Contra Costa County, CA Accessory Structures: Garage Conversions (2026)

Garages are regulated as part of the primary dwelling (when attached) or as accessory structures (when detached) under Title 8 of the Contra Costa County Ordinance Code and the California Residential Code. Attached garages must be separated from habitable dwelling space by 1/2-inch gypsum board on the garage side and a self-closing 1 3/8-inch solid-core or 20-minute rated door (CRC R302.5 and R302.6). Detached garages typically must meet a 3-foot side/rear setback in most residential zones and match the front-yard setback of the house; corner-lot garages have additional street-side setback. Converting a garage to living space (bedroom, office, gym) requires a building permit, full CRC compliance (ceiling height, emergency egress windows, insulation, smoke/CO alarms, heating), and in most zones, replacement parking—unless the conversion is to an ADU. Per AB 68 and AB 881, a garage converted to an ADU does NOT require replacement parking, even if the zoning would otherwise demand it, provided the property is within 0.5 mile of public transit or otherwise qualifies. Garages cannot be used as permanent living quarters without conversion, and cannot be sublet as bedrooms. Using a garage primarily for commercial storage or auto repair triggers Home Occupation Permit review (see home-business).