San Mateo – ADU Regulations


Both ADUs and Jr. ADUs (JADUs) are permitted in all residential zoning districts.

Number of units

The number of ADUs you’re able to build depends on how your lot is zoned:

  • Lots zoned for single family use may add 1 ADU and 1 Jr. ADU (JADU).
  • Lots zoned for multi-family may add ADUs up to 25% of the existing unit space (e.g.: 4 unit building would be allowed 1 ADU), or at least 1 ADU.

Building size

The maximum ADU you can build depends on your zoning and ADU type:

  • JADUs may reach a maximum of 500 sq. ft.
  • Attached ADUs can reach no more than 50% of the floor area of the existing dwelling
  • Detached ADUs can reach 1,200 sq. ft.

Even if requirements such as floor area ratio, proportionality (i.e. 50% threshold) to the existing or proposed primary dwelling unit’s floor area, or other zoning standards cannot be met, the City must still allow an 800 sq. ft. ADU.

Building height

Detached ADUs can reach a maximum height of 16 feet. Attached ADUs must comply with the height requirements of the applicable zoning district.


Existing unfinished basement space can be used for a conversion ADU.


Minimum setbacks of 4 feet from the side and rear lot lines, as well as 6 feet from any existing structures are required for detached ADUs. No additional setback is required if the ADU is constructed in the same location with same dimensions of an existing structure (e.g. replacing your garage).


No additional parking is required for ADUs if they are within 0.5 miles of public transportation (a bus stop, train station, etc.). When converting a garage or carport, replacement parking is not required.


Owner occupancy is not a requirement for permitting an ADU from January 1, 2020 until January 1, 2025. This is a great opportunity for rental property owners to increase their yields in a cost-effective way.


While long-term rentals are permitted for ADUs, short-term rentals (e.g. Airbnb for less than 30 days) are not permitted in San Mateo.


Impact fees are waived for ADUs less than 750 sq. ft. Utility connection and additional capacity fees are waived for ADUs that are within an existing single family dwelling; for detached ADUs, the new connection fee or capacity charge shall be proportionate to the burden of the proposed ADU.