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 Belmont
A room addition or ADU in Belmont typically runs $45,000-$130,000 for a ground-up build, while garage conversions land between $30,000 and $80,000, and a new detached ADU runs $90,000-$220,000. The number moves based on your lot, your soils report, and what we find when we open the walls. If you’re planning an addition or ADU in Belmont, call (877) 610-1318 and we’ll walk the property with you before quoting anything.
We’re Barrett & Sons Construction, and we’ve spent nearly three decades working on the same kinds of homes Belmont is full of: 1950s and 1960s ranches and split-levels cut into clay-heavy hillsides, where a simple room addition can turn into a geotechnical conversation the moment the building department gets involved. We know Belmont’s permit path, its soils, and its inspectors. That local knowledge is the difference between a project that starts on time and one that stalls for six weeks waiting on a report nobody budgeted for.
Call (877) 610-1318 any time. A live person answers, not a voicemail box.
Why Barrett & Sons Construction Is Belmont’s Preferred Home Additions & ADU Company
Tom Barrett founded this company in 1998, and his sons run the crews. When we say we’ll be on site, we mean one of us, by name, with a truck that says Barrett & Sons on the door. We’re not a franchise sending strangers from three counties over. We coordinate from our San Francisco Bay Area office, and our crews work Belmont jobs locally, week in and week out.
Belmont homeowners have told us the same story for years: a contractor from a flatland Peninsula city quotes the job without factoring in a soils report, gets surprised by the city’s geotechnical review, and the project sits for weeks while everyone argues about who pays for what. We budget for Belmont’s requirements from the first line item, not as an afterthought. Every scope, every allowance, every price change arrives as a written change order before work happens. The allowance that isn’t on paper is a surprise waiting for a date.
We pull permits on every eligible job, and we like the inspector. The permit inspector works for you, not for me, and that’s exactly how we want it. Ask anyone on your street, or look up Tom’s license number yourself. We publish it because we want you to check before you sign anything.
Our Home Additions & ADU Services in Belmont
Here’s what we handle most often in Belmont, from the flats near the Caltrain corridor up into the hills above Ralston Avenue.
ADU Design & Permitting
Most Belmont ADUs start with a soils report, not a floor plan. The city requires geotechnical review on nearly every hillside lot, and Belmont has mapped active landslide deposit zones that can add weeks to permit timelines. We handle the full permit package: drawings, soils coordination, structural review, and city submittals. If you’re building an ADU in the 94002 zip code, we’ll walk you through exactly what the building department will ask for before you pay for anything.
Garage Conversion ADU
Converting a garage into a living unit in Belmont runs $30,000-$80,000. Older garages in the postwar neighborhoods above El Camino Real often have slabs poured directly on expansive clay, which means drainage and vapor barriers become real decisions, not line items to skip. We address site prep and waterproofing from the start so your conversion doesn’t develop moisture problems two rainy seasons in.
New Detached ADU
A new detached ADU in Belmont runs $90,000-$220,000. On cut-and-fill lots, which are common throughout Belmont’s hills, the fill portion has been settling for 50 to 70 years. That means foundation work and retaining walls often carry more of the budget than the framing itself. We plan for that before breaking ground.
Junior ADU & Interior Conversion
A junior ADU, typically 500 square feet or less inside an existing home, is the most straightforward path for many Belmont owners. You’re converting existing interior space, often a bedroom wing or a lower level of a split-level, which avoids some of the site work a detached build requires. But older Belmont homes consistently turn up outdated wiring and undersized framing once we open the walls, so we write allowances into the contract for exactly that scenario.
We also handle room additions, second-story additions, and sunrooms and enclosed patios across Belmont. See our full Home Additions & ADU service breakdown for what each scope typically includes.
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 Belmont
When your Belmont addition calls for new windows, we install brands you can look up, not mystery product off a van. We regularly spec Simonton for window packages on room additions and ADUs, and Ply Gem for durable vinyl work on enclosed patios and sunrooms. For impact-rated or coastal-grade windows on exposed hillside elevations, PGT is our go-to. We carry what Belmont homes actually need, and we’re not swapping in whatever the supply house has on sale that week. You’ll see the brand name on your contract, in writing, before we order a single unit.
