Garage conversion rules in San Mateo County, CA — sometimes called garage-to-ADU or accessory living unit conversions — govern permits, ceiling height, egress, and parking replacement.
Unincorporated San Mateo County permits converting a garage into an ADU under Development Code Chapter 8.392. California ADU law (Gov. Code 66314) bars the County from requiring replacement parking when a garage is converted to an ADU, and approval is ministerial within 60 days.
Converting an existing garage or accessory structure into an ADU is allowed in unincorporated San Mateo County through the ADU process under Development Code Chapter 8.392 (non-coastal) or Chapter 8.396 (Coastal Zone). California ADU law, which preempts local rules, prohibits a local agency from requiring replacement off-street parking when a garage, carport, or covered parking structure is demolished or converted to create an ADU. State law also bars any parking requirement for an ADU created within the existing footprint of a primary residence or accessory structure, and for ADUs within a half-mile walking distance of public transit. Garage-conversion ADUs that are contained within existing space receive ministerial review with no hearing, and the County must act within 60 days of a complete application. The converted unit must still satisfy building, health, and safety codes (egress, ceiling height, fire separation). In the Coastal Zone, the converted ADU must comply with Local Coastal Program design standards so that it remains compatible in shape, materials, and style with the primary dwelling.
Converting a garage to living space without ADU permits, or without meeting building and safety code, is an unpermitted-dwelling violation enforced by Planning and Building. Enforcement can require permits, code-compliant upgrades, or reversion of the space.
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