What happens if you skip the permit (and you needed one)
- City stops work and issues a $500–$2,000 citation; forced removal of unpermitted structure can run $20,000–$100,000+ in deconstruction and remediation, especially if bay mud or foundation damage occurs.
- Title transfer or refinancing blocked by lender until unpermitted ADU is removed or retroactively permitted (retroactive permits rare and expensive in California).
- Neighbor complaint triggers code enforcement; unpermitted ADU on a bay-area lot may be deemed 'public nuisance' under local ordinance, opening you to liability for injuries and environmental damage.
- Insurance claim denied if ADU damage occurs and underwriter discovers unpermitted construction; liability coverage void in Bay Area where bay-mud subsidence is known risk.
Foster City ADU permits — the key details
California Government Code § 65852.2 (as amended by SB 9, AB 68, and AB 881) is the law that governs your ADU — not Foster City's zoning code alone. Foster City must approve any ADU on a single-family lot that meets state standards: the main unit can be owner-occupied or not; the ADU must be no larger than 850 sq ft (or 25% of the main dwelling, whichever is larger); detached ADUs need only 4-foot setbacks from side/rear property lines; and parking can be waived if the ADU is near transit, within one-half mile of a major transit stop, or on a lot with less than one parking space per bedroom on the main house. Foster City has not carved out exemptions to these state rules, which means the city cannot use 'neighborhood character' or 'lot size' as a reason to deny your ADU — if you meet state standards, the city must approve it. The 60-day deadline (AB 671) means your application, once complete, must be approved or denied within 60 calendar days; incomplete applications reset the clock. Foster City's Building Department uses an online portal (verify at fostercity.org) and accepts electronic submissions, so you can file from home.
The geotechnical report is the surprise cost that catches most ADU applicants in Foster City. The city sits on the San Francisco Bay margin, and most lots have bay mud (soft estuarine clay) within 20–40 feet of surface. Foster City's Building Department requires a Phase I Environmental Site Assessment and geotechnical engineering report for detached ADUs; this costs $2,000–$5,000 and takes 2–4 weeks. The report must address liquefaction potential (critical in Foster City, which is on the 1906 earthquake fault line), settlement risk, and groundwater conditions. If your detached ADU sits on fill or bay mud, the engineer will specify pile-and-grade-beam or reinforced-slab foundation, which drives up construction cost by 15–25% compared to a conventional slab on grade. Attached ADUs (garage conversions, above-garage units) usually bypass the full geotechnical report because they sit on the existing main house's foundation, but you still need a structural engineer to verify the foundation can carry additional load and that floor framing meets code for ADU occupancy.
Owner-builder rules are generous in California but have limits. Under B&P Code § 7044, you can pull a permit as the property owner and do the work yourself, but only for owner-occupied residential property. If you plan to rent out the ADU, you cannot be your own general contractor — you must hire a licensed general contractor to pull the permit and oversee the build. Additionally, electrical work, plumbing, and HVAC must be done by state-licensed tradespeople (electrician, plumber, HVAC tech) regardless of whether you hire a GC; you cannot do those trades yourself even as the owner. Foster City's Building Department will require proof of contractor licensing before permits are issued, and state law requires a Notice of Commencement (NOC) filing with the county recorder before any work begins. If you go the owner-builder route for an owner-occupied ADU, you'll save contractor overhead (typically 15–20% of construction cost) but will spend time managing subs and coordinating inspections — a reasonable trade if you have construction experience.
Utilities and metering are non-negotiable. Foster City and the local water/sewer agency (Foster City Municipal Utility Company or FCMUC) require the ADU to have separate utility connections from the main house: separate electric meter (via the local utility, PG&E); separate water meter and service line (often requiring a second lateral from the main line to the street, $3,000–$8,000); separate sewer connection (or, in some cases, a grinder pump and ejector system if sewer line elevation prohibits gravity drain, adding $5,000–$15,000). Separate gas meter is required if you use natural gas for cooking or heating. If separate utilities are not feasible (e.g., main house and ADU on opposite sides of the lot with long trenches), you can propose a sub-metering arrangement, but FCMUC often rejects this — the agency prefers hard-wired separate services. Permit plans must show all utility connections, elevations, and material sizes; utilities inspections happen before and after rough-in, and the city will not sign off on framing inspection until utilities are roughed in.
