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. 10, Definitions). Converting that space into habitable quarters changes its use and triggers permitting. 10, which expressly allows an accessory unit to be 'constructed within or above an existing building or detached accessory building' (Sec. 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. 10 text dates to Ord. 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.