Sonoma, CA Accessory Structures: ADU Rules (2026)

060(A)). 22, the state ADU law, which sets the floor local rules cannot fall below; where Sonoma's numbers are more restrictive than the state minimum, state law controls. 040(A), the maximum size is 850 square feet for a studio or one-bedroom ADU and 1,000 square feet for a two-bedroom unit, with no more than two bedrooms allowed; other development standards (lot coverage, floor area ratio, open space, front setbacks, minimum lot size) can never force an attached or detached ADU below 800 square feet with at least a four-foot side and rear setback.

040(C) require four feet minimum from side and rear lot lines for detached and attached ADUs, with no setback at all required when converting an existing legally built structure in place. 040(D) depends on configuration: a single-story detached ADU on a single-family lot tops out at 16 feet above grade; a two-story detached ADU may reach 25 feet to the roof peak, with one-story wall plate heights capped at 12 feet and two-story walls at 20 feet; an attached ADU cannot exceed the primary dwelling's allowed height. On a lot with an existing or proposed multifamily dwelling, a detached ADU can reach 18 feet. 040(E), (H) and (I). 060(C).