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Can I build an ADU in Mill Valley, California?

Mill Valley, Marin County · Ordinances checked 2026-10-05 · Reviewed · 15 cited sections

Allowed with a permit

Yes. Mill Valley allows ADUs on lots zoned for single-family or multifamily homes; on a single-family lot the city's handout allows up to one detached ADU, one conversion ADU and one junior ADU (JADU) inside the house. A detached ADU up to 800 sq ft, 16 feet tall and 4 feet from side and rear lines needs only a building permit; larger ADUs (up to 850 sq ft for a studio or one-bedroom, 1,000 sq ft for two or more bedrooms) need a ministerial ADU permit from the Planning Department first.

Units allowed (single-family lot)Up to 1 detached ADU, 1 conversion ADU and 1 JADU [14]
Building-permit-only detached ADU16 ft or less, 800 sq ft or less, 4 ft from side/rear lines [2]
Max ADU size850 sq ft (studio/1-bed); 1,000 sq ft (2+ bed); 1,200 sq ft (internal conversion) [3]
Max size, attached ADU (handout)50% of the existing house or 800 sq ft, whichever is larger [15]
Max height, detached ADU16 ft (25 ft combined if above a garage) [4]
Side and rear setback4 ft max required; none for conversions of existing structures [5]
Front setback15 ft [6]
JADU size150 to 500 sq ft, inside the house [10]
Owner-occupancyNot required for an ADU; the code requires it for a JADU (state law allows this only if the JADU shares a bathroom) [9]
Minimum rental term30 consecutive days [8]

Where ADUs are allowed

MVMC Chapter 20.90 allows one ADU on a lot zoned for single-family or multifamily use and one JADU per single-family lot [1]. The April 2026 handout puts the single-family limit at one detached ADU, one conversion ADU and one JADU [14]. The handout notes that where state law conflicts with the local rules, state law controls [11], and the city's ADU page says an 800 sq ft ADU is allowed regardless of site conditions [12]. Starting January 1, 2027, state law (AB 956) will require cities to approve two new detached ADUs on a single-family lot; the city's documents don't reflect that yet.

Size, height and setbacks

Studio and one-bedroom ADUs are capped at 850 sq ft; ADUs with more bedrooms at 1,000 sq ft, and internal conversions within the house at 1,200 sq ft [3]. For an attached ADU, the handout allows 50% of the existing house or 800 sq ft, whichever is larger [15]; the code instead uses 50% of the lot's maximum adjusted floor area. A detached ADU can be 16 feet tall, or 25 feet combined if built above a garage [4]. Side and rear setbacks can't exceed 4 feet, and conversions of existing structures need no setback [5]. The handout lists a 15-foot front setback; building in the front yard requires showing other locations are infeasible [6].

Which approval you need

A new detached ADU of 16 feet or less, 800 sq ft or less and at least 4 feet from side and rear lines needs only a building permit and is exempt from lot coverage and floor-area limits [2]. JADUs and conversions of existing space are also building-permit-only. Other ADUs need a ministerial ADU permit from Planning (no public hearing); ADUs tied to design review or a tree removal permit get additional review.

Parking

One 20-foot space is required, but waived for building-permit-only ADUs, conversions, ADUs within half a mile of transit, and ADUs in the H-O Historic Overlay [7].

Owner-occupancy, deed restriction and rentals

Every ADU and JADU needs a recorded deed restriction: no separate sale and no rentals under 30 consecutive days [8]. ADUs have no owner-occupancy rule. For a JADU (150 to 500 sq ft, inside the house [10]) the code requires the owner to live in the house or the JADU [9]. The handout's JADU deed-restriction rule applies this only when the JADU shares bathroom facilities with the house [13], which matches state law; state law may override the broader code wording.

Code vs. handout

Our copy of Chapter 20.90 runs through Ord. 1362 (Nov. 2025); where it differs from the April 2026 handout, confirm with Planning.

See how Mill Valley compares with 8+ other California towns on accessory dwelling units (adus) →

Next steps

  1. Check your ADU type and size against the city's April 2026 ADU Handout to see whether it qualifies for building-permit-only review (detached: 16 ft or less, 800 sq ft or less, 4 ft from side/rear lines).
  2. If it doesn't qualify, file a Ministerial ADU Permit Application with the Mill Valley Planning Department (26 Corte Madera Ave., 415-388-4033) before the building permit.
  3. Get a water service availability letter from Marin Municipal Water District, then apply online for the building permit with a full plan set (site plan, floor plans, elevations, structural, Title 24, CALGreen checklist).
  4. Record the ADU deed restriction with the Marin County Recorder and file a copy with Planning before final inspection.

