Bay Area region / California
Foster City ADU rules
& permit guide.
Planning an ADU in Foster City? Review local size, setback and parking requirements, then confirm the details for your property with the permitting agency.
Local source collected 2026-05-20 · View sources
Local requirements at a glance
Min/max ADU size
Min:150 square feet Max ADU in single-family zoning districts: 1200 square feet or 50% of main home floor area (whichever is less), unless under 800 square feet (state law allows an ADU up to 800 square feet on every property, some exceptions may apply) Max ADU in multi-family zoning districts: 850 square feet for one bedroom, 1000 square for two or more bedrooms Max JADU: 500 square feet
Height
JADU – Within existing structure Attached ADU: Shall not exceed 25 feet in height, or the height of the primary dwelling, whichever is lower. ADUs shall not exceed two stories in height. Detached ADU: 16 feet, except within ½ mile of major transit stop or high-quality transit corridor – 18 feet; 2 feet may be added to align with roof pitch of primary dwelling; 18 feet maximum for multi-family, multi-story ADU
Setbacks
Side and Rear: 4 feet Front: may not be located in the front yard.
Parking
JADUs: No parking required ADUs: One spot may be required (tandem allowed). No parking required if: The ADU is located within one-half mile walking distance of public transit. The ADU is a conversion ADU. When there is an approved car share vehicle for which a Foster City business license has been issued and remains active, located within one block of the accessory dwelling unit. ADU that only includes a studio with no bedroom.
Process
Confirm your parcel and ADU type with the local planning department, prepare the required plans, and follow the city's building permit checklist. Ask which utility, fire and site reviews apply before submitting.
Your next three steps
- Confirm the property's jurisdiction, zoning and site constraints with Foster City's planning department.
- Discuss the ADU type, dimensions and utility needs with your designer before finalizing a layout.
- Ask the permitting agency for its current application checklist, fee estimate and review process.
2026 state-law context
ADUs and JADUs have different occupancy rules
California generally prohibits local owner-occupancy requirements for ADUs. A narrow exception applies to certain separately sold affordable ADUs. For JADUs, owner occupancy generally applies when the bathroom is shared with the main home; separate-bathroom JADUs are treated differently under 2026 law. Check the HCD handbook and your project's permit conditions.
Older local summaries may not reflect the latest state changes. Read HCD’s March 2026 handbook, pages 32–33.
Questions for your first planning conversation
- Which approval path applies to my building and ADU type?
- Do easements, trees, slope, access or fire conditions affect placement?
- Which utility upgrades and agency reviews should be included in my budget?
- Are preapproved plans available, and what site work still needs review?