Building an ADU in unincorporated Marin County

Unincorporated Marin only

The County's ADU rules apply only in the unincorporated parts of Marin County. The County's ADU page asks people building in unincorporated Marin to contact its Planning team.

If your home is inside a city or town, its own ADU rules and permit office apply. See our San Rafael page; for other Marin cities and towns, check with that city's or town's planning department. Statewide rules are on our California page. [1][5]

Category 1 and Category 2 ADUs

The rules are in Section 22.32.120 of the Marin County Development Code.

Category 1 ("statewide exemption") on a single-family lot. These include an ADU entirely inside an existing house, an ADU inside an existing outbuilding (with up to 150 sq ft added for access, at least 4 feet from side and rear lot lines), or a new ADU of up to 800 sq ft. You can have up to two Category 1 ADUs if one is in an existing or proposed house or an existing outbuilding and the other is in a new building.

Category 1 on a multifamily lot. You can build up to two detached ADUs of up to 800 sq ft each, 18 feet tall, with 4-foot side and rear setbacks. You can also convert non-living space, such as storage rooms, attics, basements and garages, into at least one ADU, up to 25% of the existing units in buildings with five or more units.

Category 2 (ADU Permit). Larger ADUs, up to 1,200 sq ft, need an Accessory Dwelling Unit Permit. They must meet the zone's development standards and any building envelope. In planned districts, the R1:B3 zone standards apply unless a master plan sets a number. The County's planning pages describe the ADU Permit as a ministerial permit for units over 800 sq ft: if a project meets the standards, it gets the permit. [2][6]

Size, height and setbacks

Detached ADUs. A Category 1 detached ADU can be up to 800 sq ft and 18 feet tall, or 20 feet to match the house's roof pitch. It needs 4-foot side and rear setbacks and no front setback. A Category 2 detached ADU can be up to 1,200 sq ft if it stays within the allowed floor area ratio. Even where floor area limits would otherwise stop it, a one-bedroom ADU of up to 850 sq ft and a two-bedroom-or-larger ADU of up to 1,000 sq ft are allowed.

Attached ADUs. A Category 1 attached ADU can be as tall as the zone allows or 25 feet, whichever is greater. A Category 2 ADU in an addition to the house can be up to 50% of the existing house's floor area, within the floor area ratio and 1,200 sq ft, with the same 850 and 1,000 sq ft minimum allowances.

Category 2 setbacks. Category 2 ADUs follow the zone's normal development standards, including setbacks. [2]

Parking

Marin County's parking code requires 1 space for a studio or one-bedroom ADU and 2 spaces for an ADU with two or more bedrooms. Junior ADUs need none. Tandem spaces in an existing driveway or a setback area count. No ADU parking is required if the ADU is within half a mile of a public transit stop, is in a historic district, is where on-street permits are required but not offered to the tenant, is within one block of a car-share site, or is 500 sq ft or less and inside the same building as the house. The Department of Public Works can also approve a parking exception.

What state law says. State law says ADU parking can't exceed one space per unit or per bedroom, whichever is less, and lists more exemptions, such as ADUs built inside the house or an accessory structure. The County's parking rule dates from 2018. If you are asked for a second space, confirm with the Planning Division. [4][7]

Junior ADUs

Under Section 22.32.125, a junior ADU can be up to 500 sq ft and must be entirely inside a single-family house. It has a kitchenette, not a full kitchen, and its own entrance. If it doesn't have its own bathroom, it needs an interior door to the main living area. Only one is allowed per property. The owner must record a deed restriction that bars selling it separately from the house and limits its size. More in our junior ADU guide.

Owner occupancy. The County code says the property must be owner-occupied for any JADU (except for government agencies, land trusts and housing organizations). State law requires owner occupancy only when the JADU shares a bathroom with the house. If your JADU has its own bathroom, confirm with the Planning Division. [4][8]

Fees and the fee waiver program

Through December 31, 2026, the County waives permit fees of up to $1,500 for a JADU and up to $2,500 for an ADU, with no income limit. If you deed-restrict the unit as affordable, you can seek a waiver of up to $5,000 (moderate income, 81-120% of area median income) or up to $10,000 (low income, below 80% of area median income). You must affirm that the unit won't be a short-term rental. Staff apply the general waivers without a separate application; the affordable waivers need an online application after you consult with Planning and Building & Safety.

