San Mateo County
Foster City ADU rules
Research summary of local ADU rules on file for City of Foster City in San Mateo County. Not a determination for a specific lot.
City or county ADU page
Check any California address for freeCity of Foster City rules on file
Local rules recorded in this project for this jurisdiction. Where a city figure on file is stricter than the state baseline on file, both are shown.
ADUs that follow the state’s simple path
Some ADUs follow only the state’s basic limits: a conversion ADU plus a junior ADU inside the home or an existing accessory structure, and one new detached ADU up to 800 sq ft with 4 ft side and rear setbacks and the state height limits. The extra local design rules below do not apply to these units.
Source: City of Foster City ADU Handbook 2026 (Foster City Municipal Code Chapter 17.78, Ordinance 692), 17.78.050, accessed 2026-10-23.
Junior ADUs
Up to 500 sq ft inside a single-family home. Owner occupancy (owner lives in the main home or the junior ADU) applies only when the junior ADU has no separate bathroom. A deed restriction is recorded.
Source: City of Foster City ADU Handbook 2026 (Foster City Municipal Code Chapter 17.78, Ordinance 692), 17.78.060.B, accessed 2026-10-23.
Attached ADUs
Up to 1,200 sq ft or 50% of the main home’s floor area, but an 850 sq ft unit is allowed regardless of the 50% figure. 4 ft side and rear setbacks. Up to 25 ft and two stories. Colors, materials and roof pitch should match the main home.
Source: City of Foster City ADU Handbook 2026 (Foster City Municipal Code Chapter 17.78, Ordinance 692), 17.78.060.C, accessed 2026-10-23.
Detached ADUs
At least 850 sq ft is allowed, up to 1,200 sq ft. Height up to 16 ft (18 ft near major transit, plus 2 ft to match the main roof pitch). Not in the front yard unless an 850 sq ft unit can’t fit elsewhere.
Source: City of Foster City ADU Handbook 2026 (Foster City Municipal Code Chapter 17.78, Ordinance 692), 17.78.060.D, accessed 2026-10-23.
Conversions
Existing space in the home or an accessory structure can be converted, with up to 150 sq ft added for entry and exit.
Source: City of Foster City ADU Handbook 2026 (Foster City Municipal Code Chapter 17.78, Ordinance 692), 17.78.060.E, accessed 2026-10-23.
Extra local rules (not for state-path ADUs)
Main-home zoning rules apply unless they would block an 850 sq ft ADU. Second-floor windows larger than 2 sq ft within 10 ft of a side or rear property line must be obscured, offset or angled. No second-floor balconies facing neighbors. Exterior lights no higher than 7 ft and aimed downward.
Source: City of Foster City ADU Handbook 2026 (Foster City Municipal Code Chapter 17.78, Ordinance 692), 17.78.060.F, accessed 2026-10-23.
Multifamily lots
Conversions of non-living space up to 25% of existing units (at least one). Up to 8 detached ADUs on lots with existing multifamily buildings (not more than the existing units), or 2 with a proposed multifamily building; detached height up to 18 ft (plus 2 ft for roof pitch near transit).
Source: City of Foster City ADU Handbook 2026 (Foster City Municipal Code Chapter 17.78, Ordinance 692), 17.78.070, accessed 2026-10-23.
Parking
One space per unit or per bedroom, whichever is less, unless a state exemption applies (for example near public transit, conversions, or a garage converted to an ADU). Junior ADUs need no parking.
Source: City of Foster City ADU Handbook 2026 (Foster City Municipal Code Chapter 17.78, Ordinance 692), 17.78.080, accessed 2026-10-23.
Review timelines
The City checks completeness within 15 business days. Decisions take 30 days for ADUs using the City’s pre-reviewed plans or the state’s simple path, and 60 days otherwise. A denial can be appealed to the Planning Commission within 10 days.
Source: City of Foster City ADU Handbook 2026 (Foster City Municipal Code Chapter 17.78, Ordinance 692), 17.78.110, accessed 2026-10-23.
Impact fees
ADUs of 750 sq ft or more may be subject to impact fees proportional to the size of the main home; smaller ADUs are not. Check the City’s fee schedule for current amounts.
Source: City of Foster City ADU Handbook 2026 (Foster City Municipal Code Chapter 17.78, Ordinance 692), Fees, accessed 2026-10-23.
Contact
Community Development: cddpermit@fostercity.org, (650) 286-3227.
Source: City of Foster City Accessory Dwelling Unit (ADU) page, accessed 2026-10-23.
California state rules on file
The statewide baseline recorded from state law and the HCD ADU Handbook.
