Nearly 4,000 inland parcels in unincorporated Sonoma County can now be considered for accessory dwelling units. Permit Sonoma says the Board of Supervisors adopted an ordinance removing the county's Z (Accessory Dwelling Unit Exclusion) Combining District from those properties to bring local zoning in line with state law, and the change took effect Nov. 5 (via Permit Sonoma).
The county's state environmental filing, posted Oct. 7, puts the exact count at 3,997 parcels outside the Coastal Zone. The project runs under file numbers PLP26-0013, ORD26-0005 and ZCE26-0005. Permit Sonoma says owners of eligible parcels can submit ADU applications now.
The Z district has been on the books since 1993. In county code it was meant to keep ADUs out of areas where there wasn't enough water for drinking or firefighting, where sewer service was inadequate or groundwater could be contaminated, where another unit would add to existing traffic hazards, or where fire risk was high because of terrain, access or vegetation.
This isn't the first time the county has trimmed it. In 2019 the supervisors pulled the Z overlay off more than 1,900 agricultural parcels after a screening that left out high and very high fire hazard zones, California tiger salamander habitat, Williamson Act land and the Coastal Zone, among other things. Parcels that kept the overlay could only get ADUs through a discretionary zone change.
The push this time came from Sacramento. In a Jan. 9, 2025 review of the county's ADU ordinance, the California Department of Housing and Community Development (HCD) said the county had given no data to back the blanket ban and hadn't shown that new ADUs, as opposed to other new homes, would hurt public safety or water and sewer systems. HCD told the county to approve ADU applications under state ADU law and remove the prohibition unless it adopted detailed findings for each of the four hazard conditions. HCD added that denials over water supply have to be made case by case.
The county's environmental notice says the ordinance removes the Z district as an inconsistency with state ADU law, as HCD directed, and that the concerns behind it will now be looked at during permit review rather than as a zoning ban.
A few limits stay in place. The Z restriction still applies inside the Coastal Zone, and Permit Sonoma says some properties, including those under Williamson Act agricultural preservation contracts, may not qualify. The county's other ADU rules still apply as well. Its ADU guidance lists net-zero water use requirements in critical watershed, critical habitat and Class 4 groundwater areas, dry-weather well tests for Class 3 and 4 wells, and floodway and fault-trace limits.
Permit Sonoma's general line is (707) 565-1900. The county planner listed on the environmental filing is Isabella Wotring, (707) 565-2618.
Why it matters for Contractors
For small builders and ADU specialists in Sonoma County, this opens up a few thousand rural lots that were closed to ADUs, some of them for decades. Owners who were told no in the past may not know the rule changed, so it's worth reaching back out to past inquiries and checking current leads against the county's parcel search before assuming a lot is off the table.
Removing the overlay doesn't make every lot buildable. Water, septic, fire access and the other site standards still decide whether an ADU works, now on a parcel-by-parcel basis, so budget for well tests and septic evaluation early in any bid.