Fairfax Zoning and ADU Guide for Homeowners

Last updated November 13, 2026. Reviewed by the Cash Home Buyers CA team.

Fairfax allows one accessory dwelling unit plus one junior ADU on a single-family lot, or up to two detached ADUs on lots over 10,000 square feet, and the Town has no owner-occupancy requirement for ADUs. The rules sit in Chapter 17.048 of the Fairfax Municipal Code, which the Town Council amended in 2025 and 2026.

This guide summarizes the size, height, setback and parking standards, how long the Town has to review an application, the Town’s remedy page for older unpermitted ADUs, and what it all means if you are selling a Fairfax home.

Fairfax ADU rules at a glance

Where the rules are	Fairfax Municipal Code Chapter 17.048 (§ 17.048.010)
Recent amendments	Ordinances 897 (May 7, 2025), 902 (Dec. 3, 2025) and 908 (May 6, 2026)
Units allowed, single-family lot	One ADU and one JADU, or up to two detached ADUs on lots over 10,000 sq ft
JADU size	Up to 500 sq ft
Detached ADU (Class 1)	Up to 1,200 sq ft
Class 2 ADU	Up to 850 sq ft (studio or one bedroom) or 1,200 sq ft (two or more bedrooms); attached units limited to 50% of the primary home, but never required to be below 800 sq ft
Height	16 ft detached; 18 ft near transit, plus up to 2 ft for roof pitch; 25 ft attached, or the zone limit if lower
Setbacks	4 ft side and rear; Class 2 front setback 10 ft
Parking	One space, with seven listed exemptions
Review time	Completeness within 15 business days; decision within 60 days
Town fees listed	$250 application; $997 to $1,511 building permit

What the Town allows

On a lot with a single-family home, Chapter 17.048 allows one ADU and one junior ADU (JADU). On lots larger than 10,000 square feet, the code instead allows up to two detached ADUs. A JADU is limited to 500 square feet and sits within the existing home.

A Class 1 detached ADU can be up to 1,200 square feet. Class 2 ADUs are limited to 850 square feet for a studio or one-bedroom unit and 1,200 square feet for two or more bedrooms.

An attached ADU is limited to 50 percent of the primary home’s floor area, but the Town cannot require a unit smaller than 800 square feet.

Height, setbacks and parking

A detached ADU may be 16 feet tall, or 18 feet near transit, with up to two more feet allowed to match the roof pitch of the main house. An attached ADU can reach 25 feet or the height limit of the zone, whichever is lower. Side and rear setbacks are four feet.

A Class 2 ADU has a 10-foot front setback. The code requires one parking space and lists seven exemptions, so check the current text before you assume you need to add a space.

Objective design standards

Fairfax’s code includes objective standards that apply to ADUs, including a 40 percent floor area ratio, 50 percent lot coverage, 50 percent open space and 300 square feet of usable open space per unit. It also sets a 20-foot buffer from Fairfax Creek and San Anselmo Creek, plus architectural requirements.

These can limit what fits on a small hillside lot, so ask the Planning and Building Services office to confirm which apply to your parcel.

Occupancy and rental rules

The Town’s ADU rules do not require the owner to live on the property for an ADU. A JADU does carry an owner-occupancy requirement, with exceptions. Short-term rentals are limited: a JADU, or an ADU built after January 1, 2020, may not be rented for fewer than 31 days. Impact fees are waived for ADUs under 750 square feet.

How review works

1. Apply. The Town’s ADU page directs applicants to its application process; it lists a $250 application fee.
2. Completeness check. The Town reviews the application for completeness within 15 business days.
3. Decision. The Town acts on a complete application within 60 days.
4. Building permit. Building permit fees on the Town’s page run from $997 for a minor addition to $1,511 for a major one.
5. Inspections. Request inspections through the building inspection line at (415) 453-2263.

See our Fairfax building permits guide for office hours and the inspection process.

Unpermitted ADUs built before 2020

The Town’s remedy page says it may not deny a permit for an unpermitted ADU built before January 1, 2020 because it violates building standards, unless the Town finds that correcting the violation is necessary to protect health and safety. That can help an owner legalize an older unit.

Ask Planning and Building Services how the process works for your property before you decide whether to legalize or sell as-is. Our statewide guide to selling with unpermitted work covers disclosure.

Selling a Fairfax house with an ADU or zoning issue

An ADU that was never permitted, a JADU without an owner living on site, or a unit near a creek buffer can all raise buyer and lender questions. Some owners legalize first. Others sell as-is and let the buyer decide. Our Fairfax code enforcement guide explains the citation process if the Town has already opened a case.

How to sell a house in Fairfax

1. Gather permits, plans and any Town correspondence about the ADU or addition.
2. Note any unit built without a permit and when it was built.
3. Request a no-obligation cash offer for the home as it stands.
4. Choose your closing date.
5. Close without repairs, corrections or showings.

Our local pages cover related situations in Fairfax, including selling as-is, selling fast, relocating and selling an inherited house. See also our Fairfax page and the Marin County hub.

Frequently asked questions

How many ADUs can I build on a Fairfax lot?

One ADU plus one JADU on a single-family lot, or up to two detached ADUs on lots over 10,000 square feet.

How big can a detached ADU be?

A Class 1 detached ADU can be up to 1,200 square feet. A JADU is limited to 500 square feet.

Do I need to live on the property?

Not for an ADU. A JADU carries an owner-occupancy requirement, with exceptions.

How long does the Town take to review an ADU?

The Town checks completeness within 15 business days and decides within 60 days.

What about an ADU built without a permit?

The Town’s remedy page says it may not deny a permit for a pre-2020 unpermitted ADU for building-standard violations unless correction is necessary for health and safety.

Can I rent an ADU short term?

Not for fewer than 31 days if it is a JADU or an ADU built after January 1, 2020.

Can I sell a Fairfax house with an unpermitted ADU?

Yes. You can disclose it and sell as-is, including to a cash buyer.

More guides

• We Buy Houses in Fairfax
• We Buy Houses in Marin County
• Marin County Property Tax Guide
• Fairfax Building Permits Guide
• Fairfax Code Enforcement Guide
• All California Guides

Sources

• Fairfax Municipal Code § 17.048.010 (accessory dwelling units)
• Town of Fairfax: Apply for an ADU or JADU
• Town of Fairfax: Building Permits

This guide is general information from public sources, not legal advice. Confirm current rules, fees and hours with the city.