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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 (Within existing single-family; cannot be multi-family)

Types of ADUs:

Attached, Detached and Junior ADUs

Owner Occupancy:

Not required for the years 2020-2025. Short term rentals not allowed.


Must comply with underlying zoning district.

Lot Size:


Maximum ADU Floor Area:

Attached: No more than 50% of existing or proposed primary residence, 1,200 sq.ft. max. City must allow 800 sq.ft. minimum provided ADU follows setback requirements

Detached: 1,200 sq.ft. max. If a JADU shares the lot, then Detached max is 800 sq.ft.

Junior: 500 sq.ft. max

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 kitchen, including cooking facility with appliances (240-volt service outlets now permitted), sink (maximum 16in width/length dimensions), food preparation and storage cabinets of reasonable size in relation to JADUs size.


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 (public transit, historic district, no permits for ADU occupants, car share, attached/conversion). Parking spaces must be 10’ by 18’ without obstructions. Tandem parking allowed.

Junior ADU: No additional parking required.


Conversion 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 4’ setback is required from side and rear lot lines.

Detached: 4’ side and 4’ rear

Attached/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. If multi-family, or if JADU is on same single-family lot lot, then a Detached ADU is limited to 16’


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:



Must have similar design elements, colors, and materials as the existing dwelling, plus offset windows from neighbor’s windows to maximize privacy. Detached (with the exception of corner lots) must minimize visibility from the street. ADUs also must comply with standards for the preservation of heritage trees, as outlined in Chapter 13.52.

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!