Parking, setbacks, and lot coverage are usually the easiest hurdles in Foster City because state law is on your side. Your ADU must not exceed 850 sq ft (or 25% of the main dwelling, whichever is larger) and must comply with setback minimums (4 feet from side/rear for detached; inherit main house setbacks if attached). Parking is often waived entirely under state law; Foster City does not require parking for an ADU unless the main house is required to have parking (e.g., a 3-bed main house on a standard lot typically triggers 2 spaces, but that requirement often applies only to the main house, not the ADU). Lot coverage is typically capped at 60–65% on residential lots in Foster City, but the ADU counts against it; if your lot is small or already near the cap, a detached ADU may be denied on coverage grounds. The solution: attach it (e.g., garage conversion, second-story addition) and it will count as part of the main house's existing coverage footprint. Plan review often uncovers these issues early, so submit conceptual sketches to the Planning Department 2–3 weeks before filing a full permit to get buy-in on feasibility.
Three Foster City accessory dwelling unit (adu) scenarios
Bay mud, liquefaction, and why Foster City's geotechnical report is not optional
Foster City was built on filled Bay Area tidal marsh (bay mud) starting in the 1950s. Bay mud is soft, highly compressible marine clay with high water content and poor bearing capacity. When loaded (like with the weight of a new ADU foundation), bay mud settles and consolidates, often unevenly, causing differential settlement that cracks foundations, misaligns doors and windows, and can rupture utilities. Foster City's Building Department and most structural engineers now require a Phase I ESA (Environmental Site Assessment) and geotechnical report for any new detached structure, including detached ADUs, to assess settlement risk and design an appropriate foundation system.
The 1989 Loma Prieta earthquake highlighted another issue: liquefaction. When bay mud is saturated (Foster City's water table is often within 5–10 feet of surface due to bay proximity and storm-water percolation), earthquake shaking can cause the soil to temporarily lose strength and behave like liquid, allowing structures to sink or tilt. Foster City lies just south of the San Francisco Bay, near the San Andreas fault zone, and is categorized as a high-liquefaction-risk area by the USGS. Your geotechnical report will assess liquefaction potential and may recommend liquefaction remediation (e.g., ground improvement, engineered fill, or deep piling) that adds $10,000–$30,000 to your foundation cost.
The typical solution for detached ADUs on bay-mud lots is a pile-and-grade-beam foundation (drilled piers to 40–60 feet depth, bearing on firmer soil or bedrock, connected by grade beams above ground) or a reinforced post-tensioned slab on grade with specific concrete mix and rebar patterns. These systems are 15–25% more expensive than conventional slab-on-grade but are required by the city. You cannot argue your way out of this by saying 'the main house used a slab' — the city knows the main house was built in 1950 when standards were weaker, and current code requires current design. Factor $20,000–$40,000 for an engineered foundation system on a 600 sq ft detached ADU.
The timeline impact is also significant. A geotechnical report takes 2–4 weeks from site evaluation to final report. Once you have the report, your engineer or architect must design the foundation system (1–2 weeks more), then submit plans to the city. The city's plan review will scrutinize the geotechnical report and foundation design closely, often requesting additional clarification or revisions. If your report is incomplete or your engineer's design doesn't align with the geotech findings, the city will issue corrections and reset the 60-day clock. Start the geotechnical report early — even before you retain an architect — to avoid timeline surprises.
California's 60-day shot clock: how AB 671 and AB 881 change the game in Foster City
AB 671 (effective 2020) and AB 881 (effective 2022) imposed a 60-calendar-day timeline for cities to approve or deny ADU permit applications. The clock starts when the city deems the application complete. Once started, the city must approve or deny within 60 days; if the city doesn't act within 60 days, the permit is deemed approved by operation of law (you get the permit without the city's affirmative sign-off — unusual in California). Foster City's Building Department and Planning Department are aware of this deadline and generally try to meet it, but the clock stops and resets if the city issues a list of corrections and you resubmit your application. This means a sloppy initial application can add 10–14 days to your timeline.
The 60-day shot clock only applies to applications that meet the state definition of an ADU: one accessory unit per parcel, under 850 sq ft (or 25% of main dwelling), owner-occupied or not, meeting setback and lot-coverage standards per Government Code § 65852.2. If your application falls outside this definition (e.g., you're proposing two ADUs per parcel, or a 1,000 sq ft detached unit), the city can process it under normal discretionary review timelines, which can be 120+ days. The application must include a checklist confirming ADU compliance with state standards; if your application is incomplete or doesn't include this checklist, the city will reject it as incomplete and the clock doesn't start until you resubmit a complete application.
