ADU Builder in Foster City

Foster City is the most demanding place on the Peninsula to put a foundation in a backyard, and its own building code says so. Here is what the city actually requires, what it costs, and why a soils report comes before anything else.

Why an ADU in Foster City starts underground

Most Peninsula cities let you design an ADU and deal with the ground later. Foster City does not, and the reason is written into its own findings of fact. The city adopted local amendments to the California building code specifically because of what it is built on.

The city is built on dredged fill

Foster City describes itself, in Ordinance No. 655, as a manmade island developed over former marshland. In the early 1960s hydraulic fill of sand and clam shells was dredged from the Bay and laid to an average depth of five feet over the existing land. That is the ground your ADU sits on.

Bay mud up to 30 feet thick

Beneath the fill, the city records unconsolidated fine silty clay varying from a few feet to as much as thirty feet deep, with bedrock generally three hundred feet or more down. The city's own finding is that the weight of the fill compacts the younger bay mud underneath, and that this compaction must be accounted for in the design of structures and drainage.

Seismic Design Category E, 1,200 psf

Foster City publishes a presumptive soil bearing value of 1,200 psf and notes that anything more requires a soils or geology report. Its seismic design category is E, the most severe normally encountered in residential work, and the city places itself in Seismic Site Class D, E or F.

Soil that attacks metal

The city's findings state plainly that the salt and mineral content of the fill is very destructive to metal in contact with the soil, with chloride concentration in the highly to very highly corrosive range and adverse sulfate content. The building code amendments require a geotechnical report to recommend protection for steel and concrete against long-term deterioration from contact with on-site soils.

The code assumes your building will settle

The clearest signal of all sits in the city's roof drainage amendment. Roofs must be designed against ponding after long-term deflection and possible differential foundation subsidence, with a minimum assumed ponding of one eighth of an inch for every foot of the building's minimum dimension. On a twenty-foot-wide ADU that is two and a half inches of assumed sag, designed in from day one.

One claim you will see repeated online is that Foster City requires piles for an ADU. It does not. The code requires that a geotechnical engineer determine the foundation type and that the foundation be engineered to their report. Anyone quoting you a pile foundation before a soils report exists is guessing.

What Foster City actually allows

Governed by Foster City Municipal Code Chapter 17.78, most recently amended by Ordinance No. 692 in August 2025. Where the city's published handbook and the code disagree, the code controls.

Unit type	Maximum size	Height	Setbacks
Junior ADU (JADU)	500 sq ft, within the existing walls	n/a	n/a
Attached ADU	1,200 sq ft or 50% of the primary dwelling, whichever is less	25 ft or the primary dwelling's limit, whichever is lower, max 2 stories	4 ft side and rear
Detached ADU	1,200 sq ft or 50% of the primary dwelling, whichever is less	16 ft, or 18 ft within half a mile of a major transit stop, plus 2 ft to match the main roof pitch	4 ft side and rear
Conversion ADU	No size cap within existing space, plus up to 150 sq ft for ingress and egress	Existing	Existing

The code guarantees you at least an 800 sq ft ADU at 16 ft with 4 ft setbacks, whatever the other standards would otherwise preclude. The city's own handbook says 850 sq ft in several places; the code says 800, and the code governs.
ADUs are not permitted in the front yard unless an 850 sq ft unit is genuinely infeasible in the side and rear yards.
R-1 lot coverage is capped at 50%, and the city requires a lot coverage diagram showing the ADU, the house and any accessory structure taller than the fence line. Structures over six feet count; unroofed decks and balconies do not.
Parking is capped at one space per unit or per bedroom, whichever is fewer, and there are eight exemptions including proximity to transit, studios, JADUs, and demolishing a garage to build the ADU. The historic-area exemption exists in the code but the city notes no historically significant areas exist in Foster City.
Owner occupancy is not required for an ADU. Foster City's code still requires it for a JADU, along with a recorded deed restriction, but state law narrowed that on 1 January 2026 so it applies only where the JADU shares sanitation facilities with the main house. The city code predates the change, so confirm which governs your project before you rely on it.

Foster City rules that go beyond state law

State ADU law sets a floor, not a ceiling. Foster City adds design requirements that catch owners out when they arrive with plans drawn for a generic California lot.

