ADU rules in San Mateo County, CA — also called accessory dwelling unit regulations or granny flat ordinances — cover setbacks, owner-occupancy, parking, and permit requirements.
In unincorporated San Mateo County, ADUs are governed by Development Code Chapter 8.392 (non-coastal) and Chapter 8.396 (coastal). Ministerial approval, no minimum lot size, 4-foot side/rear setbacks, and no short-term rental. Junior ADUs under 500 sq ft require owner-occupancy.
The County of San Mateo regulates ADUs only in unincorporated areas. Chapter 8.392 of the Development Code (formerly Zoning Regulations Chapter 22.5) applies outside the Coastal Zone, and Chapter 8.396 (formerly 22.5.1) applies inside the Coastal Zone. Per the County's 2020 ordinance, no development standard may preclude an 800-square-foot ADU, 16 feet or less in height, with 4-foot side and rear setbacks. ADUs are no longer subject to minimum lot size and may be built on substandard lots. A detached ADU may be combined with a Junior ADU (JADU under 500 sq ft, located within the existing house, with a recorded owner-occupancy deed restriction). State law (Gov. Code 66310-66342, formerly 65852.2) preempts local rules: the County must approve or deny a complete ADU application ministerially within 60 days, with no public hearing. ADUs cannot be used as short-term rentals (Airbnb/VRBO). In the Coastal Zone, ADUs must meet Local Coastal Program design standards and are subject to a maximum cap of 466 units; design review is applied objectively at staff level.
Building an ADU without required ministerial permits is a zoning/building code violation handled by Planning and Building code enforcement, which can require permits, modifications, or removal. Using an ADU as a short-term rental violates the ordinance condition and may trigger abatement.
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