In practice, Foster City's Building Department uses the 60-day timeline as a framework. Plan review happens in parallel with Planning Department review; if Planning has questions about setbacks, lot coverage, or compatibility with existing structures, those corrections can be resolved in the first 21–28 days of plan review. If the city issues a 'incomplete application' letter in week 2 (e.g., missing utility details or structural calculations), you have about 7–10 days to resubmit, and then the city takes another 30–40 days to finalize approval. Most applications in Foster City are approved within 55–60 days if submitted correctly.
The practical implication: before you file your formal permit application, spend 2–3 weeks doing a pre-application review with both the Building and Planning departments. Submit preliminary sketches and utility layouts; ask the city explicitly whether your ADU meets state standards and is eligible for the 60-day shot clock. This upfront consultation prevents rejections and resets. Many applicants skip this step and file a 'complete' application that the city rejects as incomplete 10 days later — killing your 60-day timeline. The city's pre-application consultation is usually free and takes 1–2 weeks; it's worth the investment.
610 Foster City Boulevard, Foster City, CA 94404
Phone: (650) 286-3210 (Building Department main line; verify at fostercity.org) | https://www.fostercity.org/departments/community-development/building-permits (or search 'Foster City building permit online portal')
Monday–Friday, 8:00 AM–5:00 PM (closed holidays; call ahead for permit counter hours)
Common questions
Do I need owner occupancy in one of the units (main house or ADU) to qualify for an ADU permit in Foster City?
Not anymore, thanks to AB 881 (effective 2022). Previously, AB 68 required at least one unit to be owner-occupied; now, both the main house and ADU can be rented out without owner occupancy. However, if you are the property owner and do not occupy either unit, you must still hire a licensed general contractor to pull the permit — you cannot be your own GC for a non-owner-occupied property (California B&P Code § 7044). The permit itself is not denied; the contracting rules change.
Can I build a second ADU (two ADUs on one lot) in Foster City?
California law (Government Code § 65852.22, part of AB 68) allows two ADUs per parcel under limited conditions: if one is a junior ADU (shares kitchen with main house), you can build one detached ADU + one junior ADU. If both are non-junior (each has its own kitchen), you can build a maximum of one non-junior ADU per parcel. Foster City does not impose additional restrictions, so the state law is your limit. Two separate detached ADUs on a 0.3-acre lot will almost certainly violate lot coverage and setback rules, so you'd need a variance. Talk to Planning before you design a two-ADU scheme.
What if my Foster City lot is in a flood zone or an earthquake-prone area — does that kill my ADU?
Not automatically. Foster City is in both a 100-year flood zone (FEMA Flood Zone AE due to Bay proximity) and a seismic zone. Flood zones require your ADU foundation to be elevated to the Base Flood Elevation (BFE), typically 8–12 feet, which adds pilings and vertical MEP runs (cost increase of $15,000–$25,000). Seismic zones require engineered foundations per California Building Code § 1813 (similar to bay-mud requirements); your geotechnical report will specify seismic design parameters. Neither flood nor seismic risk is a permit denial — it's a design constraint. However, insurance costs for a below-BFE ADU will spike, and financeability can be tough. Confirm flood zone and BFE early; it may move your ADU to a higher cost foundation system.
If the main house is unpermitted or partially unpermitted, can I still get an ADU permit?
Legally, the city cannot deny an ADU permit solely because the main house is unpermitted. However, in practice, the city may flag the unpermitted main house during plan review and require you to legalize it (pull a retroactive permit, or demolish the unpermitted portion) before the city approves the ADU. This can delay your ADU timeline by 8–16 weeks and cost $5,000–$20,000 in retroactive plan review and fees. It's rare that the city formally blocks an ADU application for an unpermitted main house, but the delay is real. Before filing an ADU permit, disclose the main house history to the city and ask whether retroactive permits are required.
How much will separate utility connections cost me in Foster City?
