The City of San Mateo permits ADUs across most residential zones. San Mateo 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.

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

• Detached ADU: separate from the primary residence, typically placed in the backyard.
• Attached ADU: A unit that is physically connected to the main home, such as a new ground-floor addition or a side extension.
• Conversion ADU: Created by converting existing, lawful space. This includes converting a garage, basement, attic, or a large storage area into a complete living unit.
• Junior ADU (JADU): A smaller unit (maximum 500 sq ft) created entirely within the existing footprint of the primary home, such as by converting a bedroom suite or a section of the main floor. A JADU must include its own separate entrance and an efficiency kitchen.

• 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:
  
  • Front Yard: As required in the underlying zoning district.
  • Side and Rear Yards: Minimum of four feet from side and rear lot lines.
• Height Limits:
  
  • Attached ADU - 24 feet to top plate line and 32 feet to roof peak, as measured from existing grade.
  • Detached ADU - 16 feet to top plate line and 24 feet to roof peak, as measured from existing grade.
• Parking Requirements:
  
  • Additional parking is not required if the ADU is:
    
    • ADU is within one-half mile walking distance of public transit;
    • ADU is located within a significant architectural or historic district;
    • ADU is within an existing or proposed legally permitted primary residence or an existing legally permitted accessory structure;
    • If there are permits required for on-street parking in the area that are not offered to the ADU occupant;
    • If the ADU is within one block of a car share location.

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

• Zoning compliance review: Confirm the property is a residentially zoned lot with an existing single-family dwelling.
• Site plan and elevations: Clearly indicate the location and design of the separate exterior entrance required for both ADUs and JADUs.
• Architectural drawings: Detail the layout of the unit, including its creation within the existing walls for a JADU.
• Structural calculations and engineering: Provide calculations to verify the structural integrity of the new construction or conversion.
• Title 24 energy documentation: Submit compliance documentation for the California Energy Code.
•  For ADU that do not meet all standards, be prepared for an alternative discretionary review process with the Zoning Administrator, which includes public notification and potential appeal.
  

Homeowners should also review San Mateo’s current fee schedule, utility connection requirements, and fire code compliance.

Permit review times vary but are typically between 8 to 12 weeks depending on project complexity and city workload. This timeline covers the mandated 60-day ministerial review but accounts for the initial intake and resubmittal periods for corrections. Projects requiring multiple rounds of corrections will take longer. Engaging design professionals and contractors familiar with San Mateo’s specific ADU standards and process can significantly help streamline approval.

Most residential zones are eligible for ADUs, including R-1, R-2, and R-3 zones. There are no minimum lot sizes for ADUs on single-family lots in San Mateo County. Some hillside or environmentally sensitive zones may have special restrictions.

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