Common questions

Can I build an ADU in Mill Valley, California?
Yes. Mill Valley allows ADUs on lots zoned for single-family or multifamily homes. On a single-family lot, the city's handout allows up to one detached ADU, one conversion ADU and one junior ADU inside the house; the handout notes state law controls where it conflicts with local rules.
How big can an ADU be in Mill Valley, California?
Studio and one-bedroom ADUs are capped at 850 sq ft, ADUs with two or more bedrooms at 1,000 sq ft, and internal conversions at 1,200 sq ft. For an attached ADU, the handout allows 50% of the existing house or 800 sq ft, whichever is larger, while the code uses a different measure, so confirm with Planning.
Do I need a permit for an ADU in Mill Valley, California?
Yes. A detached ADU of 16 feet or less, 800 sq ft or less and at least 4 feet from side and rear lines needs only a building permit, as do JADUs and conversions. Other ADUs need a ministerial ADU permit from the Planning Department first, with no public hearing.
What are the ADU height and setback rules in Mill Valley, California?
A detached ADU can be 16 feet tall, or 25 feet combined if built above a garage. Side and rear setbacks can't exceed 4 feet, and conversions of existing structures need no setback. The handout lists a 15-foot front setback.
Can I rent out an ADU in Mill Valley, California?
Yes, but not short term. Every ADU and JADU needs a recorded deed restriction barring separate sale and rentals under 30 consecutive days. ADUs have no owner-occupancy rule; the code requires it for a JADU, though the handout and state law apply it only when the JADU shares a bathroom.

California's statewide ADU law

California's ADU law (Gov. Code §§66310–66342, recodified from §65852.2; latest amendments effective January 1, 2026) overrides conflicting city rules, including in charter cities. Cities must approve ADUs ministerially (no hearing) within 60 days of a complete application, or the permit is deemed approved.

What cities cannot require or do:

  • Owner-occupancy for an ADU.
  • A minimum lot size.
  • Side or rear setbacks over 4 feet for a new ADU, or any setback for a converted existing structure.
  • Size, lot coverage or floor-area rules that block an ADU of at least 800 sq ft. Size caps can't be under 850 sq ft (1,000 for 2+ bedrooms).
  • Parking within half a mile walking distance of public transit, or replacement parking for a garage converted to an ADU.
  • Impact fees on ADUs of 750 sq ft or less.

By right on a single-family lot: one converted ADU plus one junior ADU (JADU, up to 500 sq ft inside the house), plus one new detached ADU (up to 800 sq ft, 16 ft tall). Owner-occupancy may be required only for a JADU that shares a bathroom.

Short-term rentals: cities may bar ADU rentals under 30 days, and must for by-right ADUs, JADUs and SB 9 units.

Coming January 1, 2027: AB 956 raises the by-right count to two new detached ADUs per single-family lot.

SB 9 (§§65852.21, 66411.7) allows two homes on a single-family lot, or a split into two lots of at least 1,200 sq ft (with a 3-year owner-occupancy affidavit).

State sources: Gov. Code §66314 (ADU ordinance standards) · Gov. Code §66315 (no owner-occupancy requirement) · Gov. Code §66317 (ministerial review, 60-day deadline) · Gov. Code §66321 (size, setback and height limits on local rules) · Gov. Code §66322 (parking exemptions) · Gov. Code §66323 (by-right ADUs and JADUs) · Gov. Code §66333 (junior ADUs) · Gov. Code §66311.5 (impact fees) · AB 956 (2026), Stats. 2026, ch. 791 · Gov. Code §65852.21 (SB 9 two-unit projects) · Gov. Code §66411.7 (SB 9 urban lot splits) · HCD, Accessory Dwelling Units (ADU Handbook, updated March 2026)

What the ordinance says

Exact text from Mill Valley's published ordinances. Section numbers are as printed in the source.

