A County Guide: Where These Rules Apply.
This is a county-level guide. The standards below come from Marin County's ordinance and apply in unincorporated Marin; incorporated cities within the county set their own ADU rules, which can differ. Since 2016, California has passed a wave of laws making ADUs easier to build, and Marin County has eased its rules to align with them, with the main exception being restrictions in environmentally sensitive areas.
The county targets a 60-day review of ADU applications, and the full permitting process usually runs about three to four months. Because rules vary by jurisdiction inside the county, the fastest way to know what applies to your address is to check your lot with us.
Size, Height, And Setback Limits.
Marin County ADUs must be at least 220 square feet. Detached units can reach 850 square feet for studios and one-bedrooms; two-bedroom units are normally capped at 1,000 square feet but can go up to 1,200 square feet when they comply with floor area ratio requirements. Detached ADUs cannot exceed 18 feet in height.
Attached ADUs are limited to 50 percent of the primary home's floor area or 1,200 square feet, whichever is smaller, with a 25-foot height limit governed by the underlying zoning. All ADUs need setbacks of at least 4 feet from side and rear property lines. The full standards live in the Marin County Municipal Code, Section 20.32.140.
Coastal Zones, Fire Areas, And Parking.
Marin's geography adds a few overlays worth knowing. Additional restrictions apply in areas with sensitive habitats and in high fire severity zones, and any ADU created in the coastal zone requires a Coastal Development Permit.
Parking is straightforward: studio and one-bedroom ADUs require one space, JADUs require none, and the requirement can be waived entirely if you are within a half mile of public transit or inside a historic district.
Who Can Build, And How Many Units.
ADUs are permitted on any residentially zoned lot in unincorporated Marin that has an existing primary residence or one under development. A single-family property can have one ADU over 800 square feet plus one JADU, or alternatively two ADUs under 800 square feet each.
Lots with existing multifamily buildings can add up to eight detached ADUs, as long as the count does not exceed the number of multifamily units, and non-residential spaces like storage rooms and offices can be converted into ADUs up to 25 percent of the existing unit count. Own multifamily property in Marin? See Abodu for multifamily.
What It Actually Costs.
Custom site-built ADUs in California routinely exceed $250,000 before change orders, and hillside or coastal sites can push well past that. Abodu is transparent published pricing: homes from $234,800 plus a published installation price, an expected all-in from $298,800, covering design, permits, factory build, delivery, and installation. See how the process works and what real homeowners say in our testimonials.