California ADU rules · Marin County
Can I Build an ADU in Mill Valley?
Yes — you can build an ADU in Mill Valley. California's statewide ADU law requires every city, including Mill Valley, 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
CA state law1 ADU + 1 JADU
At least one ADU and one JADU per single-family lot (a conversion ADU may also be possible).
Units on a multifamily lot
CA state lawConversion ADUs (≥1, up to 25% of units) + up to 8 detached
Conversions of non-livable space (at least 1, or up to 25% of existing units) plus up to 8 detached ADUs on a lot with existing multifamily — never exceeding the number of existing units. JADUs are single-family only.
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 rule1,000 sq ft
Detached ADUs: 850 sq ft max for studio/1-bedroom; 1,000 sq ft max for 2+ bedrooms. Converted ADUs within existing walls may be up to 1,200 sq ft.
Max attached ADU size
Local rule50% of primary dwelling’s adjusted floor area, not less than 150 sq ft and not more than 1,000 sq ft
Attached ADUs capped at 1,000 sq ft, even if 50% of primary would be larger.
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
Local rule500 sq ft
JADU minimum is 150 sq ft; maximum is 500 sq ft.
Max height
Local rule16 ft
Detached ADU max height is 16 ft; up to 25 ft if above a garage. Attached ADUs may match primary dwelling height, up to 25 ft.
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 ruleMust comply with zoning district’s front setback; no special ADU exception
No ADU-specific front setback exception; must meet base zone.
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
1 off-street space per ADU unless exempt; no additional parking for JADUs.
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
CA state lawOnly for a JADU
No owner-occupancy requirement for ADUs (the prior sunset was removed). A JADU may require owner-occupancy, principally where it shares sanitation with the main house.
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 ruleADU permit (ministerial), Building permit, Other applicable construction permits
Explicitly lists ADU permit, building permit, and any other required construction permits.
Mill Valley-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)
ADUs must comply with all applicable building, fire, and safety standards.
Floodplain
ADUs in floodplain must comply with Title 18 requirements.
Watercourse setback
Additional agency permits required if ADU is within 30 ft of a watercourse.
Historic district
Conversions in historic districts may have additional review.
Mill Valley imposes stricter local maximum sizes for detached and attached ADUs (1,000 sq ft cap for most new construction), and requires ADUs to meet base zone front setbacks. Converted ADUs within existing structures may be up to 1,200 sq ft.
Frequently asked questions
Can I build an ADU in Mill Valley?
Yes. California's statewide ADU law requires Mill Valley 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 Mill Valley?
A detached ADU of at least 1,000 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 Mill Valley?
Side and rear setbacks are limited to 4 ft. Must comply with zoning district’s front setback; no special ADU exception.
Is parking required for an ADU in Mill Valley?
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 Mill Valley?
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 Mill Valley?
Owner-occupancy: only for a jadu. ADUs under 750 sq ft are also exempt from impact fees.
Sources
- Mill Valley Municipal Code (MVMC) Chapter 20.90, last amended by Ord. 1330, Nov. 15, 2021
- MVMC § 20.90.050(A)(2)
- MVMC § 20.90.050(A)(3)
- MVMC § 20.90.060(B)(2)
- MVMC § 20.90.050(A)(4)
- MVMC § 20.90.050(A)(1)(c)
- MVMC § 20.90.050(C)(1)-(2); § 20.90.060(C)
- MVMC § 20.90.040(C)
- MVMC Titles 14 & 15
- MVMC Title 18
- MVMC § 20.90.030(A)(7)
- MVMC § 20.90.030(A)(6)
- 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 § 66323 (by-right ADU/JADU, 800 sq ft, 4-ft setbacks)
- Cal. Gov. Code § 66333 (junior ADUs)
- Cal. Gov. Code § 66321 (size minimums & height)
- Cal. Gov. Code § 66322 (ADU parking; § 66334 covers JADU parking)
- Cal. Gov. Code § 66314 (local ordinance standards)
- Cal. Gov. Code § 66315 (owner-occupancy)
- 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 Mill Valley'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 Mill Valley Planning before relying on it.
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