Second units, also known as “accessory dwelling units,” (or “granny units,” “in-law units,” and various other names) are smaller units built on the same property as, or attached to, an existing primary residence. The County of San Mateo regulates the production of second units in the unincorporated areas of the County only (those areas that are not in the jurisdiction of incorporated cities). Find the regulating ordinance attached below.

2017 Update

On January 10, 2017, the San Mateo County Board of Supervisors adopted an update to the County’s Second Unit Regulations, Chapter 22.5 of the Zoning Regulations. The updated regulations were certified by the California Coastal Commission on September 14, 2017 and are now in effect in every area of San Mateo County, including the County’s Coastal Zone.

Key changes to the ordinance included:

  • Minor changes in the maximum allowed floor area of second units, from 1,500 to 1,200 square feet
  • Slight reductions in required parking, and changes in how and where that parking may be provided
  • Changes to the required setbacks for second units
  • Limitations on the allowed height of second units
  • A variety of other minor changes intended to comply with the most recent amendments to the California Government Code

The most significant change, required by state law, was to formalize the review and approval of those second units which comply entirely with zoning regulations as a non-discretionary approval, meaning that if the unit complies with applicable regulations, it will not be reviewed at a public hearing, will not be appealable, and will only require a building permit.

In San Mateo County's Coastal Zone

In the Coastal Zone, in addition to the Second Unit Ordinance, the following requirements of Local Coastal Program Policy 3.22 (Second Dwelling Units in R-1 Zoning Districts) apply to second dwelling units on building sites containing a single-family residence:

  1. Limit the total number of approved second units to 466 in the Coastal Zone.
  2. Limit the size of the units to 700 sq. ft. or 35% of the floor area of the existing principal residence, whichever is greater.
  3. Comply with all applicable policies and procedures as required by the LCP.
  4. Second dwelling units shall not be permitted on non-conforming parcels less than 5,000 sq. ft.