California ADU rules · Marin County
Can I Build an ADU in Fairfax?
Yes — you can build an ADU in Fairfax. California's statewide ADU law requires every city, including Fairfax, to approve a qualifying accessory dwelling unit by right, plus a junior ADU (JADU) on a single-family lot. What varies city to city are the local specifics below.
Can you build one?
ADUs allowed
CA state lawYes — by right
ADUs and JADUs are allowed by right in all residential and mixed-use zones; ministerial approval, no hearing.
Units on a single-family lot
Local rule1 ADU (attached or detached) + 1 JADU; up to 2 detached ADUs on lots >10,000 sqft (permit required)
Allows up to 2 detached ADUs on large single-family lots (>10,000 sqft) with permit.
Units on a multifamily lot
Local ruleConversions (>=1 or 25% of units); up to 2 detached ADUs on lots with proposed multifamily; up to 8 detached ADUs on lots with existing multifamily (not to exceed number of existing units)
Detached ADU cap depends on whether multifamily is proposed or existing.
Junior ADU (JADU)
CA state lawAllowed
Up to 500 sq ft, within the walls of an existing single-family home (single-family lots only).
Size & height
Max detached ADU size
Local rule1,200 sq ft
1,200 sq ft for 2+ bedrooms; 850 sq ft for studio/1-bedroom.
Max attached ADU size
Local ruleSame as detached (850/1,200 sq ft), but further limited to 50% of primary dwelling floor area
No application of percent-based size limit, FAR, or lot coverage may require ADU to be less than 800 sq ft.
Guaranteed minimum size
CA state law800 sq ft
An 800 sq ft ADU must be allowed regardless of FAR/lot-coverage caps; a city also cannot cap an attached ADU below 850 sq ft (1,000 for 2+ bedrooms).
Max JADU size
CA state law500 sq ft
Max height
CA state law16 ft
16 ft baseline; cities must allow 18 ft within ½ mi of a major transit stop or where the primary dwelling is two-story, and 20 ft on a lot with a multistory multifamily building.
Setbacks
Side setback
CA state law4 ft
A city cannot require more than 4 ft side or rear setback; conversions of existing structures are exempt.
Rear setback
CA state law4 ft
Front setback
Local rule10 ft minimum for ADUs subject to local standards
No setback required if ADU is built in same location/dimensions as existing structure.
Separation from main house
CA state lawNone required
No statewide separation requirement between the ADU and the primary dwelling.
Parking
Parking required
Local ruleUp to 1 space
One off-street space per ADU, with broad state exemptions.
Transit parking exemption
CA state lawYes — no parking required near transit & for conversions
No parking may be required within ½ mi walking distance of transit, in a historic district, for an ADU within an existing structure, within one block of car-share, or in certain permit-parking areas.
Garage-conversion replacement parking
CA state lawNot required
When a garage/carport is demolished or converted to build an ADU, replacement parking cannot be required.
Approval, timeline & cost
Approval path
CA state lawMinisterial (by right)
No public hearing or discretionary design review for a qualifying ADU.
Approval timeline
CA state law60 days
The agency must act within 60 days of a complete application — and the ADU is deemed approved if it does not.
Owner-occupancy
Local ruleOnly for a JADU
Owner-occupancy required for JADUs unless JADU has separate sanitation or is owned by public agency/land trust.
Impact-fee waiver
CA state lawWaived under 750 sq ft
An ADU of 750 sq ft or less pays no impact fees (the 750 sq ft threshold itself is exempt under § 66311.5); larger ADUs pay fees proportional to the primary dwelling. JADUs (≤ 500 sq ft) are exempt.
Permits required
Local ruleBuilding permit
ADU/JADU building permit fees reduced by 50% until Jan 1, 2036.
Fairfax-specific factors that can change the answer
State law sets the floor, but these local conditions often decide whether a specific parcel works.
Very High Fire Hazard Severity Zone (VHFHSZ)
Projects must comply with Town’s Fire Code; sprinklers required in ADU/JADU only if required in primary residence.
Historic district
No parking required for ADUs in historic districts.
Fairfax allows up to two detached ADUs on large single-family lots and sets a 10-foot front setback for ADUs subject to local standards. Detached ADUs are capped at 1,200 sq ft (2+ bedrooms) or 850 sq ft (studio/1-bedroom), and attached ADUs are further limited to 50% of the primary dwelling's floor area.
Frequently asked questions
Can I build an ADU in Fairfax?
Yes. California's statewide ADU law requires Fairfax to approve a qualifying accessory dwelling unit ministerial (by right), plus a junior ADU (JADU) on a single-family lot.
How big can an ADU be in Fairfax?
A detached ADU of at least 1,200 sq ft must be allowed, and an ADU of 800 sq ft can never be zoned out by floor-area or lot-coverage limits. A JADU can be up to 500 sq ft.
What are the setback requirements for an ADU in Fairfax?
Side and rear setbacks are limited to 4 ft. 10 ft minimum for ADUs subject to local standards.
Is parking required for an ADU in Fairfax?
Up to 1 space. No parking can be required within ½ mile of transit, for conversions, or for units inside the primary home. When a garage is converted, replacement parking is not required.
How long does ADU approval take in Fairfax?
A complete application must be acted on within 60 days, with ministerial (by right) — no public hearing or discretionary design review for a qualifying ADU.
Do I have to live on the property to build an ADU in Fairfax?
Owner-occupancy: only for a jadu. ADUs under 750 sq ft are also exempt from impact fees.
Sources
- Fairfax Municipal Code Chapter 17.048 (amended Jan 24, 2024)
- § 17.048 (E)(1)-(d)
- § 17.048 (G)(1)(a)-(c)
- § 17.048 (G)(1)(b)-(c)
- § 17.048 (G)(3)(a)-(b)
- § 17.048 (G)(7)(a)-(b)
- § 17.048 (F)(7)
- § 17.048 (B)(3), (H)(2)(a)
- § 17.048 (F)(3)
- § 17.048 (G)(7)(b)2
- Cal. Gov. Code §§ 66310–66342 (ADU Law)
- HCD 2025 Accessory Dwelling Unit Handbook
- Cal. Gov. Code § 66317 (ministerial approval, 60-day deadline)
- Cal. Gov. Code § 66333 (junior ADUs)
- Cal. Gov. Code § 66321 (size minimums & height)
- Cal. Gov. Code § 66323 (by-right ADU/JADU, 800 sq ft, 4-ft setbacks)
- Cal. Gov. Code § 66322 (ADU parking; § 66334 covers JADU parking)
- Cal. Gov. Code § 66314 (local ordinance standards)
- Cal. Gov. Code § 66311.5 (impact fees: none for an ADU ≤ 750 sq ft or JADU ≤ 500 sq ft)
Last updated 2026-06-25. This is an AI-assisted summary of Fairfax's ADU rules, drawn from its published ordinance and California ADU law — general information, not legal advice, and it may be incomplete or out of date. Verify the controlling rule for your specific parcel with Fairfax Planning before relying on it.
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