ADU rules in Sonoma County

Outside the Coastal Zone, Ordinance No. 6544 (adopted December 9, 2025, effective January 8, 2026) allows ADUs, detached new construction included, on any lot with an existing or proposed primary dwelling whose base zone permits or conditionally permits single-family, multifamily or residential mixed-use development; the use tables list ADUs as permitted in the LIA, LEA, DA, RRD, TP, AR, RR, R1, R2, R3, commercial, industrial and PC (planned community) zones, but not PF (SCC 26-24-020(F)(2); Tables 6-1 to 14-1). A single-family lot may have two ADUs of any type (detached, attached, conversion or a combination) plus one JADU; a lot with an existing multifamily building may add up to eight detached or attached units, never more than its existing units, plus conversions of non-livable space, and a proposed multifamily building two (26-24-020(F)(3)-(4)). The exceptions are parcel-specific: the Z (Accessory Dwelling Unit Exclusion) Combining District in Article 76 still bars ADUs on 3,997 inland parcels, and the County's ADU page still says so, although HCD's January 9, 2025 findings and Permit Sonoma's September 2026 staff report say state-mandated units under Gov. Code 66323 must be allowed regardless; the Board of Supervisors hears countywide inland removal on October 6, 2026. Land under a Williamson Act contract cannot take an ADU under the County's Uniform Rules, and in the Coastal Zone the certified coastal code allows one detached or attached 'second dwelling unit' only on parcels of at least 2 acres (6,000 sq ft with public water and sewer) outside the coastal Z district (26C-325.1(b), (h)(2)).

Attached additions, conversions of existing space in the house or an accessory structure (garages included, with up to 150 sq ft added for ingress and egress when converting an accessory structure on a single-family lot), and combinations of conversion and attached new construction all count toward the two ADUs a single-family lot may hold (SCC 26-24-020(F)(3)-(5)); a conversion of the house, or a unit attached to a proposed house, needs its own exterior access (26-24-020(F)(10)). One JADU of up to 500 sq ft may be created inside an existing or proposed single-family dwelling or its attached garage, with a separate exterior entrance and an efficiency kitchen, on a lot with any combination of ADUs (26-24-190(F)). The County's ADU page says JADUs are allowed in the Z Combining District while ADUs are not, and says JADUs are 'not yet provided for' in the Coastal Zone, where an attached second dwelling unit is allowed under the same coastal limits as a detached one (26C-325.1(e)).

On a lot with an existing or proposed single-family dwelling an ADU may be up to 1,200 sq ft with no percent-of-house limit; a conversion of existing space in the house may exceed that, a conversion of an accessory structure may use the whole structure plus 150 sq ft for ingress and egress, and a unit attached to a proposed house may exceed it if it stays within the house's allowable size (SCC 26-24-020(F)(6)(a)). On lots with existing or proposed multifamily dwellings ADUs have no maximum floor area, except that conversions of non-livable space are limited to that space (26-24-020(F)(6)(b)). Floor area is the interior habitable area measured along the interior perimeter, excluding garages (26-24-020(B)(3)), in line with the interior-livable-space measure of Gov. Code 66321(b); JADUs are capped at 500 sq ft (26-24-190(F)(5)). New-construction ADUs may be 35 feet tall and conversions keep the existing structure's height if taller (26-24-020(F)(8)); side and rear setbacks are 4 feet for new floor area, the front setback and lot coverage apply only to floor area beyond 800 sq ft on single-family lots, and there is no minimum lot size (26-24-020(F)(2), (5), (7)). In the Coastal Zone the certified 26C-325.1 still caps a second dwelling unit at 840 sq ft measured to the exterior walls (1,000 sq ft with a 30-year affordability agreement), limits a detached unit to 16 feet and requires a 2-acre parcel unless public water and sewer serve it, below the 850 and 1,000 sq ft floors of Gov. Code 66321(b)(2) and contrary to the 66314(b)(1) bar on minimum lot sizes, yet Permit Sonoma's coastal page applies those coastal standards.

