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

120 splits accessory dwelling units into two tracks. Category 1, the Statewide Exemption track, covers ADUs built entirely within an existing single-family dwelling or outbuilding, or a new detached unit up to 800 square feet with minimum four-foot rear and side setbacks and no required front setback; a detached unit is capped at 18 feet in height (20 feet if matching the primary roof pitch), while an attached unit may go to the zoning district's normal height limit or 25 feet, whichever is greater. Up to two Category 1 units are allowed per single-family lot if one sits in the house or an existing outbuilding and the other in new detached construction, and Category 1 units can only be rented for stays longer than 30 consecutive days.

Multi-family properties may add up to two detached 800-square-foot units, convert non-habitable space such as basements or garages into units, and, in buildings of five or more units, convert up to 25 percent of existing units into ADUs. 56. 56, applicable countywide in the unincorporated area, processes that permit as a ministerial planning decision: the Director must approve or deny within 60 days of a complete application, and any denial must list the specific defects so the applicant can fix and resubmit. An ADU cannot be sold or conveyed separately from the primary dwelling except as state law allows.