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 Martinez
Adding square footage in Martinez usually costs between $45,000 and $130,000 for a room addition, and $90,000 to $220,000 for a new detached ADU. But before you accept any quote in Martinez, you need to know one thing: the salt air off the Carquinez Strait and the adobe clay under your foundation decide the materials and the price, not the other way around. Our Home Additions & ADU team works these hillside blocks every week, and we scope every job for the soil and wind conditions actually present on your lot. If you’re weighing an addition or ADU in Martinez, call (877) 610-1318 for a free estimate and we’ll walk the property with you.
Why Barrett & Sons Construction Is Martinez’s Preferred Home Additions & ADU Company
We’re not a franchise. Tom Barrett founded this company in 1998, and his sons run the crews that show up in Martinez. That’s not marketing copy; it’s who answers the phone when you call (877) 610-1318. When a Martinez homeowner asks who’s going to be framing their second story, they get a name and a face, not a dispatch number.
Local reputation here runs on one thing: showing up and doing what you wrote down. We put every scope, every allowance, and every price change into a written change order before a single board gets cut. That’s why Martinez homeowners who’ve been burned by a national outfit that quoted one price on the phone and handed them another at the door tend to stay with us for the next project. You can verify Tom Barrett’s name, address, and license number before you sign anything. Look it up. And, if we’re in the neighborhood and a customer we’ve worked with before sees the truck, they’ll wave. That’s the kind of trust you can’t buy with a billboard.
Our Home Additions & ADU Services in Martinez
ADU Design & Permitting
Most Martinez ADU projects stall before they start because the permitting path isn’t clear. In the older hillside neighborhoods especially, the City of Martinez and Contra Costa County will want to see foundation retrofit scope before they approve framing drawings. We handle that sequence: structural assessment first, then permit drawings, then construction. Every eligible job gets permits pulled and inspections signed. You don’t pay us to argue with the building official; you pay us to walk the paperwork through correctly the first time.
New Detached ADU
A new detached ADU in Martinez typically lands between $90,000 and $220,000, depending on soil conditions, utility runs, and how far the wind exposure pushes your fastener and flashing spec. Near the Carquinez Strait, we specify hot-dip galvanized or stainless fasteners and marine-grade flashing as standard. The inland-spec version of that same ADU costs less on paper, but you’ll be re-flashing and chasing rust bleed within five years. We’d rather bid the honest number once.
Garage Conversion ADU
Converting an existing garage in Martinez’s post-WWII tracts often runs $30,000 to $80,000. The variable we look hardest at is the slab: many of those old garage floors were poured over un-compacted fill or sit on adobe clay that’s moved for fifty years. If the slab is out of level, the conversion scope starts with correction, not framing. Our garage conversions include lowering or replacing the slab where needed, running utilities legally, and tying the roof load into a system that handles the daily Carquinez wind without lifting.
Junior ADU & Interior Conversion
A junior ADU inside an existing Martinez home is the quietest path to rental income or family space. For the historic downtown core and hillside blocks, though, we won’t frame new interior walls over a floor system that’s racked. We relevel and sister first. The good news: because a JADU uses existing envelope, permitting is faster and cost stays well below a new detached build. Expect to make decisions on electrical panel capacity, egress windows, and sound separation early.
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 Martinez
We don’t stock mystery brands off a van. In Martinez, we work with named manufacturers so you can look up what you’re paying for. On ADU bathrooms we use Schluter waterproofing systems and Sterling or Grohe fixtures; on any roof or exterior surface exposed to the strait’s wind and salt, we lean on GAF roofing product lines with the upgraded fastener and flashing specs that make sense for this climate. You’ll see the brand names written in your estimate, and you’ll see the materials when they arrive. No surprises, because it’s already on paper.
Common Home Additions & ADU Problems We See in Martinez Homes
- Additions framed on unbolted, out-of-level mudsills over adobe clay. The new room’s windows bind and drywall cracks within two wet-dry seasons. We bolt and relevel the existing foundation before a single new stud goes up.
- Standard galvanized fasteners near the Carquinez Strait. Salt air and marine wind mean rust bleed, loosened metal roofing, and snapped railing hardware within five years. We specify hot-dip or stainless as standard in Martinez.