Match the house

Attached, detached and conversion ADUs must use the same exterior wall and roof colors as the primary dwelling. Attached units and JADUs must match its roof pitch and eave detail within 2:12.

Anything on a second floor is restricted

Upper windows facing a neighbor within ten feet of the line must be obscured, offset or splayed, and balconies, decks and roof terraces facing another residence are prohibited outright on new construction. Worth knowing before anyone draws a roof deck.

You must notify the neighbors

A courtesy neighbor notification report is a required application item: adjacent owners have to be told about the new ADU. Garage conversions are exempt from this one.

38 neighborhood design prototypes

Foster City's Planning Division maintains design prototypes for 38 named communities, from Shell Cove and Sea Colony to Nantucket Cove and Treasure Isle, and many of them cover accessory structures specifically. Check the prototype for your neighborhood alongside any recorded CC&Rs before design starts.

PG&E sign-off is a completeness item

Your application is not complete without a PG&E letter confirming the existing service can carry the new ADU load, or details for a new approved service. That is a lead-time item, not a formality.

No separate sale

Foster City has not adopted an AB 1033 ordinance, so an ADU here cannot be sold separately as a condominium. The only separate-conveyance route in the code is a narrow provision for qualifying nonprofit-built housing.

What Foster City charges

Figures below are from the adopted Master Fees and Service Charges Schedule for FY2026-27. The city's own ADU handbook still quotes the superseded numbers, so check the fee schedule rather than the handbook.

Parks facilities impact fee: $15,406
Public safety impact fee: $1,645
Transportation impact fee: $2,191
Total impact fees per full dwelling unit: $19,242
Plan check: 70% of the building permit fee
General plan maintenance fee for ADUs: No fee

An ADU of 750 sq ft or less is fully exempt from impact fees. Above 750 sq ft, impact fees are charged in proportion to the ADU's size relative to the primary dwelling, so a 1,000 sq ft ADU beside a 2,000 sq ft house pays roughly half.
The first plan check is included and the second costs nothing extra. Third and subsequent reviews are charged at actual cost, which is a direct argument for getting the first submittal right.
The express plan check surcharge does not apply to ADUs. The city exempts them by name.
Impact fees are adjusted every year by the construction cost index, so confirm current figures before you budget.
Water and sewer connection charges are listed in the fee schedule for dwelling units generally, but the schedule does not state how they apply to an ADU. This is a potential five-figure swing and we confirm it with the Building Division for your specific project rather than guessing.

How long Foster City takes

Foster City publishes its review clocks, which is more than most cities do. The clocks only start once your application is complete, and completeness is where most projects lose time.

15 business days
Completeness review
Before any clock starts, the city checks you have supplied every required document. The engineered foundation plan designed to a geotechnical soils report is on that list, in bold. Without it you are not in the queue.
30 days
Pre-reviewed plans
Using a plan the city has already pre-approved cuts the review to thirty days. Foster City offers four GLADUR pre-reviewed ADU designs free to download, run with HEART of San Mateo County, plus its own pre-approval program.
60 days
Standard ministerial review
A complete standard ADU application gets a ministerial decision within sixty days. If the ADU is part of a new or remodelled primary residence, that clock does not start until the house itself is permitted.
Varies
Correction cycles
Plans are resubmitted until they meet city and state code, and the city publishes no cap on the number of cycles. The first two plan checks are covered by your fee; from the third you pay actual cost. This is where a complete first submittal pays for itself.

Overlay

The levee, and what flood zone means for your foundation

Foster City sits behind a levee, and its flood designation has been in flux. Here is where it genuinely stands, including the part nobody can currently answer.

The levee was rebuilt and finished in 2024

FEMA found in 2014 that the levee did not meet its criteria. Voters passed Measure P in 2018 with about 81% support, authorising a $90 million bond. Construction ran from autumn 2020 to final completion in February 2024, designed to protect against three feet of sea level rise through 2100.

The remapping is unresolved

The city submitted a Letter of Map Revision to FEMA in February 2024 based on the as-built levee, and said the process could take six months to a year or more. Every public source goes quiet after that, and the city's own levee page still describes the completed project in the future tense. We check the effective flood map for your specific address rather than repeating a designation that may be out of date.

