The City of Millbrae permits ADUs across most residential zones. Millbrae 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.

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

• Detached ADU: Detached from an existing or proposed primary dwelling.
• Attached ADU: Attached to an existing or proposed primary dwelling.
• Conversion ADU: he conversion of an existing structure, such as a garage, into a habitable unit.
• Junior ADU (JADU): A unit up to 500 sq ft, created within the existing space of the primary dwelling.

• Maximum Size:
  
  • Detached: Up to 1,200 sq ft
  • Attached: Up to 50% of the primary dwelling’s floor area, not exceeding 1,200 sq ft
• Setbacks:
  
  • A minimum 4-foot setback from rear and side property lines is mandated.
• Height Limits:
  
  • height does not exceed 16 feet and four-foot side and rear yard setbacks are maintained.
• Parking Requirements:
  
  • Additional parking is not required if the ADU is:
    
    • Located within half a mile of public transit
    • Converted from an existing structure or part of the primary residence 
    • Served by a car share vehicle within one block

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

• Zoning compliance check
• Site plan indicating property boundaries, existing and proposed buildings, setbacks, water/sewer utilities, and trees
• Floor and architectural plans
• Structural calculations and foundation/engineering plans
• Mechanical, electrical, and plumbing plans
• Title 24 energy reports
• Permit submission via the City of Millbrae's online application portal
  

Homeowners also need to examine Millbrae's current fee schedule, utility connections, fire code compliance, and any other pre-approved ADU plan requirements if they exist.

Permit review times vary but are typically between 60 days depending on project complexity and city workload. Projects requiring corrections may take longer. Engaging professionals familiar with San Jose’s ADU process can help streamline approval.

Most residential areas in Millbrae qualify for Accessory Dwelling Units (ADUs) such as the R-1, R-2, and R-M zones. There are no lot size requirements for ADUs in these zones, although some hillside or environmentally sensitive locations may have special limitations.

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