Neither Ordinance No. 6544 nor the coastal second-unit rules require the owner to live on a lot with an ADU, and Gov. Code 66315 bars an owner-occupancy requirement for ADUs. For JADUs, SCC 26-24-190(C)(1)(b) as rewritten by Ordinance No. 6544 requires the owner to live in the JADU or the rest of the house only when the JADU shares sanitation facilities with the house, and never when the owner is a governmental agency, land trust or housing organization, matching Gov. Code 66333(b) as amended by AB 1154 from January 1, 2026; staff added that narrowing after the Planning Commission hearing, and the County's June 2026 JADU Declaration of Restrictions applies it. A recorded JADU deed restriction is still required (no separate sale, size and attributes, County as third-party beneficiary). Municode still prints the repealed 26-88-061(g)(3), which required owner occupancy for every JADU, until the next supplement.

Ordinance No. 6544 eliminated ADU parking in the inland code: 'Additional parking shall not be required for an ADU,' and no replacement parking is required when a garage, carport or covered or uncovered space is demolished for, or converted to, an ADU (SCC 26-24-020(F)(9)); JADUs need no additional parking (26-24-190(F)(9)). This replaced the one-space rule of the repealed 26-88-060(i), which the County's Development Standards for ADUs page still prints. In the Coastal Zone, 26C-325.1(i)(6) still requires one all-weather off-street space for the exclusive use of the second unit, in addition to the house's parking, plus a shared 12-foot driveway; Gov. Code 66322(a) bars any parking standard for a unit within one-half mile walking distance of public transit, in a historic district, within the house or an accessory structure, where on-street permits are required but not offered, or within one block of a car-share vehicle, and 66314(d)(10) caps any requirement at one space per unit or bedroom.

Inland, 'the primary permit required to establish a new ADU is a building permit,' and a complete application meeting SCC 26-24-020 'shall be approved ministerially without discretionary review or a hearing,' within 60 days when a primary dwelling exists, with a written list of deficiencies if denied (26-24-020(G)(1)-(4)); ADUs are exempt from design review, and JADUs follow the same path (26-24-020(F)(1), 26-24-190(G)). A zoning permit (or the building permit for any needed changes) records the re-designation of an existing house as the ADU when a new primary house is proposed (26-24-020(G)(6)), and unpermitted ADUs and JADUs built before January 1, 2020 are legalized by building permit unless substandard (26-24-020(H); Gov. Code 66311.7). Grading, well, septic and sewer permits apply as needed, and wells in Class 3 and 4 groundwater availability areas must meet the SCC 7-12 well yield standards. In the Coastal Zone the certified 26C-325.1(c) requires a zoning permit, or a use permit in the LIA and LEA districts, and Permit Sonoma says a coastal permit with or without a hearing may also be needed, although Gov. Code 66329 (as amended by AB 462, effective October 10, 2025) requires a decision on an ADU coastal development permit within 60 days with no required public hearing and no appeal to the Coastal Commission.

ADUs 'may be rented but shall not be sold or otherwise conveyed separately from the primary residence, except as specifically provided for by State law,' and 'shall not be rented for periods of thirty (30) days or less' (SCC 26-24-020(C)); JADUs carry the same term rule, may not be sold separately and need a recorded deed restriction (26-24-190(C)). That tracks Gov. Code 66323(e) and 66333(g), which require terms longer than 30 days for state-exempt ADUs and JADUs, so the County ADU page's phrase 'long-term occupancy (30-days or longer)' understates the rule by a day; the Building an ADU page and the 2026 JADU declaration say more than 30 days. Short-term use is closed off separately: a vacation rental (30 days or less, with a zoning permit and a Chapter 4 license) is not allowed in 'an accessory dwelling unit or junior accessory dwelling unit' inland or in the Coastal Zone (26-28-160(C)(1)(b)(vi); 26C-325.10(C)(3)(vi)), a vacation rental license may not issue for a single-family dwelling with a JADU (4-205(F)), ADUs may not be used as hosted rentals or bed and breakfast inns (26-88-118(c)-(d) as amended by Ordinance No. 6544), and agricultural farmstays may not use ADUs (26-88-085(b)(2)). No Gov. Code 66342 ADU-condominium ordinance appears in Ordinance No. 6544, the code or the Board record, and the County's Building an ADU page points only to the 66341 qualified-nonprofit sale.