Free written scope consultation - allowances, timeline, and change-order rules on paper before a single trade is scheduled
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AfterFast, Reliable Home Additions & ADU Across Foster City
Home additions and ADU construction in Foster City run $45,000-$130,000 for a room addition and $90,000-$220,000 for a detached ADU, and the biggest driver of that cost isn’t square footage - it’s what’s under the slab. Foster City was built on engineered fill over Bay mud, which means nearly every project here starts with a geotechnical review that wouldn’t happen a mile inland in San Mateo or Belmont. We know the lagoon-side lots on Edgewater and the 1970s tract homes off Beach Park Boulevard, and we know which ones need deep piles before the first wall goes up. If you’re planning an addition or ADU in Foster City, call (877) 610-1318 - we’ll give you a straight answer, not a sales pitch.
Why Barrett & Sons Construction Is Foster City’s Preferred Home Additions & ADU Company
Tom Barrett started this company in 1998, and his sons run the crews. That’s not a marketing line - it’s who shows up on your job site. Homeowners between 50 and 70 have told us the same story more times than we can count: a national outfit quoted one number on the phone, then handed them a different bill at the door. That doesn’t happen here. Every scope, every allowance, every price change at a Foster City job arrives as a written change order before work happens. If it’s not on paper, it didn’t happen.
Our Home Additions & ADU business in Foster City runs on the same principle Tom’s had for 29 years: pull the permit, sign the inspection, and stand behind the work. The 90-Day Done Right Promise is written on every contract. One call brings Tom back. We’re not strangers dispatching crews from three states away - Tom lives twenty minutes from your door, and if something’s wrong, he wants to know about it before you have to ask twice.
Foster City customers get a few things that are rare in this trade. Tom publishes his name, address, and license number - look it up yourself before you sign anything. A live person answers the phone 24/7, not a voicemail box. And every crew member is background-checked, uniformed, and carries the Barrett & Sons name. Ask anyone on your street, or check the license number. We’ll wait.
Our Home Additions & ADU Services in Foster City
ADU Design & Permitting in Foster City
Foster City building staff review every ADU application through the same lens: is this slab-on-fill foundation going to move, and what are we doing about liquefaction risk? We handle the full design and permitting path, including the geotechnical report, structural drawings, and the inevitable back-and-forth with city reviewers who know Bay mud better than most engineers. A detached ADU here is not a shed with a bathroom - it’s an engineered structure on the same ground that’s been slowly consolidating since the 1960s.
Garage Conversion ADU in Foster City
Most garages in Foster City’s 1964-1985 tract homes sit on the same post-tension slab as the house, which is actually good news for conversion work - the slab is already there and already engineered. The challenge is usually less about structure and more about the lagoon moisture. Salt-laden air off the tidal channels accelerates corrosion of any metal we’re not careful about, so we spec stainless and coated fasteners on every conversion. A typical garage conversion in Foster City runs $30,000-$80,000 depending on whether we’re adding plumbing, a kitchenette, or just a clean studio space.
New Detached ADU in Foster City
New detached ADUs in Foster City range from $90,000 to $220,000, and the difference between the low and high end often comes down to the foundation system. On lots near the lagoons or on the older fill sections, a deep pile or grade-beam foundation can add $15,000-$30,000 before framing starts. It’s not optional - the permit inspector works for you, not for us, and they’ll flag it. We’d rather tell you that number on day one than surprise you with a change order six weeks into the job.
Junior ADU & Interior Conversion in Foster City
Junior ADUs are the quiet workhorse of Foster City additions - 500 square feet or less, converted from an existing bedroom or attached space, no separate utility hookups required. These usually run $45,000-$80,000 all-in and move faster through permitting because they don’t trigger the same foundation review as a detached unit. That said, the same Bay mud rules apply if you’re expanding the building envelope at all, even by a few feet. We’ll tell you honestly whether your JADU is simple or needs a geotech letter.
What happens when you call
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A real person answersNo phone trees - you reach a local pro.
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You get an upfront price rangeHonest numbers before anyone is dispatched.
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A background-checked tech heads outLicensed & insured, dispatched right away.
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You approve before work beginsNothing starts until you say go.
Trusted Brands We Service in Foster City
We carry brands you can look up, not mystery materials off the back of a van. For ADU kitchenettes and baths, we use Schluter waterproofing systems and Grohe fixtures - both hold up to Foster City’s salt-laden air better than bargain-bin alternatives. For any Foster City Roofing tie-in on a sunroom or enclosed patio, we use GAF underlayment and flashing systems rated for coastal moisture. If a brand doesn’t have a spec sheet we can hand you, we don’t install it. Foster City customers deserve that.
