A free site visit, a check of what your lot allows and a written, line-item price.
Get your free estimateADU Company in Foster City, CA
Adding an ADU in Foster City means working with a lagoon city that has stricter rules than most of the Peninsula. Benson ADU San Francisco builds ADUs, home additions, and room additions here, and we know what the lagoons, levees, and 1960s housing stock actually require. Most Foster City homeowners call us because they want a written price before anything starts, plus a clear read on what the city will and won’t approve. Call (424) 328-5746 and you’ll reach a live person who can walk you through it.
Why Foster City Homes Are Different
Foster City isn’t like Redwood Shores or San Mateo when it comes to ADU work. The entire city sits on engineered lagoons and levees, which means nearly every ADU project here has to clear an elevation requirement that simply doesn’t exist two miles inland. Homes on the lagoon side, especially around neighborhoods like Dolphin Bay and the areas off Edgewater Boulevard, often need their ADU floor raised above the 100-year flood plain line. That adds real cost and engineering time. Neighbors on the hillside lots near the western edge don’t face the same flood plain requirement, but they do deal with liquefaction zones, so their foundations need different detailing.
The housing stock itself is unusually uniform. Foster City was master-planned in the 1960s and 1970s, and most lots have a wood-frame single-family home on a concrete slab with a predictable footprint. That consistency is actually helpful; once we’ve worked a few Foster City jobs, we know what the framing, setbacks, and utility runs look like before we open the walls. What’s less predictable is the zoning code. Foster City’s code historically favored detached ADUs over attached ones, a policy that state mandates are now pushing the city to loosen. For homeowners, that means your ADU path can look very different from a friend’s project in Burlingame, even if the homes look identical.
Our Main Foster City Services
Most Foster City homeowners start with one of three projects: a detached ADU in the backyard, an addition that extends the main house, or a room addition that repurposes existing square footage. Each has its own Foster City page with local pricing and permit specifics.
ADU Builders in Foster City
A detached ADU in Foster City usually makes the most sense on the standard 5,000 to 6,000 square foot lots, but the lagoon setback and elevation rules shape the design from day one. We build both site-built and prefab ADUs here, and on many Foster City lots a prefab panel system from brands like Abodu or Cover shortens the on-site timeline significantly. If you’re considering a backyard unit, start with our ADU Builders in Foster City page for local pricing and a plain-English breakdown of the setback and flood plain rules.
Home Addition Contractor in Foster City
Home additions in Foster City most often mean going out, not up, because so many of the original 1960s homes were built as single-story ranches on slab. A ground-floor extension usually works with the existing foundation logic, but the bay wind means we detail the new roof and wall connections differently than an inland job would. For a written breakdown of what an addition costs per square foot in Foster City, see our Home Addition Contractor in Foster City page.
Room Addition Contractor in Foster City
A room addition in Foster City is often the fastest way to gain a bedroom or home office without triggering the full ADU review. Many of these jobs involve converting an attached garage or extending into the side setback, both of which are common in the older Foster City tract layouts. We handle the structural work and the permit path, and our Room Addition Contractor in Foster City page covers the costs and timelines specific to this city.
What happens when you call
- 1
A real person answersNo phone trees - you reach a local pro.
- 2
You get an upfront price rangeHonest numbers before anyone is dispatched.
- 3
A background-checked tech heads outLicensed & insured, dispatched right away.
- 4
You approve before work beginsNothing starts until you say go.
Brands We Service in Foster City
We build with and service ADUs and additions from our brands, including Boxabl, Plant Prefab, and Tuff Shed, plus James Hardie cladding on site-built work. Because Foster City jobs are mostly slab-on-grade with standard lot geometry, we can usually quote a prefab option quickly and keep parts in the Bay Area rather than waiting on long-haul freight.
Pricing for ADU Company in Foster City, CA
| Service | Typical Range in Foster City |
|---|---|
| Detached ADU (site-built) | $280,000 – $420,000 |
| Detached ADU (prefab) | $220,000 – $340,000 |
| Attached ADU (conversion) | $150,000 – $240,000 |
| Garage conversion to ADU | $130,000 – $210,000 |
| Home addition (ground-floor) | $180,000 – $320,000 |
| Room addition (single room) | $70,000 – $140,000 |
| Foundation work (liquefaction zone) | $12,000 – $30,000 |
| Flood plain elevation lift | $15,000 – $40,000 |
| Wind-resistant roof and wall detailing | $4,000 – $9,000 |
| Permit and engineering package | $6,000 – $15,000 |
Who gets the high end of these ranges in Foster City? Lagoon-side lots that trigger the flood plain elevation requirement, homes on liquefaction soils that need deeper foundations, and any project with wall or roof upgrades for the bay wind. Every price we quote is written down before work starts, and it doesn’t change at the door.
Neighborhoods We Serve in Foster City
We cover all of Foster City, and because the city is only about four square miles, we can usually reach any address within a short drive from the Bay Area office.
- Dolphin Bay
- Edgewater
- Isle Cove
- Beach Park
- Lantern Bay
Service Area – Cities Near Foster City
Benson ADU San Francisco also serves homeowners in nearby Bay Area cities. If you’re just outside Foster City, you can find our service pages for San Francisco, CA, Berkeley, CA, Vallejo, CA, and Piedmont, CA.
Serving Foster City, CA - Our Local Coverage Area
We’re based in the Foster City area and know this community well. Use the map below to see our service coverage - if you’re nearby, we can almost certainly help.
FAQs About ADU Company in Foster City
We schedule Foster City estimates every week, and because the city is compact, a site visit often happens within a few business days of your call. You’ll get a specific appointment window, and a live person answers the phone when you call (424) 328-5746.
Yes. Lagoon-side lots often require flood plain review and elevation certification as part of the permit package, which most inland lots skip. We handle that documentation as part of the job and put the engineering cost in the written quote up front, so there’s no line item surprise later.
No. The price we put in writing before any work begins is the price we build too, including the Foster City flood plain, wind, and foundation items we already flagged in the plan. If the city requires a change after plan review, we put that in writing before proceeding. Call (424) 328-5746 for your free Foster City estimate.
What happens when you call
- 1A real person answersNo phone trees - talk to a local adu pro.
- 2You get an upfront price rangeHonest numbers before anyone is dispatched. No surprises.
- 3We come out and measureLicensed & insured, at a time that suits you - the visit and the quote are both free.
- 4You get the price in writingLine-itemed, yours to keep, and nothing starts until you say go.
What Foster City Customers Say
"Plumbing and electrical passed on the first inspection."
- Alan B."Communication was the best part. I always knew what was happening that week without having to ask."
- Dolly N."Room addition off the kitchen. Framing, drywall, and paint all done well."
- Wade L.
