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.

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.

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