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

Colusa County Code Chapter 44 defines a 'private garage' as 'a fully enclosed accessory building, either attached or detached, or a portion of a building designed and/or used primarily for the shelter or storage of vehicles, trailers, recreational vehicles, and boats by the occupants of the dwelling' (Sec. 44-0.10, Definitions). Converting that space into habitable quarters changes its use and triggers permitting. If the conversion creates an independent dwelling unit, it is an 'accessory unit' governed by Section 44-4.10, which expressly allows an accessory unit to be 'constructed within or above an existing building or detached accessory building' (Sec. 44-4.10.060(b)); the unit must have its own kitchen, bathroom, and separate entrance and meet the size, setback, and parking standards of that section. A conversion that simply adds habitable rooms to the existing single-family residence (not a separate unit) still requires a building permit and must comply with the zone's setback, height, and lot-coverage rules and the adopted building codes. The county's Section 44-4.10 text dates to Ord. 765/788 and references former Government Code Section 65852.2; current state ADU law in Government Code Sections 66310-66342 controls and requires the county to approve a qualifying garage-to-ADU conversion ministerially. Under that state law, conversion of an existing legal structure is generally exempt from minimum setbacks, and the county cannot require replacement off-street parking when a garage is converted to an ADU. Confirm the applicable pathway with Colusa County Planning & Building before starting work.