Sonoma County Accessory Structures: ADU Rules Rules (2026)

Sonoma County's current ADU and JADU ordinance was adopted as Ordinance No. 6422 by the Board of Supervisors on December 5, 2023 and became effective January 4, 2024, repealing and replacing the prior versions of Sec. 26-88-060 and Sec. 26-88-061 in Chapter 26 (Zoning) of the Sonoma County Code. The ordinance was a substantial rewrite required by the wave of California ADU legislation - principally Government Code Section 65852.2 (general ADU statute) and 65852.22 (JADU statute) as amended by AB 68 (2019), SB 13 (2019), AB 881 (2019), AB 670 (2019, HOA pre-emption), AB 671 (2019), AB 3182 (2020), AB 345 (2021), SB 9 (2021), SB 897 (2022, height/non-conforming standards), AB 2221 (2022, denial limits), and AB 1033 (2023, separate-condominium sale option) - and to harmonize the County's ADU rules with the post-Tubbs Fire fire-safe building standards in Chapter 13 (Building Regulations) and the Vegetation Management ordinance. ADUs are ministerially permitted under Sec. 26-88-060 in any zoning district that allows a single-family or multi-family dwelling as a permitted use, including residential (R1, R2, R3), agricultural (LIA Land Intensive Agriculture, LEA Land Extensive Agriculture, DA Diverse Agriculture, RRD Resources and Rural Development), and rural residential (AR, RR) districts. On agricultural and resource parcels (LIA, LEA, DA, RRD), Sec. 26-88-060(C)(2) provides that adding an ADU reduces the maximum number of allowed agricultural employee dwellings by one. The Z Accessory Dwelling Unit Exclusion Combining District historically prohibited ADUs on parcels with public safety, water, or geologic hazard concerns; in 2019 the Board of Supervisors removed the Z designation from over 1,900 parcels. Detached new-construction ADUs are capped at 1,200 square feet of usable floor area; attached ADUs are limited to 50% of the primary dwelling's floor area or 1,200 square feet (whichever is less), with state law guaranteeing an at-least-800-square-foot ADU regardless of local size limits if base-zoning standards would otherwise prevent that footprint. Conversion ADUs (within an existing single-family or accessory structure) are limited to the dimensions of the existing structure plus an addition of up to 150 square feet for ingress/egress. JADUs under Sec. 26-88-061 are capped at 500 square feet, must be contained within the walls of a proposed or existing single-family dwelling (or its attached garage), must include an efficiency kitchen and a separate exterior entrance, may share sanitation with the primary dwelling, and require owner-occupancy of either the primary dwelling or the JADU plus a recorded deed restriction submitted to the County Recorder. Height limits are: detached ADU on a single-family lot - 18 feet (or up to 25 feet if attached to multi-family within ½ mile of major transit per SB 897); detached ADU on a multi-family lot - 18 feet; ADU within an existing accessory structure - the existing structure's height. Side and rear yard setbacks are 4 feet (0 feet for ADUs converted from existing legal structures); front setbacks follow the base zone unless that would prevent an 800-square-foot ADU. There is no minimum lot size for an ADU under state law, but the parcel must demonstrate adequate water (well yield testing per Permit Sonoma standards or public water service confirmation) and adequate sewer or septic capacity (a Well & Septic site evaluation is typically required before a building permit can issue). One off-street parking space is required per ADU unless the parcel is within ½ mile walking distance of public transit, located in a historic district, the ADU is part of an existing primary residence or accessory structure, an on-street parking permit is required but unavailable, the parcel is within one block of a car-share, or the ADU is a studio. JADUs require no additional parking. Owner-occupancy is not required for ADUs permitted between January 1, 2020 and January 1, 2025 (Government Code 65852.2(a)(6)); under SB 1211 (2024) and AB 2533 (2024), additional protections took effect January 1, 2025 limiting local ability to deny ADUs based on legal-nonconformity of the primary dwelling and prohibiting certain development standards on multi-family ADUs. ADUs and JADUs in unincorporated Sonoma County may not be rented for periods shorter than 30 days - meaning Airbnb, Vrbo, and other short-term-rental use is prohibited; long-term tenancies of 31 days or longer are permitted, and the County's separate Vacation Rental ordinance (Sec. 26-88-120) does not authorize STR licensing of ADUs. Fire-safe construction is significant in Sonoma County after the October 2017 Tubbs Fire (which destroyed roughly 5,600 structures, mostly in Santa Rosa's Fountaingrove and Coffey Park neighborhoods and unincorporated Mark West Springs / Larkfield-Wikiup), the October 2019 Kincade Fire near Geyserville, and the September 2020 Glass Fire on the Sonoma-Napa border: ADUs in CAL FIRE State Responsibility Areas, Local Responsibility Area Very High Fire Hazard Severity Zones, or designated Wildland-Urban Interface areas must comply with California Building Code Chapter 7A (ignition-resistant exterior materials, ember-resistant vents, multi-pane tempered windows, Class A roofing) and Public Resources Code Section 4291 / Sonoma County Code Chapter 13A defensible space (a 0-30 foot 'lean, clean, and green' zone and a 30-100 foot 'reduced fuels' zone). ADUs in Scenic Resources Combining Districts (the SR, RC, and VOH overlays governed by Article 64 of the Sonoma County Zoning Code) must meet objective siting and visual-resource standards (ridgeline avoidance, color, materials), but discretionary design review is no longer required for ADUs under state law. Coastal Zone parcels (those with a CC suffix along the Pacific coast and Russian River mouth) are governed instead by Sec. 26C-325.1 of the Local Coastal Program, which retains some discretionary review under the California Coastal Act. Sonoma County has not yet adopted a local AB 1033 ordinance, so ADUs cannot currently be sold separately as condominiums in unincorporated areas (only the cities of San Jose, Berkeley, Santa Monica, and unincorporated San Diego County have opted in as of early 2026).