California ADU rules · San Mateo County
Can I Build an ADU in Foster City?
Yes — you can build an ADU in Foster City. California's statewide ADU law requires every city, including Foster City, to approve a qualifying accessory dwelling unit by right, plus a junior ADU (JADU) on a single-family lot. What varies city to city are the local specifics below.
Can you build one?
ADUs allowed
CA state lawYes — by right
ADUs and JADUs are allowed by right in all residential and mixed-use zones; ministerial approval, no hearing.
Units on a single-family lot
Local rule1 JADU (max 500 sq ft) AND 1 conversion ADU AND/OR 1 newly constructed ADU (detached or attached); never more than one JADU per lot
Allows both a JADU and up to two ADUs (conversion and/or new construction) on a single-family lot, subject to size and configuration limits.
Units on a multifamily lot
Local ruleUp to 8 detached ADUs (not to exceed number of existing units); 1 attached ADU per lot; conversion ADUs within existing structure up to 25% of existing units
Detached ADUs capped at 8 or number of existing units, whichever is less; one attached ADU per lot.
Junior ADU (JADU)
CA state lawAllowed
Up to 500 sq ft, within the walls of an existing single-family home (single-family lots only).
Size & height
Max detached ADU size
Local rule850 sq ft
Detached ADU max is 850 sq ft for studio/1-bedroom; 1,200 sq ft for 2+ bedrooms. This override applies to the lower cap for smaller units.
Max attached ADU size
Local rule1,200 sq ft OR 50% of primary dwelling, whichever is less
Attached ADUs are capped at the lesser of 1,200 sq ft or 50% of the primary dwelling.
Guaranteed minimum size
CA state law800 sq ft
An 800 sq ft ADU must be allowed regardless of FAR/lot-coverage caps; a city also cannot cap an attached ADU below 850 sq ft (1,000 for 2+ bedrooms).
Max JADU size
CA state law500 sq ft
Max height
CA state law16 ft
16 ft baseline; cities must allow 18 ft within ½ mi of a major transit stop or where the primary dwelling is two-story, and 20 ft on a lot with a multistory multifamily building.
Setbacks
Side setback
CA state law4 ft
A city cannot require more than 4 ft side or rear setback; conversions of existing structures are exempt.
Rear setback
CA state law4 ft
Front setback
Local ruleADUs generally not allowed in front yard unless 850 sq ft ADU is infeasible elsewhere
Front yard placement is restricted except when no feasible alternative exists for an 850 sq ft ADU.
Separation from main house
Local ruleNone required
No local separation requirement; defaults to California Residential Code.
Parking
Parking required
Local ruleUp to 1 space
Max 1 space per unit or per bedroom, whichever is less; multiple local exemptions apply.
Transit parking exemption
CA state lawYes — no parking required near transit & for conversions
No parking may be required within ½ mi walking distance of transit, in a historic district, for an ADU within an existing structure, within one block of car-share, or in certain permit-parking areas.
Garage-conversion replacement parking
Local ruleNot required
No replacement parking required for garage/carport/parking space conversion.
Approval, timeline & cost
Approval path
CA state lawMinisterial (by right)
No public hearing or discretionary design review for a qualifying ADU.
Approval timeline
Local rule30 days
30-day approval timeline applies if using pre-approved ADU plans; otherwise, 60 days.
Owner-occupancy
Local ruleOnly for a JADU
Owner occupancy required only for JADUs, not for ADUs.
Impact-fee waiver
CA state lawWaived under 750 sq ft
An ADU of 750 sq ft or less pays no impact fees (the 750 sq ft threshold itself is exempt under § 66311.5); larger ADUs pay fees proportional to the primary dwelling. JADUs (≤ 500 sq ft) are exempt.
Permits required
Local ruleBuilding permit, Demolition permit (for garage conversion)
Demolition permit must be issued concurrently for garage conversions.
Foster City-specific factors that can change the answer
State law sets the floor, but these local conditions often decide whether a specific parcel works.
California Fire Code compliance
ADUs must comply with California Fire Code as amended by San Mateo Consolidated Fire Department; sprinklers not required unless primary dwelling requires them.
Foster City allows both a JADU and up to two ADUs (conversion and/or new construction) on single-family lots, with specific caps for detached and attached ADUs. Height, parking, and front yard placement have detailed local rules, and a pre-approved plan program is being implemented.
Frequently asked questions
Can I build an ADU in Foster City?
Yes. California's statewide ADU law requires Foster City to approve a qualifying accessory dwelling unit ministerial (by right), plus a junior ADU (JADU) on a single-family lot.
How big can an ADU be in Foster City?
A detached ADU of at least 850 sq ft must be allowed, and an ADU of 800 sq ft can never be zoned out by floor-area or lot-coverage limits. A JADU can be up to 500 sq ft.
What are the setback requirements for an ADU in Foster City?
Side and rear setbacks are limited to 4 ft. ADUs generally not allowed in front yard unless 850 sq ft ADU is infeasible elsewhere.
Is parking required for an ADU in Foster City?
Up to 1 space. No parking can be required within ½ mile of transit, for conversions, or for units inside the primary home. When a garage is converted, replacement parking is not required.
How long does ADU approval take in Foster City?
A complete application must be acted on within 30 days, with ministerial (by right) — no public hearing or discretionary design review for a qualifying ADU.
Do I have to live on the property to build an ADU in Foster City?
Owner-occupancy: only for a jadu. ADUs under 750 sq ft are also exempt from impact fees.
Sources
- Foster City Municipal Code Chapter 17.78 (Ord. 684, 2024; Ord. 692, 2025)
- FCMC § 17.78.060(A)
- FCMC § 17.78.070(B)-(C)
- FCMC § 17.78.060(D)(1)
- FCMC § 17.78.060(C)(1)
- FCMC § 17.78.060(C)(3), (D)(3), (E)(1)
- FCMC § 17.78.060(D)(3)(b)
- FCMC § 17.78.080(A)-(B)
- FCMC § 17.78.080(B)(8)
- FCMC § 17.78.090(A)
- FCMC § 17.78.060(B)(2)
- FCMC § 17.78.090(C)
- FCMC § 17.78.100(E)
- Cal. Gov. Code §§ 66310–66342 (ADU Law)
- HCD 2025 Accessory Dwelling Unit Handbook
- Cal. Gov. Code § 66317 (ministerial approval, 60-day deadline)
- Cal. Gov. Code § 66333 (junior ADUs)
- Cal. Gov. Code § 66321 (size minimums & height)
- Cal. Gov. Code § 66323 (by-right ADU/JADU, 800 sq ft, 4-ft setbacks)
- Cal. Gov. Code § 66322 (ADU parking; § 66334 covers JADU parking)
- Cal. Gov. Code § 66311.5 (impact fees: none for an ADU ≤ 750 sq ft or JADU ≤ 500 sq ft)
Last updated 2026-06-25. This is an AI-assisted summary of Foster City's ADU rules, drawn from its published ordinance and California ADU law — general information, not legal advice, and it may be incomplete or out of date. Verify the controlling rule for your specific parcel with Foster City Planning before relying on it.
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