If your parcel is in a flood hazard area, it changes the foundation

Foster City requires residential construction in a mapped special flood hazard area to have its lowest floor two feet above base flood elevation. That is a foot more freeboard than the federal minimum, and it pushes an ADU onto a raised or stem-wall foundation. After construction the elevation must be certified by a civil engineer or licensed surveyor.

An ADU does not get the shed exemption

Detached garages and storage structures can sit below base flood elevation with flood vents and resistant materials. An ADU is habitable space, so it does not qualify for that allowance and is treated as residential construction.

What Foster City does not have: fire hazard zoning

San Mateo Consolidated Fire confirms Foster City is not located within a fire hazard severity zone. That is a real advantage over the hill cities on this side of the county. Belmont carries 971 acres of moderate zone and Millbrae was remapped in 2025 from none to 353 acres of very high, and both now face wildland-urban interface construction standards that simply do not apply here.

Flood zone is parcel-specific and it is the single largest unknown in a Foster City ADU budget. It is the first thing we check, before any design work.

Where else we build

ADU builder in Belmont

Belmont is a hillside city, and on a hillside the slope decides your ADU before the architect does. Here is how Belmont's own code actually works, including the two rules that catch almost every owner out.

Detached ADUs

A standalone backyard home, what it costs and what decides the size.

Testimonials & Reviews

Real reviews from Bay Area homeowners we’ve built for.

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Foster City ADU questions

Here are some common questions along with their answers to help clear up any confusion.

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Do I really need a soils report for an ADU in Foster City?

For a new detached ADU, yes. Foster City's ADU submittal checklist lists an engineered foundation plan designed to a geotechnical soils report as a completeness requirement, which means your application is not even accepted without it and the sixty-day review clock does not start. The code also requires a stamped letter from the project engineer confirming the report's recommendations were built into the design before plans are approved. The building official can waive the geotechnical investigation for additions to single-story detached homes, but a new detached ADU is a new structure rather than an addition, so that waiver does not obviously reach it. We confirm the position for your specific project.

How big an ADU can I build in Foster City?

A detached or attached ADU is capped at 1,200 sq ft or 50% of your primary dwelling, whichever is smaller, so on a modest house the 50% rule usually binds before the 1,200 does. Whatever the other standards would otherwise allow, the code guarantees you at least 800 sq ft at 16 feet in height with four-foot side and rear setbacks. A junior ADU is capped at 500 sq ft and has to sit inside the existing walls of the house. Converting existing space has no size cap at all, plus up to 150 sq ft for access.

Will I lose my parking if I convert the garage?

No. Foster City's code exempts an ADU from replacement parking where a garage or carport is demolished or converted to build it, and it caps ADU parking at one space per unit or per bedroom in any case, with eight separate exemptions on top. Several of those are easy to meet on the Peninsula, including being within half a mile's walk of transit and building a studio with no bedroom.

What will the permit fees actually be?

If your ADU is 750 sq ft or smaller it is fully exempt from impact fees, which are $19,242 for a full dwelling unit at FY2026-27 rates. Above 750 sq ft you pay those fees in proportion to the ADU's size against the main house. On top of that sit a valuation-based building permit fee, plan check at 70% of it, and smaller processing and technology charges. Foster City charges ADUs no general plan maintenance fee and exempts them from the express plan check surcharge.

How long does the whole thing take?

Plan on fifteen business days for the city to confirm your application is complete, then thirty days of review if you use a pre-approved plan or sixty days for a standard design, then correction cycles until the plans clear. Construction follows permit issuance. The published clocks are the reliable part; the correction cycles are where projects drift, which is why the first submittal matters more here than in most cities.

Can I sell the ADU separately later?

Not in Foster City. Selling an ADU as a separate condominium requires the city to have adopted an ordinance under AB 1033, and Foster City has not. No San Mateo County city had adopted one as of early 2026. The one separate-sale provision in Foster City's code applies to qualifying nonprofit-built affordable housing and is not available to an ordinary homeowner.

Where these figures come from

Every rule and fee on this page is taken from Foster City’s own published code and fee schedule. Ordinances change, so confirm anything you are budgeting against.

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