The City of Portola Valley permits ADUs across most residential zones. Portola 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.

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

• Detached ADU: ADU that is structurally detached from the main residence.
• Attached ADU: ADU that is structurally attached to the main residence but which has independent, direct access from the exterior.
• Internal ADU: ADU that is created by converting existing interior space (such as bedrooms, garages, basements, or attics)
• Junior ADU (JADU): ADU that is created by converting existing interior space (such as bedrooms, garages, basements, or attics), up to 500 sq ft. A JADU must have an efficiency kitchen and separate entrance, but can have a bathroom shared with the main residence. Either the JADU or the main building must be owner occupied.

• Maximum Size:
  
  • Detached: 800 to 1,500 square feet depending on proposed setbacks.
  • Attached: 1700 square feet Not more than 50% of primary dwelling square footage, unless 50% of primary dwelling is less than 800 square feet.
• Setbacks:
  
  • A minimum setback of 4 feet from the rear and side property lines is required for both detached and attached ADUs.
    
• Height Limits:
  
  • Typically, 1 acre or more, the general height limit is 28 feet, with a possible maximum of 34 feet in certain cases.
• Parking Requirements:
  
  • Additional parking is not required if the ADU is:
    
    • Located within half a mile of public transit
    • Part of a garage conversion or converted from another existing structure
    • Within a historic district or architecturally designated area
    • When on-street parking permits are required but not offered to the ADU occupant
    • When a car share vehicle is located within one block of the ADU

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

• Zoning compliance review with the Planning & Building Department Site plan and elevations
• Site plan, floor plans, and elevations showing ADU placement Structural calculations and engineering
• Architectural drawings (for new construction or conversions)
• Structural calculations and engineering reports (if applicable)
• Building permit submission through the Town of Portola Valley’s Online Permit Center
  

Homeowners should also review Portola Valley’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 Town workload. Projects requiring corrections, resubmittals, or additional reviews (e.g., Fire District or ASCC design review) may take longer. Engaging design and engineering professionals familiar with Portola Valley’s ADU process can help streamline approval.

Most residential zones are eligible for ADUs, including single-family districts. There are no minimum lot sizes for ADUs under state law, but Portola Valley applies adjusted maximum floor area (AMFA) limits based on parcel size and slope. Parcels 3.5 acres or larger may allow up to two ADUs, with at least one detached. Some hillside, floodplain, and environmentally sensitive areas may have additional restrictions and require Architectural & Site Control Commission (ASCC) review.

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