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

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

• Detached ADU: is an ADU that is constructed as a separate structure from the primary dwelling unit and does not share any walls with the primary dwelling unit or an existing attached accessory structure
• Attached ADU: is an ADU that is constructed as a physical expansion (i.e., addition) of an existing single-family residence
• Conversion ADU: is an ADU that is created within existing space, including a portion of a primary dwelling unit or existing non-habitable space in an existing primary or accessory structure
• Junior ADU (JADU): Maximum 500 Sq. ft

• Maximum Size:
  
  • Detached: no more than 1, 200 square feet
  • Attached: no more than 1,200 square feet or 50% of the existing or proposed primary dwelling
• Setbacks:
  
  • Detached: 4’ setback to side and rear yards.
  • Attached: 4’ setback to side and rear yards.
• Height Limits:
  
  • Detached: Maximum height for detached ADUs is 16 feet. However, on lots with an existing or proposed multifamily multistory dwelling unit, the height limit is 18 feet
  • Attached: Maximum height for attached ADUs is 25 feet or the maximum allowed by the underlying zoning, whichever is lower
• Parking Requirements:
  
  • Additional parking is not required if the ADU is:
    
    • ADU located within ½ mile of public transit
    • Converted With approved car share with issued Foster City Business License,
    • Proposed ADU is a studio only (no additional bedrooms),
    • ADU is located in a architecturally and historically significant area

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

• Zoning compliance review: Courtesy neighbor notification (if required) and confirmation of compliance with ADU regulations and standards
• Site plan and elevations: Required plans and drawings submitted to the Building Division showing site layout, structure placement, and elevations
• Architectural drawings: Detailed design documents and floor plans as required for ADU construction or conversion
• Structural calculations and engineering: Included within the plans when structural modifications (such as garage conversions or new builds) are proposed
• Title 24 energy documentation: Energy efficiency documentation provided as part of the building permit requirements.
  

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

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 Foster City’s ADU process can help streamline approval.

Most residential zones in Foster City, including R-1 (Single-Family Residence), R-2 (Two-Family Residence), and R-M (Multiple-Family Residence), are eligible for ADUs. There are no minimum lot size requirements for ADUs in these zones. However, certain hillside or environmentally sensitive areas may have additional restrictions.

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