Marin County

Town of Fairfax ADU rules

Research summary of local ADU rules on file for Town of Fairfax in Marin County. Not a determination for a specific lot.

Last checked October 23, 2026.

City or county ADU page

Fairfax ADU page

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Town of Fairfax rules on file

Local rules recorded in this project for this jurisdiction. Where a city figure on file is stricter than the state baseline on file, both are shown.

Local ADU rules
Fairfax's ADU and JADU rules are in Town Code § 17.048.010, last amended by Ordinance 908 (May 6, 2026). Review is ministerial, with no discretionary review or hearing. The Town decides whether an application is complete within 15 business days and acts on a complete application within 60 days.
Source: Fairfax Municipal Code (American Legal), § 17.048.010(D), accessed 2026-10-23.

How to apply
Submit a building permit application with plans and a copy of the property deed. Plans also go through review by the Ross Valley Fire District, and the reviewed plans come back to the Town before the permit is issued.
Source: Town of Fairfax Apply for an ADU or JADU page, Steps, accessed 2026-10-23.

Fee reduction for new and legalized units
Town ADU processing fees and building permit fees for ADUs and JADUs are reduced by 50% under an incentive program that runs until January 1, 2036. While it runs, legalizing a non-permitted ADU or JADU carries no penalty for the missing permit.
Source: Fairfax Municipal Code (American Legal), § 17.048.010(D)(2), accessed 2026-10-23.

How many units on a single-family lot
A converted ADU and a JADU can be combined with one or two new detached ADUs (up to 1,200 sq ft, 4 ft side and rear setbacks). The code says a second detached ADU is available on lots larger than 10,000 sq ft.
Source: Fairfax Municipal Code (American Legal), § 17.048.010(E)(1), accessed 2026-10-23.

Height of detached and attached ADUs
Detached ADU: up to 16 ft; up to 18 ft within a half mile walk of a major transit stop or high-quality transit corridor, plus up to 2 ft more to match the main home's roof pitch. Attached ADU: up to 25 ft or the zone's height limit, whichever is lower, and no more than two stories.
Source: Fairfax Municipal Code (American Legal), § 17.048.010(F)(2), accessed 2026-10-23.

Size of other ADUs
ADUs outside the state's building-permit-only categories may be up to 850 sq ft (studio or one bedroom) or 1,200 sq ft (two or more bedrooms). An attached ADU is also held to 50% of the main home's floor area, but no size, FAR, setback, coverage or open-space limit can push an ADU below 800 sq ft.
Source: Fairfax Municipal Code (American Legal), § 17.048.010(G)(1), accessed 2026-10-23.

Creek setback
No ADU may be built closer to the top of the bank of Fairfax Creek or San Anselmo Creek than 20 ft or twice the average depth of the creek bank, whichever is greater.
Source: Fairfax Municipal Code (American Legal), § 17.048.010(G)(9), accessed 2026-10-23.

Septic systems
If the ADU or JADU will connect to a septic (onsite wastewater) system, the application must show compliance with County of Marin Environmental Health Services requirements.
Source: Fairfax Municipal Code (American Legal), § 17.048.010(F)(6), accessed 2026-10-23.

Owner occupancy for junior ADUs
ADUs have no owner-occupancy rule. A JADU needs an owner living on the property only if the JADU shares sanitation facilities with the main home.
Source: Fairfax Municipal Code (American Legal), § 17.048.010(F)(7), accessed 2026-10-23.

Short-term rentals
JADUs, and ADUs built after January 1, 2020, may not be rented for terms shorter than 31 days.
Source: Fairfax Municipal Code (American Legal), § 17.048.010(F)(4), accessed 2026-10-23.

Legalizing a pre-2020 unit
The Town will not deny a permit to legalize an unpermitted ADU or JADU built before January 1, 2020 for building-code or ADU-rule violations unless a correction is needed to protect health and safety (Health and Safety Code § 17920.3). Plans by a qualified professional must show the unit meets, or will be brought up to, those standards. You may first get a confidential third-party inspection from a licensed contractor or design professional.
Source: Town of Fairfax Remedy for Un-permitted ADUs page, Gov. Code § 66332, accessed 2026-10-23.

California state rules on file

The statewide baseline recorded from state law and the HCD ADU Handbook.

Detached size cap described in state law
1,200 sq ft. Recorded in this rules file from Government Code section 66314.
Source: California Government Code section 66314, 66314, accessed 2026-10-16.