Paid planning help. Walk-in and phone help from the Planning Division is free. A one-hour Planning Consultation costs $383, and a Pre-submittal Plan Review costs $139.

State limits. No impact fees may be charged on an ADU of 750 sq ft or less of interior livable space; larger ADUs pay in proportion to the size of the house. ADUs under 500 sq ft don't pay school impact fees. See our ADU cost guide. [3][4][9]

Legalizing an existing ADU

The County's ADU Legalization Program helps owners permit unpermitted ADUs and JADUs built before January 1, 2020. Under AB 2533, the County can't deny a permit for such a unit unless corrections are needed to fix substandard conditions.

You can review the inspection checklist anonymously first. To apply, hire a licensed design professional to prepare a site plan, floor plan and elevations, and email the application form. Once you apply, you have one year to finish; otherwise your file goes to Code Enforcement. After the inspection you receive a Letter of Legalization, not a Certificate of Occupancy. [10]

Renting it out

Category 1 ADUs can be rented only for terms longer than 30 consecutive days, and an ADU can't be sold or conveyed separately from the main home except as state law allows. The fee waiver also requires that the unit not be used as a short-term rental.

The County links a Guide on Fair Housing Laws Applied to ADUs and its Renting in Marin page for landlords. [1][2][3]

Where County rules and state law differ

In July 2023, the state housing department (HCD) found that the County's 2021 ADU ordinance didn't comply with state law, for example by allowing only one ADU on a single-family lot and limiting new detached ADUs to 16 feet. The County amended Section 22.32.120 in 2023 (Ordinance 3806); the current code allows two Category 1 ADUs and 18-foot detached ADUs.

Some current provisions still differ from the state rules HCD describes in its 2026 handbook: the County's two-space parking rule for larger ADUs, its owner occupancy rule for every JADU, and its limit of two detached ADUs on a multifamily lot (state law now allows up to eight on a lot with an existing multifamily building, but no more than the number of existing units). State ADU law generally sets the limits counties must follow. If one of these rules affects your project, ask the Planning Division how it applies. [2][4][7][8][11]

Permit steps in Marin County

1. Confirm you're in unincorporated Marin and read the rules

   Check that your property is outside city and town limits, then read Section 22.32.120 to see whether your ADU is Category 1 or Category 2. [1][2]

2. Talk to the Planning Division

   Get free walk-in or phone help, or pay for a Planning Consultation ($383) or Pre-submittal Plan Review ($139). The County's fee waiver program also asks you to consult Planning first to determine all fees and permits. [3][9]

3. Apply for an ADU Permit if your ADU is Category 2

   ADUs over 800 sq ft need a ministerial Accessory Dwelling Unit Permit. Planning Permit applications must be submitted in person at 3501 Civic Center Drive, Suite 308, San Rafael. [5][6][9]

4. Get a decision within 60 days

   The Director must approve or deny an ADU Permit within 60 days of finding the application complete. A denial must list what is deficient and how to fix it. [5]

5. Get your building permit and any fee waiver

   Work with the Building & Safety Division on the building permit. General fee waivers are applied by staff; for the $5,000 or $10,000 affordable waiver, submit the online application. [3]

Who to call

• Marin County Planning Division (Planning Customer Service Team)

  ADU rules and ADU Permits for unincorporated Marin; Monday-Thursday 8 a.m.-4 p.m., Friday 8 a.m.-12 p.m.

  415-473-6269

  3501 Civic Center Drive, Suite 308, San Rafael, CA 94903

  [1][9]

• Marin County CDA Housing Team

  ADU fee waiver program and housing questions

  415-473-7309

  [1]

Questions people ask