Napa County, CA Accessory Structures: Garage Conversions (2026)

Napa County Code § 18.08.550(B) (as amended by Ordinance 1456) defines a 'second unit, detached' as 'a second unit that is not attached to or located within a proposed or existing primary dwelling unit, including a second unit attached to, or located within, a detached garage, outbuilding, or other accessory structure.' Garage conversions to ADUs are explicitly authorized. Critically, § 18.104.180(A)(12) provides: 'If the construction of a second unit replaces an existing garage, carport, or covered parking structure, no replacement spaces need be provided' — mirroring the parking exemption codified in California Gov. Code § 65852.2(d)(1). Section 18.104.180(A)(16)(a) provides that 'No setback is required for an existing living area or an existing accessory structure converted to a second unit,' meaning a legally permitted existing garage that converts to a second unit retains its existing footprint without triggering new setback compliance. The conversion must still meet California Building Code occupancy, egress, fire-rating, ventilation, and energy-code standards. All reviews are ministerial under § 18.104.180(A)(7). Building code requirements that apply to single-family dwellings also apply to converted ADUs (§ 18.104.180(A)(8)).