Detached size cap described in state law
1,200 sq ft. Recorded in this rules file from Government Code section 66314.
Source: California Government Code section 66314, 66314, accessed 2026-10-16.
Side and rear setback cap for a new ADU
4 ft. Recorded in this rules file from Government Code section 66314, which describes a cap on side and rear setbacks for a new ADU.
Source: California Government Code section 66314, 66314, accessed 2026-10-16.
Setback for a conversion of an existing structure
No additional setback, as recorded in this rules file from Government Code section 66314 for a conversion of an existing structure.
Source: California Government Code section 66314, 66314, accessed 2026-10-16.
Minimum lot size
A city may not impose a minimum lot size for an ADU, as recorded in this rules file from Government Code section 66314.
Source: California Government Code section 66314, 66314, accessed 2026-10-16.
Minimum unit the handbook says cities must allow
At least an 800 sq ft ADU with 4 ft side and rear setbacks. The HCD ADU Handbook (updated March 2026) describes this under section 66321(b)(3).
Source: HCD ADU Handbook, 66321(b)(3); handbook updated March 2026, accessed 2026-10-16.
Detached height described in the handbook
16 ft, or 18 ft or 20 ft in some cases. The HCD ADU Handbook (updated March 2026) describes this under section 66321(b)(4).
Source: HCD ADU Handbook, 66321(b)(4); handbook updated March 2026, accessed 2026-10-16.
Permit decision deadline
60 days. Recorded in this rules file from Government Code section 66317(a)(3): the permitting agency must decide on (approve or deny) a completed ADU application within 60 days when an existing single-family or multifamily dwelling is on the lot. Section 66317(a)(1) says the application is considered ministerially, without discretionary review or a hearing.
Source: California Government Code section 66317, 66317(a)(1), (a)(3), accessed 2026-10-17.
Owner-occupancy requirement
None. Recorded in this rules file from Government Code section 66315: a local agency may not impose an owner-occupancy requirement on an ADU. It may require that the property be used for rentals of terms 30 days or longer. The HCD ADU Handbook (updated March 2026) describes the same rule.
Source: California Government Code section 66315, 66315; HCD ADU Handbook updated March 2026, accessed 2026-10-17.
Parking near public transit
No parking standards within one-half mile walking distance of public transit. Recorded in this rules file from Government Code section 66322(a)(1). Section 66322(a) lists several other cases where parking standards may not be imposed.
Source: California Government Code section 66322, 66322(a), accessed 2026-10-17.
Ministerial categories
The HCD ADU Handbook (updated March 2026) states that Government Code section 66323 sets four ministerial categories. This rules file does not restate those four categories.
Source: HCD ADU Handbook, 66323; handbook updated March 2026, accessed 2026-10-16.
Fire Hazard Severity Zones
If your lot is in a Fire Hazard Severity Zone, the CAL FIRE Office of the State Fire Marshal says in Information Bulletin 25-004 (issued March 3, 2025): “Fire Hazard Severity Zones (FHSZs) should not restrict the development of Accessory Dwelling Units (ADUs). However, additional building standards may be necessary, including but not limited to California Fire Code Chapter 49 and California Building Code Chapter 7A.” State ADU law does not itself bar an ADU there. A city or county may still apply its building code, including wildfire-resistant construction (Gov. Code 66314(d)(8)); require side and rear setbacks “sufficient for fire and safety” for a unit converted under 66323(a)(1); and decline parking in setbacks on specific fire and life safety findings (66314(d)(10)(B)). It may also consider public safety when it designates where ADUs may be permitted (66314(a)). Sprinklers can't be required in an ADU or JADU if the primary residence doesn't need them (66314(d)(12), 66323(d)). Some cities and counties still apply their own fire-zone limits. Where this summary has one, it is listed with the local rules. This summary does not look up your lot. Check the CAL FIRE Fire Hazard Severity Zone map viewer linked from the State Fire Marshal's Fire Hazard Severity Zones page: https://experience.arcgis.com/experience/5065c998b4b0462f9ec3c6c226c610a9
Source: CAL FIRE Office of the State Fire Marshal Information Bulletin 25-004, page 4, issued March 3, 2025; Gov. Code 66314, 66323, accessed 2026-10-20.
State baseline on the directory → · State ADU rules on the homepage →
Can I build an ADU in Foster City?
State law sets a baseline for ADUs in Foster City, and the local standards on file are listed on this page. Whether a specific lot can add an ADU depends on zoning, utilities, and other site facts this page does not measure. Confirm with City of Foster City Community Development Department, or run a free address summary.
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