Separate electric meter: $1,500–$3,000 (PG&E will run a new service line from the pole to a new meter outside the ADU, or to a dedicated breaker panel if the meter bank is at the main house). Separate water meter and lateral: $3,000–$8,000 (the local water utility, FCMUC, will drill a new connection from the water main to the ADU; if the main water line is far from the ADU, costs spike). Separate sewer: $2,000–$5,000 if gravity drain is feasible; $5,000–$15,000 if you need a grinder pump and ejector system (common if the ADU is downhill from the main sewer or slab elevation is below street level). Separate gas meter: $500–$1,500. Total for all utilities on a typical 600 sq ft detached ADU: $8,000–$30,000 depending on lot layout. For a garage conversion or junior ADU that shares water/sewer, costs drop to $2,000–$4,000 (electric and gas sub-panels only).
Can I use pre-approved ADU plans from a state list to speed up my permit?
California's pre-approved ADU plan list (via the Department of Housing and Community Development) includes designs that may streamline review in some cities, but Foster City's Building Department will still require full plan review and geotechnical assessment for detached ADUs. Pre-approved plans can save 1–2 weeks in design time (you don't have to hire an architect from scratch), but the 60-day shot clock starts from your permit application, not from when you download a pre-approved plan. Using a pre-approved plan does not waive geotechnical requirements or lot-specific design work (setbacks, foundation depth, utilities). The advantage is consistency and reduced architect fees ($500–$1,000 instead of $3,000–$5,000), not a faster permit timeline.
What if my ADU is a prefabricated or modular unit — does it change the permit process?
Modular ADUs are becoming common in California. They're factory-built, inspected at the factory by the state, then transported and assembled on-site. They still require a building permit in Foster City, but the city's plan review is slightly faster because the structural and MEP systems are pre-engineered and UL-listed. However, you still need a geotechnical report, foundation design, utility connections, and site-plan review specific to your lot. Timeline savings: typically 2–3 weeks on plan review. Cost savings: roughly 10–15% over stick-built ADU due to factory efficiency. The modular unit itself costs $60,000–$120,000 depending on size and finishes, plus $20,000–$40,000 for site work (foundation, utilities, permitting). Modular units do not bypass Foster City's 60-day shot clock or geotechnical requirements — they just streamline the structural and MEP review.
Do I need a separate property survey or easement if I'm running utilities across a neighbor's land?
If your water, sewer, gas, or electric laterals require a utility easement across neighboring property, you must obtain a written easement agreement from the neighbor and record it with San Mateo County. Foster City's Building Department and the utility companies will not allow underground utilities without recorded easements. Easement costs vary widely: a neighbor might grant it for free, or demand $500–$2,000. If the neighbor refuses, you'll need to redesign your utilities to avoid their land (e.g., longer trenches around the property), which adds cost and may be infeasible on a small lot. A professional land surveyor ($1,500–$3,000) can identify easement locations and help draft language. Survey and easement work should happen during pre-design, before you pay for full architectural plans.
What's the difference between a 'junior ADU' and a 'detached ADU,' and does it affect my permit timeline in Foster City?
A junior ADU is a bedroom + bathroom carved out of the main house (or a garage conversion) that shares a kitchen and common area with the main house. It does NOT have a separate kitchen (no separate stove/oven). A detached ADU is a stand-alone structure (or an above-garage unit) with its own kitchen and separate entrance. California law allows junior ADUs even without owner occupancy, and they count toward your one-ADU-per-parcel limit. In Foster City, junior ADUs have a faster permit timeline (no geotechnical report, simpler structural review) and lower fees ($1,500–$3,000 vs. $2,500–$4,500 for detached). Construction cost is also lower (garage conversion: $45,000–$85,000; detached: $120,000–$200,000+). If you're undecided on ADU type, junior ADU (garage conversion) is usually the fastest and cheapest path in Foster City.
If the city doesn't approve my ADU within 60 days, do I automatically get the permit?
Yes, but with caveats. If the city fails to act within 60 calendar days and hasn't issued a correction list (which resets the clock), you can claim the permit is deemed approved by operation of law under AB 671. However, you still need a final certificate of occupancy before you can occupy the ADU, and the city will require inspections and sign-offs regardless of the deemed approval. In practice, claiming a deemed approval is rare and contentious — the city will likely continue working on plan review and inspections, arguing that the deemed approval does not waive code compliance. If you hit the 60-day mark, contact the city's permit manager in writing, cite AB 671, and request confirmation that the permit is approved. Have a real estate attorney draft the letter if you want it to carry weight. Most cities push hard to finalize approval before day 60 to avoid the deemed-approval trap.