The City of Mill Valley permits ADUs across most residential zones. Mill Valley follows both state and local ADU laws, allowing most single-family homes to build one detached ADU and one Junior ADU (JADU) per lot. Multi-family properties may be eligible for multiple ADUs, depending on site conditions.

Mill Valley aligns with California’s broader ADU legislation but has its own requirements for height, setbacks, and design review.

• Detached ADU: Detached ADUs do not share any walls with the primary dwelling and must be separated by at least 6 feet.
• Attached ADU: Attached ADUs share wall(s) with the primary dwelling but may not have an internal connection between units.
• Conversion ADU: An ADU converted from existing space in a primary dwelling or accessory building in the same footprint, with small modifications allowed for egress/ingress purposes. 
• Junior ADU (JADU): Up to 500 sq ft, built entirely within the existing primary dwelling, with an efficiency kitchen and separate exterior entrance.

• Maximum Size:
  
  • Detached: Up to 1,000 sq ft
  • Attached: Up to 50% of the primary dwelling’s floor area, not exceeding 1,000 sq ft.
• Setbacks:
  
  • Minimum 4 ft from side and rear property lines
• Height Limits:
  
  •   Typically, 16 ft for detached ADUs, up to 25 ft when built above a garage
• Parking Requirements:
  
  • Additional parking is not required if the ADU is:
    
    • Located within half a mile of public transit
    • Converted from an existing structure (e.g., garage, basement)
    • In a historic district or designated area
    • Within a block of a car share location

To build an ADU in Mill Valley, the following approvals are typically required:

• Zoning compliance review to ensure the project meets Mill Valley Municipal Code (MVMC 20.90) and state ADU law.
• Site plan and elevations showing the location, setbacks, and dimensions of the ADU.
• Architectural drawings illustrating floor plans, exterior materials, design, and roof structure. 
• Structural calculations and engineering for foundation, framing, and seismic requirements.
• Title 24 energy documentation demonstrating compliance with California energy standards.
• Permit submission through the City of Mill Valley’s online portal, including required forms, fees, and deed restriction documentation.
  

Homeowners should also review Mill Valley’s current fee schedule, utility connection requirements, fire code compliance, and any additional planning review if the project exceeds objective standards or requires design exceptions.

Permit review times vary but are typically between 6 to 12 weeks depending on project complexity and city workload. Projects requiring corrections or additional documentation may take longer. Engaging professionals familiar with Mill Valley’s ADU process and submitting complete plans can help streamline approval.

Most residential zones are eligible for ADUs, including R-1, R-2, and R-M zones. There are no minimum lot sizes for ADUs in most areas of Mill Valley. Some hillside or environmentally sensitive zones may have special restrictions related to setbacks, height, or lot coverage.

For the most current and complete information, refer directly to the City of Mill Valley.