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 between $45,000 and $130,000 for an addition and $90,000 to $220,000 for a detached ADU, with garage conversions landing around $30,000 to $80,000. Those are the real numbers we quote to homeowners in the 95046 ZIP, and they include permits pulled through Santa Clara County, not a city office. If you’re on acreage off Masten Avenue or near San Martin Avenue and you’re wondering whether your property can even support an ADU under county rules, call us at (877) 610-1318. We’ll walk the site, check the soils, the well, the septic, and the airspace, and give you a written scope before you spend a dollar on design.
We’ve been building in unincorporated Santa Clara County since 1998, and the one thing we’ve learned is that San Martin is not Gilroy and it’s not Morgan Hill. County permits, private wells, expansive clay, and an airport right in the middle of the community all change how an addition gets built here. If your contractor treats San Martin like any other town, the schedule will bite you. We don’t. Home Additions & ADU work here demands a different playbook, and that’s what this page is about.
Why Barrett & Sons Construction Is San Martin’s Preferred Home Additions & ADU Company
Tom Barrett founded Barrett & Sons Construction in 1998, and his sons run the crews that show up on San Martin job sites. That’s not a talking point, it’s the business model. When you call, a live person answers 24/7, and the person who eventually shows up at your door is someone whose name is on the truck. We don’t dispatch strangers from a call center. We live twenty minutes away. If something’s wrong with your addition three months after we finish, you call Tom and he comes back. That’s the 90-Day Done Right Promise, and it’s written on every contract, not tucked into fine print.
San Martin homeowners have a right to be skeptical. Most of the people who call us have already been burned once by an outfit that quoted one number on the phone and handed them a different invoice at the end. That’s why every allowance, every change order, every scope adjustment arrives in writing before a single nail moves. The permit inspector works for you, not for me, and that’s exactly why we like him on site. Ask anyone on your street, or look up Tom’s license number yourself before you sign anything.
We know the county’s planning desk, the Environmental Health department, and the FAA process near San Martin Airport because we’ve navigated all three on jobs across the 95046 ZIP. That local fluency is why our permits get through faster and why our schedules hold. San Martin isn’t a satellite market for us. It’s home turf.
Our Home Additions & ADU Services in San Martin
New Detached ADU
A detached ADU on San Martin acreage is usually the smartest way to add rentable square footage or a multigenerational suite without tearing into the main house. Most of the parcels we work on are one to five acres or more, so there’s room to build, but county setbacks, well capacity, and septic field sizing all constrain where the structure can actually go. We handle the site planning, the county permit package, the foundation engineered for Pacheco-series clay, and the full build. A detached ADU in San Martin generally runs $90,000 to $220,000 depending on finish level, with custom millwork and architectural glass pushing toward the upper end.
Second Story Additions
Going up instead of out makes sense when your lot is already spoken for by barns, arenas, or irrigation lines. But a second-story addition near San Martin Airport can trigger FAA Part 77 airspace review, and that’s a step most San Martin homeowners have never heard of until the county holds the permit. We run that check early, before you’ve paid for drawings. The foundation question is equally local: the expansive adobe clay under most of the valley floor demands pier-and-grade-beam or post-tension slab work, not a plain perimeter footing that will crack within a decade. A second-story addition in San Martin typically falls within the $45,000 to $130,000 range for the addition itself, with structural upgrades itemized separately.
ADU Site Prep & Utilities
This is where San Martin ADUs get complicated and where out-of-area builders stall. There is no municipal water or sewer grid across much of the 95046 ZIP. Your new ADU needs its own water service from the existing private well, and its waste must tie into a septic system that Santa Clara County Environmental Health has approved for the added load. We coordinate both tracks in parallel with the building permit, because if you wait until after design to test well yield and septic capacity, you may discover the ADU has to shrink or the septic system has to be replaced entirely. That’s the change order that resets the whole schedule. We price site prep and utilities as their own line item, in writing, before we start.
Garage Conversion ADU
San Martin has no shortage of detached garages, workshops, and outbuildings from the 1960s through the 1990s that would make fine ADUs if they weren’t sitting on cracked slabs and wired for a different era. A garage conversion ADU in San Martin runs $30,000 to $80,000, but the honest range depends on what we find when we open the walls. Many of these structures predate modern energy and seismic codes, so the conversion often includes a new foundation, seismic reinforcement, a subpanel upgrade, and insulation work before finish carpentry ever starts. On a one-acre ranch property off a rural road in the 95046 ZIP, our crew converted a 1970s detached workshop into a garage conversion ADU. The existing slab-on-grade had cracked from expansive Pacheco-series adobe clay, so we installed a pier-and-grade-beam foundation and coordinated the private well and septic approval with Santa Clara County Environmental Health before any interior rough-in. The project included a full KraftMaid kitchen, a Kohler bath, and a subpanel upgrade, all priced under a fixed scope of work agreement before the long-lead cabinetry was ordered.
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 carry premium brands across every trade because San Martin homeowners want materials they can look up, not mystery products off a van. On windows and doors for additions and ADUs, we install Simonton, Ply Gem, and PGT lines, including impact-rated options where county or wildfire overlay requirements come into play. We stock local parts and trim profiles for San Martin jobs, which means a missing jamb extension or a short piece of flashing doesn’t idle a crew for a week. The brand names go on the contract in writing, just like everything else.
