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

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

• Detached ADU: A newly constructed unit that is freestanding or separate from the primary single-family home, like a cottage, pool house, or guesthouse.
• Attached ADU: A newly constructed unit that is connected to the primary single-family home by at least one wall.
• Conversion ADU: Space within an existing or proposed home (like an attached garage or attic) or within the footprint of a permitted accessory structure (like a detached garage or outbuilding) can be converted into an ADU.
• Junior ADU (JADU):  A converted space of up to 500 square feet, contained entirely within a single-family residence.

• 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-foot side-and rear-yard setbacks.
    
• Height Limits:
  
  • Typically, 16 ft for standard detached or attached ADUs, up to 32 ft depending on location, building envelope, and topography.
    
• Parking Requirements:
  
  • Additional parking is not required if the ADU is:
    
    • The ADU is located within half a mile of public transit.
    • The ADU is converted from an existing structure.
    • The ADU is in a historic district or designated area.
    • The ADU is entirely within the existing or proposed primary dwelling.
    • There is a city-approved and dedicated parking space for a car share vehicle located within one block of the ADU.

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

• Zoning Compliance Review: Ensure the ADU complies with Hillsborough Municipal Code.
• Site Plan and Elevations: Submit architectural plans, including site plan, floor plans, and elevations, indicating materials and colors matching the primary dwelling.
• Architectural drawings: Provide detailed construction drawings for review.
• Structural calculations and engineering: Include structural engineering calculations as part of the permit application
• Title 24 energy documentation: Ensure compliance with California Title 24 energy efficiency standards.
• Permit submission: Submit the completed application and required documents to the ADU Specialist
  

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

Permit review times in Hillsborough for Accessory Dwelling Units (ADUs) typically range between 30 to 60 days, depending on the project's complexity and the completeness of the submitted application. Projects requiring corrections or that do not meet the objective standards may take longer. Engaging professionals familiar with Hillsborough’s ADU process can help streamline approval.

Most residential zones in Hillsborough are eligible for ADUs. Minimum lot size requirements generally apply, with lots typically needing at least 2,400 sq. ft. for an ADU. Some lots may have special setback or frontage requirements, and certain environmentally sensitive or hillside areas may have additional restrictions.

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