Side and rear setback cap for a new ADU
4 ft. Recorded in this rules file from Government Code section 66314, which describes a cap on side and rear setbacks for a new ADU.
Source: California Government Code section 66314, 66314, accessed 2026-10-16.

Setback for a conversion of an existing structure
No additional setback, as recorded in this rules file from Government Code section 66314 for a conversion of an existing structure.
Source: California Government Code section 66314, 66314, accessed 2026-10-16.

Minimum lot size
A city may not impose a minimum lot size for an ADU, as recorded in this rules file from Government Code section 66314.
Source: California Government Code section 66314, 66314, accessed 2026-10-16.

Minimum unit the handbook says cities must allow
At least an 800 sq ft ADU with 4 ft side and rear setbacks. The HCD ADU Handbook (updated March 2026) describes this under section 66321(b)(3).
Source: HCD ADU Handbook, 66321(b)(3); handbook updated March 2026, accessed 2026-10-16.

Detached height described in the handbook
16 ft, or 18 ft or 20 ft in some cases. The HCD ADU Handbook (updated March 2026) describes this under section 66321(b)(4).
Source: HCD ADU Handbook, 66321(b)(4); handbook updated March 2026, accessed 2026-10-16.

Permit decision deadline
60 days. Recorded in this rules file from Government Code section 66317(a)(3): the permitting agency must decide on (approve or deny) a completed ADU application within 60 days when an existing single-family or multifamily dwelling is on the lot. Section 66317(a)(1) says the application is considered ministerially, without discretionary review or a hearing.
Source: California Government Code section 66317, 66317(a)(1), (a)(3), accessed 2026-10-17.

Owner-occupancy requirement
None. Recorded in this rules file from Government Code section 66315: a local agency may not impose an owner-occupancy requirement on an ADU. It may require that the property be used for rentals of terms 30 days or longer. The HCD ADU Handbook (updated March 2026) describes the same rule.
Source: California Government Code section 66315, 66315; HCD ADU Handbook updated March 2026, accessed 2026-10-17.

Parking near public transit
No parking standards within one-half mile walking distance of public transit. Recorded in this rules file from Government Code section 66322(a)(1). Section 66322(a) lists several other cases where parking standards may not be imposed.
Source: California Government Code section 66322, 66322(a), accessed 2026-10-17.

Ministerial categories
The HCD ADU Handbook (updated March 2026) states that Government Code section 66323 sets four ministerial categories. This rules file does not restate those four categories.
Source: HCD ADU Handbook, 66323; handbook updated March 2026, accessed 2026-10-16.

Fire Hazard Severity Zones
If your lot is in a Fire Hazard Severity Zone, the CAL FIRE Office of the State Fire Marshal says in Information Bulletin 25-004 (issued March 3, 2025): “Fire Hazard Severity Zones (FHSZs) should not restrict the development of Accessory Dwelling Units (ADUs). However, additional building standards may be necessary, including but not limited to California Fire Code Chapter 49 and California Building Code Chapter 7A.” State ADU law does not itself bar an ADU there. A city or county may still apply its building code, including wildfire-resistant construction (Gov. Code 66314(d)(8)); require side and rear setbacks “sufficient for fire and safety” for a unit converted under 66323(a)(1); and decline parking in setbacks on specific fire and life safety findings (66314(d)(10)(B)). It may also consider public safety when it designates where ADUs may be permitted (66314(a)). Sprinklers can't be required in an ADU or JADU if the primary residence doesn't need them (66314(d)(12), 66323(d)). Some cities and counties still apply their own fire-zone limits. Where this summary has one, it is listed with the local rules. This summary does not look up your lot. Check the CAL FIRE Fire Hazard Severity Zone map viewer linked from the State Fire Marshal's Fire Hazard Severity Zones page: https://experience.arcgis.com/experience/5065c998b4b0462f9ec3c6c226c610a9
Source: CAL FIRE Office of the State Fire Marshal Information Bulletin 25-004, page 4, issued March 3, 2025; Gov. Code 66314, 66323, accessed 2026-10-20.

State baseline on the directory → · State ADU rules on the homepage →

Can I build an ADU in Town of Fairfax?

State law sets a baseline for ADUs in Town of Fairfax, and the local standards on file are listed on this page. Whether a specific lot can add an ADU depends on zoning, utilities, and other site facts this page does not measure. Confirm with the Town of Fairfax Planning and Building Services Department, or run a free address summary.

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