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 Los Altos
A home addition or ADU in Los Altos typically runs $45,000 to $130,000 for an addition and $90,000 to $220,000 for a new detached ADU, and the defining variable here isn’t labor or lumber. It’s the land. Los Altos sits on some of the most expensive residential dirt in the country, and the city’s permitting process, its heritage oak protections, and its aging post-WWII housing stock all shape what you can build, where you can build it, and what it costs before a single board is cut.
We’re Barrett & Sons Construction, and we’ve been building Home Additions & ADU for over 29 years. Tom Barrett’s name is on the truck, his sons run the crews, and every permit, allowance, and change order arrives on paper before work starts. If you’re planning an addition or ADU in Los Altos, call (877) 610-1318 for a free estimate and a straight conversation about your lot.
Why Barrett & Sons Construction Is Los Altos’s Preferred Home Additions & ADU Company
Ask anyone on your street, or look up the license number yourself. Tom Barrett publishes his name, his address, and his license number, because the customer who got burned by a national outfit that quoted one price on the phone and showed up with another deserves to verify who they’re hiring. We answer our own phone, 24/7. Not a call center, not a voicemail box.
Around Los Altos, word travels fast. Homeowners on Covington Road, Rosita Avenue, and Arastradero Road talk to each other, and the contractor who shows up with a written change order instead of a surprise invoice gets mentioned at the hardware store. That’s how we’ve built our reputation here. Every completed job carries the 90-Day Done Right Promise in writing: if it’s not done right, one call brings Tom back.
Our Home Additions & ADU Services in Los Altos
ADU Design & Permitting
Most Los Altos lots fall under city of Los Altos jurisdiction, but a meaningful share of larger parcels, especially those on septic and well rather than municipal sewer and water, are county-permitted through Santa Clara County. That distinction changes everything about the permit path. We handle the designs, the submittals, and the revisions for both, and we’ll tell you in the first meeting which one governs your property. A county-permitted ADU on septic carries setback rules for the leach field that a city-permitted ADU on sewer never faces.
Garage Conversion ADU
A garage conversion in Los Altos typically runs $30,000 to $80,000, and it’s often the smartest play on a flat 94022 or 94024 lot where a detached structure already exists. The catch is usually the sewer or septic connection. On sewer, tying into the house line is usually straightforward. On septic, a separate connection question comes up, and the county may require its own lateral or a system capacity review before sign-off. We sort that out before we quote, not after rough-in.
New Detached ADU
Building a new detached ADU in Los Altos means grading, foundation, utilities, and on many lots, working around heritage oaks whose drip lines are non-negotiable. A $90,000 to $220,000 budget has to account for site prep that a flat, treeless Sunnyvale lot never sees. We scope the pad, the drainage, and the utility trenching with the oak locations marked before we write the first number.
Junior ADU & Interior Conversion
A JADU inside the existing footprint of a Los Altos home usually involves converting a portion of the main house into a self-contained unit with its own entrance, kitchenette, and bath. On a 1950s ranch with a shallow concrete-perimeter foundation, that’s often the lowest-cost ADU path, because no new foundation is required. We’ll tell you honestly whether your layout supports a JADU without compromising the main house.
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 Los Altos
Where the fixture or finish matters, we install brands you can look up yourself. On ADU bathrooms and kitchens, that means Schluter waterproofing systems behind the tile, Sterling and Grohe fixtures in the wet areas, and GAF materials on any roofing work tied to the addition. If you’re paying Los Altos prices for an ADU, you shouldn’t get mystery-brand materials off a van. Call (877) 610-1318 and we’ll walk you through the spec sheet line by line.
Common Home Additions & ADU Problems We See in Los Altos Homes
- Heritage oak constraints freeze the permit. Los Altos’s tree-preservation ordinance treats protected native and heritage oaks as non-negotiable site constraints. A detached ADU’s foundation and septic lines must be rerouted around drip lines, and one unpermitted root cut can stop a Santa Clara County permit sign-off cold. This doesn’t apply across the border in Palo Alto or Sunnyvale.
- Septic and well setbacks force redesigns. A homeowner assumes city of Los Altos rules apply to their ADU, then discovers Santa Clara County requires well and septic setbacks that force a redesign after rough-in has started. The quote is dead, and the budget is blown. We verify jurisdiction before we quote.
- Post-and-pier foundations can’t carry a second story. Pricing a second-story addition without a load-bearing survey of a 1950s post-and-pier ranch is a mistake we see far too often. The original shallow concrete perimeter can’t carry the new load, and the quote dies at the framing inspection. We survey the structure before we write the number.