- Garage conversions built over moving slabs. The adobe clay under your lot expands and contracts seasonally, cracking old concrete and taking the new framing with it. We check the slab before we ever quote finish work.
- Second-story additions tied into roofs not built for current wind uplift. The gap in the Coast Ranges funnels daily wind through Martinez harder than Concord or Pleasant Hill feels. Roof connections have to be engineered for it.
Pricing for Home Additions & ADU in Martinez, CA
Here’s what you can expect to pay in Martinez’s current market:
| Project | Typical Range in Martinez |
|---|---|
| Home Addition | $45,000-$130,000 |
| ADU Construction | $90,000-$220,000 |
| Garage Conversion | $30,000-$80,000 |
What moves the number in Martinez specifically is foundation scope: whether the existing structure is bolted, whether the slab has moved, and whether the perimeter concrete is square. A second variable is wind exposure: a hillside lot near the strait requires heavier fasteners and flashings than a sheltered lot east of downtown. You’ll see all of it itemized. Call (877) 610-1318 and we’ll give you a written line-item quote with every allowance named in full.
We Also Serve Cities Near Martinez
Our crews work all over Contra Costa County’s older core. In addition to Martinez, you’ll find us on jobs in Pleasant Hill, Contra Costa Centre, Waldon, and Benicia. Same written change orders, same 90-Day Done Right Promise, same named brands on every estimate.
Serving Martinez, CA - Our Local Coverage Area
We’re based in the Martinez 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 Martinez
Because the foundation retrofit scope is real and unavoidable. In Martinez’s pre-1920 hillside and downtown core, most homes sit on original pier-and-beam or unreinforced masonry foundations laid over expansive adobe clay. Before we can frame an addition or ADU, we typically relevel floors, sister sagging joists, and hot-dip galvanize new anchor bolts through the mudsill. In Concord or Pleasant Hill, the newer slab-on-grade stock often skips that whole phase. That structural phase is the price gap. Call (877) 610-1318 and we’ll do a site walk so you know your number before you commit.
Hot-dip galvanized or stainless steel, always, within the wind exposure zone near the strait. Standard electroplated fasteners corrode fast in salt air and loosened panels follow within a few years. We specify hot-dip as our base in Martinez, and we write the brand and finish into the contract so there’s no value-engineering later. If the job is within a mile of the waterfront, we may push to stainless on the most exposed edges. Estimates are free; call (877) 610-1318 and we’ll tell you exactly what applies to your lot.
It’s usually the first thing we address. The clay expands when wet and shrinks when dry, and over decades that movement racks old garage slabs. Before we run any new subfloor or load-bearing walls, we check level and crack patterns. If the slab has moved, correction is written into the original scope of work, not sprung as a change order. That’s the honest way and it’s the only way we’d build a living space over it. Call (877) 610-1318 to schedule a free estimate.
In the historic district, wind and salt air hit second-story windows far harder than anything at street level. We recommend fiberglass or marine-grade aluminum frames; they hold dimension better than vinyl in sustained marine wind and they don’t corrode like raw aluminum. If the house is a contributing structure, we’ll also talk about matching original sightlines and trim profiles so the addition doesn’t fight the home’s character. Want a written recommendation for your exact exposure? Call (877) 610-1318.
Because the permit inspector works for you, not for the contractor. If the retrofit is covered over before it’s signed off, no one can verify the bolting, leveling, and joist work were done to code. We want that inspection to happen, and we sequence the draw schedule so it does. That’s how our ADU and addition projects in Martinez stay clean all the way to final. Have questions about your specific lot? Call (877) 610-1318 and we’ll walk you through it.
Call Barrett & Sons Construction for Home Additions & ADU in Martinez
You’re not shopping for the cheapest bid. You’re looking for a contractor who will put their name on the work, pull the permits, and show up when you call. That’s Barrett & Sons. We live here, we work here, and we stand behind every job with a written 90-Day Done Right Promise: if it’s not right, one call brings Tom back. Call (877) 610-1318 today for your free estimate and we’ll start with a foundation you can trust.
Written by Tom Barrett, Owner at Barrett & Sons Construction, serving Martinez and Contra Costa County since 1998.