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San Mateo City, California: ADU Regulations

City or County:

San Mateo City, California

Date Updated:



ADU: R1-A, R1-B, R1-C, R2, R3, R4, R5, R4D, R5D, R6D and BMSP

Junior ADU: R1-A, R1-B, R1-C, and R2

Types of ADUs:

Attached, Detached and Junior ADUs


Must comply with underlying zoning district.

Lot Size:


Maximum ADU Floor Area:

ADU: 640 sq.ft. max

Junior ADU: 500 sq.ft. max

Note: floor area counts against total floor area allowed on a parcel. Total floor area of both ADU and primary dwelling must not exceed max. allowed for underlying zoning district.

Lot Coverage:

Must comply with underlying zoning district.






ADU: Requires separate bathroom and kitchen facilities (permanent range, counters, refrigerator, and sink).

Junior ADU: Requires separate “efficiency” food preparation area.


ADU: Require separate bathroom facilities

Junior ADU: May provide a separate or shared bathroom.

Note: Drainage and sewer connections should be assessed on a project by project basis.


ADU: One off-street parking space per unit or bedroom, whichever is less. State parking exceptions apply. Parking spaces must be 10’ by 18’ without obstructions. Tandem parking allowed.

Junior ADU: No additional parking required.


ADU: must comply with underlying zoning district. No setbacks required for existing legally converted garages or other accessory structures. For ADUs constructed above a garage, a 5’ setback is required from side and rear lot lines.

Junior ADU: must comply with underlying zoning district.

Note: Fire Department regulations should be assessed on a project by project basis.


Must comply with underlying zoning district.


ADU: Must be oriented to keep windows offset from neighboring dwellings to preserve privacy. Must be designed to minimize visibility from the street, except on corner lots.

Junior ADU: By definition is located wholly within the walls of an existing dwelling, and must include the conversion of an existing bedroom.

Rear Yard Coverage:

Shall not occupy more than 50% of rear yard and must adhere to requirements of underlying zoning district.


Must have similar design elements, colors, and materials as the existing dwelling.

Note: Historic Neighborhoods or Districts should be assessed on a project by project basis.


ADU: Must have independent access from primary dwelling and cannot be closer to the street than the primary dwelling.

Junior ADU: Must have an independent entrance from primary dwelling and must have access to the common living areas of the primary dwelling which can be provided by a double lock door.

Source Code:

Important Note: Rules and regulations are updated and changed frequently. It’s one of our jobs to be up to speed and navigate those regulations. Let us know if our website isn’t up to date or if you have any information that could help. Cheers!