Sonoma County will stop banning backyard cottages and in-law units on 3,997 unincorporated parcels Nov. 5. Nearly 500 of them have Monte Rio addresses, and 248 of those touch the area the county expects the Russian River to cover in a moderate flood.
The Board of Supervisors voted 5-0 on Oct. 6 to scrap the county’s accessory dwelling unit exclusion, a zoning overlay known as the Z combining district, on every parcel outside the coastal zone. The Planning Commission had recommended the change unanimously Sept. 3.
The state Department of Housing and Community Development told the county in January 2025 that the ban broke state ADU law. State law now requires counties to allow a minimum number of ADUs anywhere they allow houses. County planning staff told supervisors the state’s guidance was that a county can’t adopt findings to block that minimum. Sonoma County allows only the minimum, so the overlay had to go. State law allows the housing department to refer violations to the state attorney general.
A ban with no record behind it
The county created the Z district in 1985 as an urgency measure, according to a February 2025 county letter to the state. The code says it was meant to keep second units out of areas short of water for drinking or firefighting, areas with poor sewer service or a risk of groundwater contamination, areas with traffic hazards and areas with a significant fire hazard.
The state housing department found “no data, substantiation, or explanation given for the prohibition,” and the archived ordinance behind it offered none either, the department wrote.
In their report to supervisors, county planning staff wrote that the district “has historically been applied for a variety of reasons, not always for the purposes described” in the code, “and often without documented reasoning or evidence to support its application.”
The county told the state in February 2025 that most of the parcels in the district have no water or sewer service provider.
Fire and flood
Of the 3,964 parcels on the county’s list that match its current parcel map, 2,043 touch land Cal Fire rates as high or very high fire hazard, according to a comparison of the list with the county’s own map layers. The county’s flood layer shows 481 parcels touching the area the county projects the river would cover at 38 feet on the Guerneville gauge. The National Weather Service calls 35 to 40 feet there a moderate flood. Thirty-three parcel numbers on the county’s list didn’t match the current map.
All 494 parcels with Monte Rio addresses touch a high fire hazard zone, and 248 touch the 38-foot flood area. The typical lot is 0.13 acre, about 5,700 square feet. The county assessor lists 65 of them as vacant lots that “may not be buildable” and 24 as “totally unusable.”
Unincorporated parcels with Healdsburg addresses make up the largest group, 587, and 402 of them touch high or very high fire hazard land. In the Sonoma area, 344 of 493 do.
What owners still face
Removing the zoning does not approve a single unit. Owners still need permits, and the county will judge water supply, wastewater disposal and fire safety parcel by parcel under existing building and fire codes, staff told supervisors. The state housing department told the county that any denial for lack of water must be made case by case, not across an area.
In December 2025 supervisors adopted a separate ordinance fixing most of the state’s other objections to the county’s ADU rules. The Z district was the last one left.
The change doesn’t apply to parcels in the coastal zone.