5 county-level rules, plus city-specific rules for 4 cities in Sonoma County, California.
Verified from official government sources
Unincorporated Sonoma County permits ADUs ministerially under County Code Sec. 26-88-060, implementing California ADU law (Gov. Code 66310+). Detached ADUs are generally allowed up to 1,200 sq ft with 4-foot side/rear setbacks. No zoning permit is required, but building and any related septic/water permits are.
Sonoma County Ordinance No. 6222, Exhibit A (amending Sonoma County Code § 26-88-060, Accessory Dwelling Units)
Except as otherwise provided by this section, accessory dwelling units shall be ministerially permitted only in compliance with the requirements of this section, and all other requirements of the applicable zoning district in the following agricultural and residential zoning districts: LIA (Land Intensive Agriculture), LEA (Land Extensive Agriculture), DA (Diverse Agriculture), RRD (Rural Resou...
In unincorporated Sonoma County, detached residential accessory structures such as sheds need a building permit at 120 square feet or larger, per Permit Sonoma's policy. Sheds generally observe minimum side and rear setbacks and height limits, and must comply with the underlying zoning district's standards.
Permit Sonoma Administrative Policy 1-4-6 (Detached Residential Accessory Structures)
Accessory structure is a structure not greater than 3,000 square feet (sq. ft.) in floor area, and not over two stories in height, the use of which is customarily accessory to and incidental to that of the dwelling(s) and which is located on the same lot.
Converting a garage into living space in unincorporated Sonoma County is most commonly done as an ADU or Junior ADU under County Code Secs. 26-88-060 and 26-88-061. A building permit is required to change the use from a garage to habitable space, and an existing-structure conversion ADU is allowed ministerially.
Permit Sonoma Administrative Policy 1-4-6 (Detached Residential Accessory Structures), Section 5(a)
Any accessory structure with a kitchen, as defined in Policy 1-4-5, is considered a dwelling unit and must either be classified as a second unit limited to 840 sq. ft. in size or meet the allowable density under zoning for another dwelling.
Carports are treated as detached residential accessory structures in unincorporated Sonoma County. A building permit is required at 120 square feet or larger under Permit Sonoma's policy, and carports must meet the setback, height, and lot-coverage standards of the parcel's zoning district under Chapter 26.
Permit Sonoma BPC-002 Residential Plan Checklist, Sec. 5 (Garages and Doors)
5.1 Common wall between garage and dwelling shall have 1/2 inch gypsum board applied on the garage side. Garage ceiling with habitable space above shall have 5/8 inch type X gypsum wallboard applied to the ceiling. Carports with no enclosed uses above do not require protection. (R302.6) 5.3 Garage and carport floor surfaces shall be of approved noncombustible material. Asphaltic surfaces shall ...
A permanently sited tiny home on a foundation is typically permitted in unincorporated Sonoma County as an ADU under County Code Sec. 26-88-060 (max generally 1,200 sq ft), processed ministerially. Movable tiny homes on wheels and RVs are not treated as permanent dwellings and face separate restrictions.
Permit Sonoma, Tiny Houses; Policy 8-1-1; Sonoma County Code Sec. 26-88-010(P)
According to the 2018 International Residential Code (IRC), a tiny house is a dwelling that is 400 square feet or less in floor area excluding lofts. Stick-built/traditional/site-constructed homes: A small home constructed entirely at the building site. Manufactured houses (commonly referred to as mobile homes): Homes built in a factory mounted on a non-removable steel chassis or frame. Factory...
4 cities in Sonoma County have their own accessory structures rules. Each link goes to that city's dedicated page with code citations.
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