Colusa County, CA Accessory Structures: ADU Rules (2026)

Colusa County's Zoning Code (Chapter 44, Article 44-4) sets standards for 'accessory units' at Section 44-4.10. Only one accessory unit is allowed per legally created parcel, and it is not permitted on a parcel already containing two or more dwelling units (Sec. 44-4.10.010). The unit may be within, attached to, or detached from the primary dwelling and must have its own kitchen, bathroom, and separate entrance (Sec. 44-4.10.030). Under Section 44-4.10.060, the maximum floor area is 1,200 square feet, except on agricultural classifications (F-A, E-A, U-C, U-T, and A-T) where it may reach 2,500 square feet; the minimum is 150 square feet (or the state efficiency-unit size, whichever is greater). Detached accessory units require a 3-foot side setback and 8-foot rear setback, with at least 10 feet between buildings. A one-story detached unit is limited to 16 feet and a two-story unit to 28 feet in height. Sections 44-4.10.040 and 44-4.10.100 of the county code require the owner to occupy the primary or accessory unit and to record a deed restriction barring separate sale. However, current state ADU law - recodified into Government Code Sections 66310-66342 - preempts conflicting local rules: it bars owner-occupancy requirements for standard ADUs, requires ministerial (no-hearing) approval, guarantees at least an 800-square-foot unit at 16 feet with 4-foot side and rear setbacks, and allows a junior ADU up to 500 square feet within an existing single-family home. Where the county's older Ord. 765/788 text conflicts with that state law, state law controls. Confirm current requirements with Colusa County Planning & Building.