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 San Martin
Home additions and ADU construction in San Martin typically run $45,000-$130,000 for a room addition and $90,000-$220,000 for a new detached ADU, depending on square footage, foundation type, and finish level. The biggest variable in San Martin isn’t lumber or labor, it’s the county permitting path: every permit flows through Santa Clara County, not a city building office, which changes timelines, septic requirements, and even airspace rules near San Martin Airport. We’ve been navigating that process since 1998, so when you call (877) 610-1318, you’re talking to someone who knows exactly what the county will ask for before the first hole is dug.
Why Barrett & Sons Construction Is San Martin’s Preferred Home Additions & ADU Company
Tom Barrett founded this company in 1998 and his sons run the crews today. That matters in San Martin because you’ll see the same faces from the first estimate to the final inspection, not a rotating cast of subcontractors. When we quote a job on a 2.5-acre ranch off New Avenue, Tom is usually the one walking the property line, checking the well head, and asking about the septic leach field before he ever talks price. That’s not a sales tactic, it’s just how you have to work in unincorporated Santa Clara County.
We carry the premium brands our San Martin customers expect: KraftMaid and Cambria in kitchens, Andersen and Marvin on windows, Belgard and Techo-Bloc on hardscapes. Materials you can look up, not mystery brands off a van. And every completed job is backed by the written 90-Day Done Right Promise. If it’s not done right, one call brings Tom back. No fine print, no runaround.
Our pricing is fixed in writing before we order anything. Every allowance, every change order, every scope adjustment arrives on paper with a signature line before a single nail moves. The allowance that isn’t on paper is a surprise waiting for a date. That’s how we’ve kept our reputation intact in San Martin for over two decades.
Our Home Additions & ADU Services in San Martin
New Detached ADU
A detached ADU on San Martin acreage usually means pier-and-grade-beam or post-tension slab work because of the expansive Pacheco adobe clay under most of the 95046 ZIP. We design around that from day one, not after the first crack appears. Expect $90,000-$220,000 for a complete build, including county plan checks, septic coordination, and the FAA airspace determination if your parcel falls near San Martin Airport’s Part 77 surface.
ADU Design & Permitting
We handle the Santa Clara County paperwork end to end: building permits, Environmental Health septic sign-off, well compliance, and any FAA obstruction review. On a recent job off New Avenue, the county’s Environmental Health septic sign-off and a FAA airspace determination triggered by a planned second-story addition set the schedule, not our framing crew. We managed the draw schedule so long-lead millwork arrived exactly when the foundation passed inspection. That sequencing is the whole game in San Martin.
Garage Conversion ADU
Converting an attached or detached garage into a junior ADU runs $30,000-$80,000 in San Martin, and the county treats it as a change of use, not a fresh build. That carries different fire separation and energy code requirements than a from-scratch ADU. We’ve converted garages on ranches, hobby farms, and vineyard estates across the 95046 ZIP, and we know which county reviewers will flag what. Permits pulled and inspections signed on every eligible job, because the last contractor most customers used told them permits were optional.
Junior ADU & Interior Conversion
A junior ADU inside an existing home, or a conversion of a bonus room, den, or attached studio, is the most cost-efficient path for San Martin homeowners who don’t want another foundation pour. These projects often straddle the line between a room addition and an ADU under county code, and getting the classification right on the front end saves months. We’ll help you decide what makes sense for your parcel and how the county will view it.
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 San Martin
We install and stand behind brands you can look up: Simonton, Ply Gem, and PGT windows and doors, plus the KraftMaid and Cambria lines we put in our San Martin ADUs. Because we stock local parts for these product lines and our crews serve San Martin directly, we can handle warranty claims and follow-up work without the multi-week back-and-forth you get with national installers. If a Simonton sash fails on a San Martin sunroom, we’re the ones who come back and fix it, not an out-of-state call center.
Common Home Additions & ADU Problems We See in San Martin Homes
- Expansive clay foundation failure: The Pacheco-series adobe clay under San Martin’s valley floor swells and shrinks through the wet-dry cycle. A conventional perimeter-slab footing without piers or grade beams cracks and heaves within the first winter. We specify pier-and-grade-beam or post-tension slabs as standard practice, not an upsell.
