City of San Mateo Accessory Dwelling Unit (ADU) Standards

        New laws were recently passed, and updates will be coming soon


650.522.7212 – This email address is being protected from spambots. You need JavaScript enabled to view it.


Julia Klein, Principal Planner, 650.522.7216 – This email address is being protected from spambots. You need JavaScript enabled to view it.

ADU Ordinance

Zones allowed

Residential zoned properties with a single-family residential use (R1-A, R1-B, R1-C, R2, R3, R4, R5, R4D, R5D, R6D, and BMSP).
Lot / use restrictions
The residential property where approval of an ADU is requested shall not be part of a condominium, townhouse, or other multi-family development. Only one ADU per residential zoned property is allowed.
Minimum lot size


Min / Max ADU size

None / 640 sf. The square footage of the ADU, when combined with the square footage of all of the other buildings on-site, must still comply with the maximum Floor Area Ratio, or FAR, for the underlying zoning district.

ADU building height

Detached and attached ADUs must comply with same height standards of the underlying zoning district.

Note: ADUs proposed to be attached to existing legally permitted accessory structures must comply with same height standards of the underlying zoning district and NOT those standards outlined in the Accessory Buildings and Structures Ordinance.

Daylight Plane

Detached and attached ADUs must comply with same daylight plane standards of the underlying zoning district (applicable to R1 & R2 zoning districts only)


Detached and attached ADUs must comply with same setback standards of the underlying zoning district. However, no setbacks are required if an existing legally permitted garage is converted to an ADU, whether attached or detached and regardless of location, provided replacement parking can be provided. Additionally, a minimum setback of 5 feet from rear and side lot lines for an ADU constructed above an existing legally permitted garage is required.
Maximum coverage of required rear yard
All accessory buildings, including the ADU, shall not occupy more than 50 percent of a required rear yard (SMMC 27.19.050)
Parking requirements

In order to construct the ADU, the parking requirements for the primary residence must comply with the current code requirements of the underlying zoning district, including the number of stalls and stall dimensions. However, if a garage, carport, or covered parking structure is demolished or converted to construct an ADU, even if the existing garage is substandard, the required off-street parking spaces for the primary residence may be replaced as follows, provided it is on the same parcel as the ADU:

Garage parking spaces; Carport Parking spaces, Uncovered parking spaces; or Tandem parking spaces.

The required replacement parking for the primary residence may be located within the required front yard setback, street side yard setback, and rear yard setback areas. The required parking may also be located within a portion of the interior side yard setback provided that a minimum 5 foot wide side yard clearance with no obstruction is maintained.

For the ADU, one space per bedroom is required unless exemptions are met:

  • ADU is within 1⁄2 mile of public transit stop;
  • ADU is part of the existing legally permitted primary residence or an existing legally permitted accessory structure;
  • ADU is in an architecturally and historically significant district; or
  • ADU is in an area with residential parking permit rules that do not offer permits to occupants of the ADU

For the ADU, garage, carport, uncovered, or tandem parking spaces are allowed, and may be located in the rear 1/3rd of the property including the required rear yard setback (inclusive of the portion of the required street side yard and interior side yard setback areas). The required parking shall NOT be located within the required front, interior side yard, or street side yard setback areas outside of the rear one-third of the property.

All required parking spaces shall be a minimum of 10 ft x 18 ft long without any obstructions. All parking must comply with the limits on paving standards for R1 & R2 Districts only (SMMC 27.18.070).

Owner occupancy

Owner must live in the main dwelling unit or the ADU.

Deed Restriction: A deed restriction will be required mandating owner occupancy, limiting rental turnover, prohibition of separate sale of the ADU, and documentation of the approved size and attributes of the ADU.

Sample fees from recently built projects*
ADU 1(800 sf, detached): $21,409
ADU 2 (500 sf, garage conversion): $10,169
Other requirements

Heritage Tree ordinance: Building permits for ADUs need to demonstrate consistency with all of the City’s other applicable ordinances, including the Heritage Tree ordinance.

Sale & rental: The ADU shall be prohibited from being sold separately from the primary residence. Rental of the ADU shall not be for less than 30 consecutive calendar days (Deed Restriction).

Fire safety: Generally, if an existing detached ADU is proposed and the primary residence is sprinklered, the ADU must be sprinklered; if the primary residence is not sprinklered, the ADU does not require sprinklers. However, all new detached ADUs do require sprinklers. For attached ADUs, if the primary residence is sprinklered, the ADU requires sprinklers; if the primary residence is not sprinklered, the ADU will only need to be sprinklered if 50% of the existing floor is repaired, altered, or added. Please contact the Fire Department at 650.522.7940 for any additional questions.

Shape, materials and style: The ADU, whether attached or detached, must be of the same architectural style as the primary structure, and constructed of similar exterior materials, finishes, and family of colors as the primary residential dwelling unit, and the windows must be offset from the neighbor’s windows to maximize privacy. Detached ADUs must be designed to minimize the visibility of the ADU from the street(s), with the exception that corner lots are excluded from this requirement (also, refer to “Process” below).

Utility connections: A separate water, sewer service, or power service connection is not required. ADUs are not considered new residential units for the purposes of calculating City connection fees or capacity charges for utilities.

Architectural and design standards are included in city’s ADU ordinance and are reviewed as part of the building permit (ministerial) review process. No separate discretionary design review is required. Making an appointment to meet with a planner in the initial stages of the project can help avoid challenges and is highly recommended.


Disclaimer: standards and process may vary. Consultation with the contact is recommended before starting any project.

*Fees are specific to each project and will vary. Fees change regularly. These are provided as examples only.

Download ADU Standards PDF

Updated: 12/11/2017