  1. MVMC 20.90.030(A)(1)-(2) General Requirements, Municipal Code Chapter 20.90 - Accessory Dwelling Units, p. 1
    One ADU may be constructed on a lot zoned to allow single family or multiple family dwelling residential use. 2. One JADU may be constructed per residential lot in zoning districts permitting single family dwelling residential use.
  2. Review Process - Building Permit Only, Planning Department Handout Material for Accessory Dwelling Units (updated April 2026), p. 1
    New Detached ADUs that are 16' or less in height*; 800 SF or less in Gross Floor Area (measured from the outside walls); and setback 4' or more from the side (interior) and rear property lines.
  3. MVMC 20.90.050(A)(2) Development Standards, Municipal Code Chapter 20.90 - Accessory Dwelling Units, p. 1
    Studio and one-bedroom ADUs shall not exceed 850 square feet of gross floor area. ADUs with more than one bedroom shall be limited to a maximum of 1,000 square feet of gross floor area, except for those internal converted ADUs that are within the existing walls of a primary single-family residence, which shall be limited to a maximum of 1,200 square feet of gross floor area.
  4. MVMC 20.90.050(A)(4) Development Standards, Municipal Code Chapter 20.90 - Accessory Dwelling Units, p. 1
    A detached ADU shall not exceed 16 feet in height; or, when located above a garage, the combined height of the ADU and the garage shall not exceed 25 feet.
  5. MVMC 20.90.050(A)(1)(c) Development Standards, Municipal Code Chapter 20.90 - Accessory Dwelling Units, p. 1
    No setback shall be required for a converted ADU that is within an existing structure or that is within a structure constructed in the same location and to the same dimensions as an existing structure. For all other ADUs, the maximum required setback from side and rear lot lines shall be four feet.
  6. Summary of ADU Design and Development Standards - Setbacks, Planning Department Handout Material for Accessory Dwelling Units (updated April 2026), p. 3
    Front setback: 15 feet. Applicants seeking to build an ADU within front yard setback must demonstrate that other locations are infeasible.
  7. Summary of ADU Design and Development Standards - Parking, Planning Department Handout Material for Accessory Dwelling Units (updated April 2026), p. 3
    ADU parking waived if the ADU is: 1) a detached or attached ADU subject to a building permit only; 2) part of an existing or rebuilt structure (ADU conversion); 3) located 1/2 mile from transit; 4) located in the H-O Historic Overlay District.
  8. MVMC 20.90.030(B)(2)(b) Restrictions - deed restriction, Municipal Code Chapter 20.90 - Accessory Dwelling Units, p. 1
    If the ADU or JADU is rented, it shall not be rented for a period of less than 30 consecutive days.
  9. MVMC 20.90.060(A)(1) JADU Owner Occupancy, Municipal Code Chapter 20.90 - Accessory Dwelling Units, p. 1
    The owner of a parcel proposed for a JADU shall occupy as a principal residence either the single family dwelling unit or the JADU.
  10. MVMC 20.90.060(B)(1) JADU Development Standards, Municipal Code Chapter 20.90 - Accessory Dwelling Units, p. 1
    The JADU shall be a minimum of 150 square feet and a maximum of 500 square feet of gross floor area.
  11. Summary of Information - State ADU law, Planning Department Handout Material for Accessory Dwelling Units (updated April 2026), p. 1
    Where the provisions of state laws conflict with Mill Valley’s ADU and JADU regulations, the state laws supersede local regulations.
  12. Overview, Accessory Dwelling Units (Planning page), p. 1
    However, regardless of existing site conditions, an 800-square-foot ADU is allowed.
  13. Summary of Junior ADU Design and Development Standards - Deed Restriction, Planning Department Handout Material for Accessory Dwelling Units (updated April 2026), p. 4
    Same as ADU restrictions, with the addition of owner occupancy requirement if the JADU has shared sanitation facilities with the primary dwelling, and the owner is not a governmental agency, land trust, or housing organization.
  14. Summary of Objective ADU Design and Development Standards - Land Use Allowances, Planning Department Handout Material for Accessory Dwelling Units (updated April 2026), p. 2
    Single Family Zoning Districts: A maximum of 1 detached ADU; 1 conversion ADU and 1 Junior ADU based on Standards summarized below.
  15. Summary of Objective ADU Design and Development Standards - Size of ADUs, Planning Department Handout Material for Accessory Dwelling Units (updated April 2026), p. 2
    Attached: May not exceed 50% of the existing primary dwelling unit or 800 sf, whichever is larger, not to exceed the maximum square footages stipulated above.
This page summarizes Mill Valley's posted ordinances as of 2026-10-05. Ordinances are amended, and individual lots can have overlays, subdivision covenants, or HOA rules that are stricter. Confirm with Planning & Building Department (Building Division), City Hall, 26 Corte Madera Avenue (415-388-4033) before you build. Spot an error?