- County permit timeline surprises: Santa Clara County plan checks and rural septic/well reviews routinely add 6-10 weeks before framing can start. Homeowners who assume a city-style timeline end up with framing crews idle and materials stacked in the weather. We build that lead time into the schedule from the first conversation.
- FAA airspace determinations near San Martin Airport: Santa Clara County enforces FAA Part 77 airspace obstruction surfaces over parcels near E16. A tall barn, grain silo, or two-story addition in the airport’s vicinity may need an FAA airspace determination before the county will issue a permit. Out-of-area builders get blindsided by this. We check it before you buy materials.
- Septic and well compliance stalls: Most of San Martin isn’t on municipal water or sewer. ADUs and additions that increase bedroom count or square footage can trigger Santa Clara County Environmental Health review of your private well and septic system. That’s a separate permitting track, and if it’s not anticipated early, it stalls the whole build.
Pricing for Home Additions & ADU in San Martin, CA
You’ll see honest ranges here because we’d rather you call with a realistic number in mind than waste a trip. The table below covers the line items that drive most San Martin addition and ADU budgets. Every price is fixed in a written contract before we order materials, and any change arrives as a written change order before work proceeds. No surprise invoices at the end.
| Service | San Martin Price Range |
|---|---|
| Home Addition | $45,000 - $130,000 |
| ADU Construction | $90,000 - $220,000 |
| Cabinet Installation | $4,000 - $13,000 |
| Countertop Installation | $2,000 - $6,500 |
What moves the number most in San Martin is the permitting path: a single-story room addition on flat, stable ground prices differently than a two-story addition near the airport corridor or an ADU on a parcel with a marginal septic system. We’ll give you a line-item quote, not a ballpark. Call (877) 610-1318 to schedule your free estimate.
We Also Serve Cities Near San Martin
Our Home Additions & ADU team works throughout the South County, including San Martin and the surrounding communities of Morgan Hill, Gilroy, Interlaken, and Watsonville. If you’re outside the 95046 ZIP, give us a call, chances are we already work in your area.
Serving San Martin, CA - Our Local Coverage Area
We’re based in the San Martin 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 San Martin
Yes, in most cases. An ADU that increases bedroom count or connected square footage usually triggers Santa Clara County Environmental Health review of your existing private well and septic system, on top of the building permit. The septic review is a separate permitting track with its own timeline, and we coordinate both from the start so your ADU doesn’t stall mid-build. Call (877) 610-1318 for an exact quote on your parcel, estimates are free.
You may. Santa Clara County enforces FAA Part 77 airspace obstruction surfaces over parcels in the airport’s vicinity, and a planned second-story addition or tall structure can trigger an airspace determination before a permit issues. We check your parcel against those surfaces before you commit to anything. Call (877) 610-1318 and we’ll walk your property with that question in mind.
Because the expansive Pacheco adobe clay under San Martin’s valley floor swells and shrinks with the wet-dry cycle, and a conventional perimeter-slab footing cracks and heaves within the first winter. Pier-and-grade-beam and post-tension slabs handle that movement without failing. It’s standard practice for us on any San Martin addition, not an upgrade.
We lock the allowance in writing in the contract before ordering, then back the schedule to those long-lead items. On a recent San Martin ADU off New Avenue, the Kohler rough-in fixtures and Cambria countertop allowances were fixed in writing before ordering, and the millwork arrived exactly when the post-tension slab passed inspection. That sequencing keeps your build moving. Call (877) 610-1318 for a written quote, estimates are free.
Sometimes.Santa Clara County treats barn and workshop conversions as a change of use, and the existing structure has to meet current energy, seismic, and fire separation codes before it qualifies as a dwelling unit. Older outbuildings that predate modern codes often need structural upgrades first. We’ll tell you honestly whether your barn is a candidate before you spend money on plans.
Written by Tom Barrett, Owner at Barrett & Sons Construction, serving San Martin since 1998.