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

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

• Detached ADU: A detached accessory dwelling unit is a unit that is an independent structure, entirely separated from the structure of the primary residence
• Attached ADU: An attached accessory dwelling unit is a unit that is built as an addition to, extension of, or within the primary residence
• Conversion ADU: Converted from a garage, basement, or internal space
• Junior ADU (JADU): Up to 650 sq ft, carved out of existing space within the home (such as a bedroom or attached garage)
  

• Maximum Size:
  
  • Detached:  Up to 800 sq ft, or 35% of the primary residence
  • Attached: Up to 800 sq ft, or 50% of the primary residence, up to 1,500 sq ft
• Setbacks:
  
  • Minimum 4 ft from side and rear property lines
• Height Limits:
  
  • The maximum height of the accessory dwelling unit shall be 26 feet
• Parking Requirements:
  
  • Additional parking is not required if the ADU is:
    
    • Within ½ mile walking distance of public transit
    • In a historic / architecturally significant district
    • ADU is a conversion of an existing structure or part of the primary residence

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

• Zoning compliance review: ADUs are allowed in all principally residential zones in the county
• Site plan and elevations: You must submit a plot plan showing existing and proposed structures, plus complete floor plans and elevation views
• Architectural drawings: Required to include construction details (foundation, framing, plumbing, electrical, mechanical) plus signed engineering / geotechnical calculations
• Title 24 energy documentation: Energy calculation data must be submitted showing compliance with California energy code
• Permit application submission: Submit the application (with all drawings and documents) to the County Building Department; four sets of plans are required
  

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

Permit review times vary but are typically between 6 to 12 weeks depending on project complexity and county workload. Projects requiring plan check corrections or multiple resubmittals may take longer. Engaging professionals familiar with County of San Mateo’s ADU process can help streamline approval and reduce delays.

Most residential zones are eligible for ADUs, including R-1, R-2, and R-M zoning districts. There are no minimum lot size requirements for constructing an ADU in most areas of the County of San Mateo. However, parcels located in hillside areas, coastal zones, or environmentally sensitive districts may be subject to additional development standards, design review, or special restrictions.

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