Common Home Additions & ADU Problems We See in Belmont Homes
- Soils report requirements on hillside lots. Belmont’s building department wants a geotechnical review for most additions and ADUs on sloped parcels, and the city has mapped active landslide deposit zones. Contractors who don’t work Belmont regularly get stuck here. We don’t.
- Original wiring still live in the walls. When we opened a 1960s split-level above El Camino Real for a second-story addition, our crew found original knob-and-tube wiring still hot in the wall cavities and a load-bearing beam that had been notched by a previous owner for ductwork. We wrote an allowance into the contract for exactly this scenario, got the city inspector’s sign-off on the new beam and rough-in before closing anything, and issued a change order with photos and line-item pricing for the rewire before touching a single wire.
- Saturated clay and failed drainage on cut-and-fill lots. Belmont sits at the base of the Santa Cruz Mountain foothills and gets noticeably more winter rain than Foster City or Redwood Shores. Expansive Franciscan clay on sloped lots absorbs that water, which accelerates soil creep and stresses retaining walls and drainage systems every rainy season. Ignore drainage during a room addition and your new subfloor sits on saturated clay, cupping and molding within two winters.
- Split-level framing that’s been compromised over decades. The 1950s and 1960s splits that define Belmont’s hillside neighborhoods have been through multiple remodel cycles. We routinely find cut studs, notched beams, and load paths that stopped making sense two renovations ago. When you open the wall, you own what’s behind it. We make sure you know what that costs before we touch it.
Pricing for Home Additions & ADU in Belmont, CA
Belmont’s market isn’t San Mateo’s, and it isn’t Foster City’s. Hillside lots, soils reports, and aging foundations push costs toward the upper end of the ranges you’ll see from flatland contractors. Here’s what we quote, in writing, before any work begins:
| Project Type | Belmont Price Range |
|---|---|
| Garage Conversion | $30,000-$80,000 |
| ADU Construction | $90,000-$220,000 |
| Room Additions | $45,000-$130,000 |
What moves the number in Belmont: whether your lot triggers a soils report, whether you’re in a mapped landslide deposit zone, what we find when we open the walls of a 1950s ranch, and how much drainage work your cut-and-fill lot needs. We price every allowance in writing before you sign, and the price doesn’t change unless you approve a written change order. Estimates are free. Call (877) 610-1318 and we’ll come walk the property.
We Also Serve Cities Near Belmont
In addition to our work across Belmont, we serve Home Additions & ADU in Belmont‘s neighboring communities, including San Carlos, San Mateo, Redwood Shores, and Foster City. Each city has its own building department, its own soils, and its own permit quirks. We know them all, and we quote accordingly.
Serving Belmont, CA - Our Local Coverage Area
We’re based in the Belmont 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 Belmont
Belmont’s building department requires a soils report for most additions and ADUs on hillside lots, and the city has mapped active landslide deposit zones that can add weeks of geologic review to your permit timeline. Call (877) 610-1318 and we’ll tell you whether your specific lot triggers the requirement, in plain English, before you spend a dime on drawings.
We write an allowance for it into the contract before we open the first wall. That allowance covers the likely scope: cloth-wrapped wiring, knob-and-tube remnants, undersized panels. If what we find exceeds the allowance, you get a written change order with photos and line-item pricing before we touch a single wire. You approve it or you don’t. The price never moves without your signature.
Because Belmont gets more winter rainfall than bayfront cities, and the expansive Franciscan clay on sloped lots saturates and moves. Your neighbor in San Mateo is likely building on flatter ground with different soils. Your lot in Belmont’s hills needs drainage designed into the project, not bolted on after the first winter. Any contractor who skips that line item is pricing the job wrong.
You see one any time the scope changes, and only after you’ve approved it in writing. The document includes photos of what we found, line-item pricing for materials and labor, and an updated total. It arrives before work proceeds, never after. That’s the rule, and it’s written on every Barrett & Sons contract.
If your property falls within a mapped landslide deposit zone, expect additional geologic review during permitting, which can add weeks to the timeline. We identify that early, coordinate the geotechnical work, and build the review time into the schedule so you’re not surprised. Call (877) 610-1318 for a free estimate and we’ll check your specific parcel.
Written by Tom Barrett, Owner at Barrett & Sons Construction, serving Belmont and the surrounding Peninsula communities since 1998.