urban 5 1,389
Unincorporated County Total 250 14,825
7 Data Source: San Mateo County Assessor, San Mateo County Information Services, National Elevation Dataset
15,075 parcels with building coverage to accomodate
detached ADU
112 parcelswith building
coverage to accomodate
attached ADU
15,187 parcels with zoning,
current land use, and floor area to accomodate ADU
37
Building Constraint Layer 6: Parking Requirements Under the Second Unit Ordinance, property owners are not required to provide a new parking space for the ADU if (1) the unit is within a ½ mile of a public transit stop or station, (2) the unit is located within a historic district, (3) the unit is part of the existing primary residence or an existing accessory structure, or (4) the unit is within 1 block of the car share vehicle pick-up/drop-off location. If none of these situations apply, then 1 parking space needs to be provided for studio, 1-bedroom, or 2-bedroom ADUs. If the ADU has 3 or more bedrooms, then 2 parking spaces need to be provided. According to our review of the Assessor Parcel Database, all 15,187 feasible properties have enough remaining open space square footage to accommodate at least one additional parking space of 171 square feet (minimum parking space area defined in the Zoning Ordinance). Of these 14,037 can accommodate up to two parking spaces and 1,150 are limited to one ADU parking space.
Figure 10: ADU Parking Accommodation
14,825 parcels with building coverage and
slope to accomodate
detached ADU
362 parcels with building
coverage and slope to
accomodate attached ADU
15,187 parcels with zoning,
current land use, and floor area to accomodate ADU
4,810 4,528
3,273
794 386
1,396
4,483 4,308
2,969
640 276
1,361
-
1,000
2,000
3,000
4,000
5,000
6,000
Bayside Flat Bayside Hill Coastal Rural Open Rural SmallTown
San FranciscoUrban
Parcels that can support at least one ADU parking space (#)
Parcels that can support up to two ADU parking spaces (#)
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While all 15,187 properties have the ability to provide at least one parking space, we mapped our final set of feasible properties with a half-mile buffer around all SamTrans stops and BART stations to determine how many properties would ultimately be exempt from providing additional parking. We found that 12,671 of the properties that we have preliminarily identified as feasible for ADU construction are located within a half-mile of transit (see Figure 3 below).
Figure 11: ADU Eligible Properties with Parking Exemption
Data Source: San Mateo County Transit District, Caltrans GIS Data Library
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Google Earth Parcel Measurements Analysis While the Assessor Parcel Database analysis allowed us to develop estimates for the number of properties that can accommodate ADU construction countywide, certain physical constraints could not be tested with the data available. Because the Assessor data has limited lot dimension and building foot print data, we conducted additional analyses using Google Earth measurements for a random sample of properties from our selected communities to estimate the effect of setback and parking restrictions on detached ADU construction feasibility in different community types. We selected 66 properties at random from the pool of 15,075 properties preliminarily identified as feasible for detached ADU construction given zoning, current land use, floor area, and building coverage restrictions. Assuming that property owners will most likely build a detached ADU in their backyard and provide parking for that ADU in their front and side yards, our research team measured the dimensions of each property’s backyard, front yard, and side yards in Google Earth. These measurements were then used to determine 1) what percentage of properties in each selected community could build a one-story or two-story detached ADU in the backyard given setback requirements and 2) what percentage could provide one or two ADU parking spaces in the front and side yards given parking requirements.
Setback Requirements The Assessor Parcel Database Analysis used aggregate lot size and building square footage data to determine the number of parcels that can accommodate a detached ADU. This, however, did not take into account that the Second Unit Ordinance requires all detached ADUs to maintain a 5-foot side setback and a 5-foot distance from all other structures on the property. Additionally, one-story backyard ADUs should have a rear setback of 5 feet and two-story backyard ADUs should have a rear setback of 10 feet. Our analysis used Google Earth measurements of backyard dimensions to determine the impact of these regulations on detached ADU construction feasibility. Please see Appendix 11 and 12 for a detailed description of the methodology to determine setback constraints. As illustrated in the following tables, we found that a total 98.2% of properties, or about 14,803 propttiecould still accommodate a one-story ADU after factoring in set-back restrictions and 95.7% of properties could still construct a two-story ADU after factoring in set-back restrictions.
