Mendocino County, CA Accessory Structures: ADU Rules (2026)

166 of the County Code makes ADUs and junior ADUs a ministerial, by-right use on any parcel in a zoning district that allows single-family or multi-family homes, per Sec. 010. No conditional use permit, variance, or planning approval is required, only a building permit. Sec. 015 requires the Department of Planning and Building Services to approve or deny a complete application within 60 calendar days, or the application is deemed approved; the clock pauses if the applicant requests a delay or if the ADU application is bundled with a new single-family dwelling application.

Sec. 030(B) caps a detached ADU at 1,200 square feet unless it qualifies for the expanded standards in Sec. 040, and sets side and rear setbacks at four feet each, with the front setback following the underlying zoning district (the Director must reduce or waive the front setback if it would block an 800-square-foot ADU). Attached ADUs are capped at 50 percent of the primary dwelling's floor area under Sec. 030(A). One attached or detached ADU is allowed per parcel under Sec. 025, plus one junior ADU, so long as no unexpired Second Residential Unit permit exists on the parcel.

Every ADU needs Division of Environmental Health sign-off on water and sewage disposal, and none is required to add fire sprinklers if the primary residence lacks them.