- Adobe clay moves under shallow slabs. Los Altos sits on expansive Adobe clay soils that swell and shrink with seasonal wet and dry cycles. Differential foundation movement is a persistent problem for slab-on-grade and shallow perimeter foundations across the flatlands. Any addition tied to an existing slab needs a soils review.
Pricing for Home Additions & ADU in Los Altos, CA
Here’s what you’re looking at in Los Altos, straight up. Home additions run $45,000 to $130,000. Detached ADU construction runs $90,000 to $220,000. Garage conversions run $30,000 to $80,000. Those ranges shift based on foundation type, septic versus sewer, oak tree constraints, and whether the existing structure can carry a second story without reinforcement.
| Project Type | Los Altos Price Range |
|---|---|
| Home Addition | $45,000-$130,000 |
| ADU Construction | $90,000-$220,000 |
| Garage Conversion ADU | $30,000-$80,000 |
Every scope, every allowance, every price change arrives as a written change order before work happens. An allowance that isn’t on paper is a surprise waiting for a date. Call (877) 610-1318 for a free estimate with line-item pricing, on paper.
Building on Los Altos Acreage: What Actually Changes the Price
If you own a larger parcel in the 94022, 94023, or 94024 zip codes, you already know that county rules, not city rules, often govern what happens on your lot. For a detached ADU or a substantial addition, that means well and septic setbacks, long material hauls, equipment access across open ground, and grading that satisfies Santa Clara County stormwater requirements during the wet months from November through March.
We don’t guess at these. On a 94024 lot off Arastradero Road, we scoped a new detached ADU where the county-mandated septic setback pushed the building pad forty feet from the original stake, adding an $18,000 grading and drainage allowance before we even wrote the first draw schedule. Our crew reset the pad, pulled a revised ADU permit with the county, and kept the rough-in inspection on track without touching a protected oak’s drip line. That’s what scoping in writing looks like.
If your lot in Los Altos Hills, or on the unincorporated edges of Los Altos proper, has a well, a leach field, open drainage, or protected oaks, the price is not what a tract-home builder in Mountain View would quote you. We walk the lot before we quote. Every time.
We Also Serve Cities Near Los Altos
Our crews work across the South Peninsula every week. We’re active in Los Altos Hills, Mountain View, Stanford, and Sunnyvale, and we know how the permitting and site conditions change from one city to the next. If you’re in Home Additions & ADU in Los Altos or a neighboring town, call (877) 610-1318 and Tom will tell you straight what your project needs.
Serving Los Altos, CA - Our Local Coverage Area
We’re based in the Los Altos 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 Los Altos
Yes. A detached ADU on a septic property in the Los Altos area typically requires Santa Clara County permits, and the septic system must be reviewed for capacity and setback compliance before the building permit is issued. The setback requirements can move your building pad significantly off the original stake. Call (877) 610-1318 and we’ll verify your jurisdiction and setback before you spend a dollar on design work.
A heritage oak on a 94022 lot in Los Altos becomes a fixed design constraint. The ADU foundation, septic lines, and utility trenches must be routed around the tree’s drip line, and a single unpermitted root cut can halt the permit sign-off completely. You’ll likely need an arborist report and canopy-protection fencing before ground breaks. We include that in the quote, not as a surprise change order later.
Rarely. Most 1950s ranches near downtown Los Altos sit on original post-and-pier or shallow concrete-perimeter foundations that were never designed to carry a second story. Without a load-bearing survey and structural reinforcement, the framing inspection will fail and the project stops. We survey the existing foundation before we quote a second-story addition, so the price you see is the price you get.
On a city-sewer lot in Los Altos, a garage conversion ADU can usually tie into the existing house sewer line, subject to the city’s plumbing code and a clean-out requirement. On a septic lot, the county may require a capacity review or a separate connection, which changes the scope and the price. We verify which situation you have before we write the quote. Call (877) 610-1318 for a free evaluation.
Allowances are dollar amounts set aside in a quote for materials that haven’t been selected yet, like fixtures, flooring, or tile. A low allowance makes a quote look cheaper and turns into a surprise invoice when the homeowner picks the finishes they actually want. At Barrett & Sons, every allowance is written on paper, and every allowance change arrives as a written change order before work proceeds. If it’s not on paper, it’s a surprise waiting for a date.
Get Your Los Altos Home Addition or ADU Scoped Today
You don’t need a contractor to guess. You need someone to walk the lot, check the oak drip lines, verify the jurisdiction, and put the scope in writing. Call (877) 610-1318 to schedule your free estimate. Tom Barrett answers his own phone, and he’ll tell you straight what your Los Altos project needs and what it costs, before any work begins.
Written by Tom Barrett, Owner at Barrett & Sons Construction, serving Los Altos and the South Peninsula since 1998.