Common Home Additions & ADU Problems We See in San Martin Homes
- The FAA Part 77 surprise. A second-story addition or tall detached ADU within the airspace obstruction surfaces near San Martin Airport (E16) can stall at the county permit desk pending an airspace determination. We run this check before you spend design fees, because discovering it after drawings are done is a costly and avoidable delay.
- Well and septic capacity discovered too late. On acreage in the 95046 ZIP, there’s no municipal hookup. If you finalize ADU plans before testing the existing well yield and septic field capacity, you may learn the septic system has to be replaced or the ADU downsized. That’s a change order that resets everything.
- Expansive clay cracking a standard slab. The valley floor beneath San Martin sits on Pacheco-series adobe clay that swells and shrinks with the wet-dry Mediterranean cycle. Plain perimeter-slab footings fail here. We engineer for pier-and-grade-beam or post-tension slabs from the start, and we see the cracked slabs other builders left behind.
- Long-lead finishes ordered before the scope is fixed. Custom millwork, architectural glass, and specialty cabinetry have production lead times measured in weeks and months, not days. If the scope isn’t locked in writing before those orders go out, allowances spiral and the calendar stretches. We fix the price in writing before ordering, period.
Pricing for Home Additions & ADU in San Martin, CA
Here are the numbers we actually quote for San Martin work. They account for county permits, engineered foundations for expansive clay, and the documentation required by Environmental Health.
| Project Type | Typical Price Range in San Martin |
|---|---|
| Home Addition | $45,000 - $130,000 |
| ADU Construction | $90,000 - $220,000 |
| Garage Conversion | $30,000 - $80,000 |
What moves the number within those ranges comes down to finish level, foundation conditions, and the state of your well and septic system. On high-end additions and ADUs, lead times on custom millwork and architectural glass, not labor, set the calendar. That’s why we price everything as a fixed scope of work agreement before the cabinetry order goes out. Change orders come in writing, before work proceeds, so the final invoice should never surprise you. Call (877) 610-1318 for a free estimate on your San Martin project. We’ll walk the site and put the numbers on paper.
We Also Serve Cities Near San Martin
Our crews work throughout the southern Santa Clara County and into the Pajaro Valley. In addition to San Martin, we regularly serve Home Additions & ADU in San Martin and the surrounding communities of Morgan Hill, Gilroy, Interlaken, and Watsonville. If you’re nearby and planning an addition or ADU, call us, same phone, same fixed-scope pricing.
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
You need a Santa Clara County permit, full stop. San Martin is unincorporated, so there is no city building office, and the county does not treat an ADU as an exempt agricultural accessory structure. An ADU is a dwelling unit, which means building permits, setbacks, septic and well approval, and inspections all apply. Call (877) 610-1318 and we’ll map out exactly which permits your parcel will need before you commit to anything.
It can stop your permit until an airspace determination is issued by the FAA. San Martin Airport, also known as South County Airport and coded E16, sits within the community, and any tall structure within its obstruction surfaces needs that determination before the county will issue a building permit. A second-story addition on the wrong parcel can trigger this requirement, and it’s a step most out-of-area builders miss until the county flags it. We check this early. Call (877) 610-1318 to find out if your parcel is affected.
Maybe, but you won’t know until the well yield and septic capacity are actually tested, and across much of the 95046 ZIP there is no municipal water or sewer to fall back on. Santa Clara County Environmental Health must approve the added load, and some older septic systems on San Martin acreage can’t handle another bedroom. If the system has to be replaced, that’s a significant cost and a schedule reset. We run these tests early so you can price the project honestly. Call (877) 610-1318 to schedule a site assessment.
Because the expansive Pacheco-series adobe clay under San Martin swells and shrinks with every wet winter and dry summer, and a plain perimeter-slab footing will crack under that movement. Pier-and-grade-beam or post-tension slab designs are standard practice in this part of the valley for a reason. We’ve seen what ordinary slabs look like after a decade on this soil. Call (877) 610-1318 and we’ll explain what your foundation will need and what it will cost.
Yes, and that’s the only way we’ll build it. Custom millwork, architectural glass, and specialty cabinetry have long production lead times, and if the scope isn’t fixed in writing before those orders go out, your allowances will drift and your schedule will stretch. We price the job as a fixed scope of work agreement before the long-lead materials are ordered, so the cabinet number you see on paper is the cabinet number you pay. Call (877) 610-1318 for a free written estimate.
Ready to Build in San Martin? Call Barrett & Sons.
If you’re planning a home addition, a detached ADU, or a garage conversion on your San Martin property, call us at (877) 610-1318 for a free estimate. Tom Barrett and his sons will walk the site, check the soils, the well, the septic, and the airspace, and give you a written scope of work before you spend a dollar on design. Over 29 years, founded in 1998, owner-operated from day one. Permits pulled, inspections signed, every price change in writing before a nail moves, and the 90-Day Done Right Promise on every contract. No trip charge inside the county. Senior and Veteran discount of 10% on every job.
Written by Tom Barrett, Owner at Barrett & Sons Construction, serving San Martin since 1998.