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Table 27: Randomized Set-Back Review for One-Story Backyard ADU8
Community Type Community Name Backyard
accommodates one-story ADU
Backyard does not
accommodate one-story ADU
Property unclear
% of all properties that accommodate one-story ADU
Bayside Flat
North Fair Oaks 66 0 0 100.0%
Menlo Oaks 65 0 1 98.5%
West Menlo Park 63 2 1 95.5%
Bayside Hill San Mateo Highlands
66 0 0 100.0%
Coastal El Granada 65 0 1 98.5%
Rural Open Area North Skyline* N/A N/A N/A N/A
Rural Small Town La Honda* N/A N/A N/A N/A
San Francisco Urban Broadmoor 64 2 0 97.0%
TOTAL 389 4 3 98.2% *Note: Google Earth views of properties in the rural communities were significantly obstructed by dense tree cover, so we were
unable to conduct the setback analysis in North Skyline and La Honda.
Table 28: Randomized Set-Back Review for Two-Story Backyard ADU9
Community Type Community Name Backyard
accommodates two-story ADU
Backyard does not accommodate two-
story ADU
Property unclear
% of all properties that accommodate two-story ADU
Bayside Flat
North Fair Oaks 61 5 0 92.4%
Menlo Oaks 65 0 1 98.5%
West Menlo Park 63 2 1 95.5%
Bayside Hill San Mateo Highlands
66 0 0 100.0%
Coastal El Granada 61 4 1 92.4%
Rural Open Area North Skyline* N/A N/A N/A N/A
Rural Small Town La Honda* N/A N/A N/A N/A
San Francisco Urban
Broadmoor 63 3 0 95.5%
TOTAL 379 14 3 95.7% *Note: Google Earth views of properties in the rural communities were significantly obstructed by dense tree cover, so we were
unable to conduct the setback analysis in North Skyline and La Honda.
Parking Requirements The Assessor Parcel Database analysis concluded that all parcels have sufficient lot area for the provision at least one ADU parking space on the property. However, this assumed that the parking space can be placed anywhere on the property and did not take into account the existing building footprint. For this reason, we conducted an additional Google Earth parking analysis for our random sample of properties to test the feasibility of providing additional parking within the front and side yards. We measured the dimensions of the front yard and any side yard greater than 10 feet wide and determined what percentage of properties could accommodate one additional space and two additional spaces. Please see Appendix 11 and 12 for a detailed description of the methodology to determine parking constraints.
8 Data Source: Google Earth Measurements 9 Data Source: Google Earth Measurements
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The analysis found that 98.0% of properties can accommodate one ADU parking space within the front and side yards and 97.0% can accommodate two ADU parking spaces.
Table 29: Randomized Parking Review for One Front or Side Yard ADU Parking Space10
Community Type
Community Name
Front yard accommodates
one ADU parking space
Front yard does not accommodate one ADU parking
space
Property unclear
% of all properties that accommodate
one ADU parking space
Bayside Flat
North Fair Oaks 66 0 0 100.0%
Menlo Oaks 65 0 1 98.5%
West Menlo Park 64 1 1 97.0%
Bayside Hill San Mateo Highlands
66 0 0 100.0%
Coastal El Granada 64 1 1 97.0%
Rural Open Area
North Skyline* N/A N/A N/A N/A
Rural Small Town
La Honda* N/A N/A N/A N/A
San Francisco Urban
Broadmoor 63 3 0 95.5%
TOTAL 388 5 3 98.0%
Table 30: Randomized Parking Review for Two Front or Side Yard ADU Parking Space11
Community Type
Community Name
Front yard accommodates
two ADU parking spaces
Front yard does not accommodate two ADU parking
spaces
Property unclear
% of all properties that accommodate
two ADU parking spaces
Bayside Flat
North Fair Oaks 66 0 0 100.0%
Menlo Oaks 65 0 1 98.5%
West Menlo Park 64 1 1 97.0%
Bayside Hill San Mateo Highlands
66 0 0 100.0%
Coastal El Granada 64 1 1 97.0%
Rural Open North Skyline* N/A N/A N/A N/A
Rural Small Town
La Honda* N/A N/A N/A N/A
San Francisco Urban
Broadmoor 59 7 0 89.4%
TOTAL 384 9 3 97.0%
10 Data Source: Google Earth Measurements 11 Data Source: Google Earth Measurements