We also handle Kitchen Remodeling in Foster City when an addition grows into a full kitchen refresh, and Bathroom Remodeling in Foster City for ADU baths and primary-suite expansions. Same crews, same written change orders, same Done Right Promise.
Common Home Additions & ADU Problems We See in Foster City Homes
- Slab cracking from Bay mud consolidation. The fill under Foster City has been settling for 60 years, and post-tension slabs from the 1970s and early 1980s frequently show hairline cracking that homeowners mistake for normal wear. Before we frame an addition, we map the existing slab condition - if it’s moving, the new work has to accommodate that movement or the cracks migrate into the new space.
- Second-story additions that the original foundation can’t carry. This is the Foster City-specific surprise: a 1970s slab-on-fill foundation that’s fine for a single story often can’t hold a second story without engineered reinforcement. Contractors pulling permits for second-story additions routinely learn this from the city’s structural reviewer. We plan for it from the first walkthrough.
- Salt corrosion on exterior fasteners and flashing. The tidal lagoons that make Foster City beautiful also push salt-laden air across every structure in 94404. Metal fasteners, flashing, and even reinforcing steel corrode faster here than a few miles inland. On additions and ADUs, we spec hot-dipped galvanized or stainless hardware as standard, not as an upgrade.
- Permit delays from geotechnical scrutiny. Foster City’s building department requests geotechnical reports on projects that would sail through San Mateo or Belmont with a standard soils letter. If your contractor tells you permits are ‘just a formality’ here, they haven’t worked in this city. We pull permits on every eligible job and we don’t fight the review - the inspector works for you, not for us.
Pricing for Home Additions & ADU in Foster City, CA
Here’s the honest picture for Foster City’s market. A single room addition runs $45,000-$130,000 depending on whether it’s a ground-floor bump-out or a full second-story build. A garage conversion ADU lands at $30,000-$80,000. A new detached ADU starts around $90,000 and tops out near $220,000 for a two-bedroom unit with a full kitchen and bath. The Foster City wildcard is the foundation: deep piles or grade beams on Bay mud can add five figures to any of these numbers. We’ll put that in writing before you commit - every allowance, every scope line, every change order, on paper. No surprise invoices, because an allowance that isn’t on paper is a surprise waiting for a date. Call (877) 610-1318 for a free estimate.
We Also Serve Cities Near Foster City
Beyond Foster City itself, our crews cover Redwood Shores, San Mateo, Belmont, and San Carlos. The foundation rules change as soon as you leave the fill zone - San Mateo and Belmont sit on solid ground, so if you’re comparing quotes between cities, make sure you’re comparing apples to apples. For more about our work in this area, see Foster City service details or call us directly at (877) 610-1318.
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 - Home Additions & ADU in Foster City
A home addition in Foster City typically runs $45,000-$130,000, with the final number driven by square footage, whether it’s a ground-floor or second-story addition, and what the geotechnical review finds under your slab. A garage conversion ADU runs $30,000-$80,000, and a new detached ADU runs $90,000-$220,000. Call (877) 610-1318 for an exact quote - estimates are free.
Yes. We work throughout 94404, including the lagoon-side streets off Edgewater Boulevard, the tracts near Beach Park Boulevard, and the older sections closest to the San Mateo border. If you’re in Foster City, you’re in our service area. No trip charge inside the county, ever.
Plan on 6-10 months for a detached ADU in Foster City, with the permitting and geotechnical phase taking 2-3 months of that. Junior ADUs and interior conversions move faster, often 3-5 months. We’ll give you a written schedule before work starts, and we’ll update it in writing if anything changes.
Not without a written change order that you sign first. That’s our rule on every Foster City job. If the geotechnical report comes back requiring deep piles or grade beams, you’ll see exactly what that costs, in writing, before we order a single piece of steel. If it’s not on paper, it doesn’t happen. We also publish Tom Barrett’s name, address, and license number, so you can verify who you’re dealing with before you sign.
Yes, every eligible job. Foster City’s building department is strict about geotechnical review, particularly on the engineered fill that most of the city sits on. We don’t treat permits as optional, and we don’t ask you to pull them yourself. The inspector works for you - that’s why we like him. When the final inspection signs off, you’ll have a permit record that protects the value of your addition if you ever sell. Call (877) 610-1318 to schedule your free estimate.
Written by Tom Barrett, Owner at Barrett & Sons Construction, serving Foster City since our company’s founding in 1998.