The City of San Mateo has adopted Chapter 27.19 to describe its regulations for ADUs in its Municipal Code.


An accessory dwelling unit, as defined in Section 27.04.165, shall comply with the following development standards:

(a)   Permitted Zoning Districts. Accessory dwelling units shall only be constructed on residential zoned properties, specifically R1-A, R1-B, R1-C, R2, R3, R4, R5, R4D, R5D, R6D and BMSP, that have been developed with a single-family dwelling unit or an accessory dwelling unit is proposed to be built in conjunction with a single-family dwelling unit, where the single-family dwelling is the primary residential dwelling unit. The residential property shall not be part of a condominium, townhouse, or other multi-family development.

(b)   Number of Units. No more than one accessory dwelling unit may be constructed on any site. An accessory dwelling unit shall not be allowed on a site with more than one unit.

(c)   Owner Occupancy. One of the dwelling units on the site shall be owner-occupied.

(d)   Maximum Unit Size. The floor area of an accessory dwelling unit shall not exceed 640 square feet including attic and basement areas as defined in Section 27.04.200 for the applicable underlying zoning district, provided that the total floor area for the parcel or lot does not exceed the maximum allowable floor area.

(e)   Setback and Other Zoning Regulations. For purposes of setbacks and other zoning regulations, the accessory dwelling unit (both attached and detached) shall be considered to be a part of the principal use of subject site and shall be subject to the same requirements of the underlying zoning district, including standards for lot coverage, setbacks, and height, unless otherwise provided in this Article.

(f)   Maximum Coverage of Rear Yard. Accessory buildings shall not occupy more than 50% of a required rear yard.

(g)   Existing Accessory Structure. When any accessory dwelling unit is proposed to be attached to an existing accessory structure, the entire structure must comply with all standards for the principal use of the subject site and underlying zoning district, and not those standards for an accessory structure.

(h)   Separate Entry, Cooking Facility/Kitchen and Bathroom. The accessory dwelling unit, attached or detached, shall contain a separate entrance, cooking facility/kitchen, and bathroom.

(i)    Wetbar. One wetbar is permitted in a dwelling unit in a common living area room only. A single half-size or small refrigerator is permitted, but a microwave is not.

(j)    Location of Accessory Dwelling Unit. The accessory dwelling unit may be within, attached to, or detached from the primary dwelling unit.

(1)     Setback Exceptions.

  1. No setback shall be required for an existing legally permitted garage or accessory structure that is converted to an accessory dwelling unit.
  2. A minimum setback of five feet shall be required from the side and rear lot lines for an accessory dwelling unit that is constructed above an existing legally permitted garage.   (k)   Number and Type of Required Parking.(1)     General Requirements. There shall be a minimum of one standard size off-street parking space for an accessory dwelling unit with up to one bedroom and one standard size parking space shall be required for each additional bedroom. The required accessory dwelling unit parking may be located in a garage, carport, uncovered or tandem space. In addition, the required parking for the primary residential dwelling unit must comply with current code standards, as specified by the underlying zoning district.(2)     Location of Required Parking for Accessory Dwelling Unit. The required parking for the accessory dwelling unit may be located on the rear one-third of the property, including the required rear yard setback and a portion of the required side yard and street 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.


City of San Mateo ADU Application Checklist

City of San Mateo website

City of San Mateo Community Development