Free written scope consultation - allowances, timeline, and change-order rules on paper before a single trade is scheduled
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After
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AfterFast, Reliable Home Additions & ADU Across Los Altos
Home additions and ADU construction in Los Altos typically run $45,000-$130,000 for a room addition and $90,000-$220,000 for a detached ADU, with most Los Altos projects taking four to nine months from first permit submittal to final inspection. That’s the honest range before anyone has walked your lot. What actually drives the price in Los Altos isn’t square footage alone. It’s the heritage oaks on your parcel, the county’s septic and well setback rules, and whether your ranch home’s foundation can carry a second story or needs the kind of grading work that only shows up once the rains hit. Call (877) 610-1318 and we’ll come walk the property with you. No charge for the visit, no charge for the straight answer.
Why Barrett & Sons Construction Is Los Altos’s Preferred Home Additions & ADU Company
Tom Barrett founded this company in 1998, and his sons run the crews that work Los Altos job sites. You’ll see the same faces from first walkthrough to final walkthrough. That matters more in Los Altos than most places, because a lot of what happens on your addition or ADU depends on people who know this city’s building department, its tree ordinance, and its soil. We do. When we quote a detached ADU near O’Keefe Lane or a second-story addition in the Country Club area, we’re quoting from actual experience with Los Altos permitting, not a generic county checklist.
Every contract we write carries the 90-Day Done Right Promise. If something isn’t right after we’re finished, Tom comes back and makes it right. One call. We also put every scope change in writing before work proceeds. The allowance that isn’t on paper is a surprise waiting for a date, and we don’t do surprises. Our Home Additions & ADU team has built enough in Los Altos to know which questions to ask before the first stake goes in the ground.
You can look up Tom Barrett’s name, address, and license number yourself. We want you to. Ask anyone on your street, or call the building department. We’ve been doing this long enough that our name stands on its own in Los Altos.
Our Home Additions & ADU Services in Los Altos
ADU Design & Permitting
Los Altos has its own municipal building department, and detached ADUs on lots with heritage oaks or septic systems trigger review paths that a Mountain View or Sunnyvale permit runner won’t see. We handle the full design and permitting package: site plan, arborist report coordination, Title 24 energy calcs, structural drawings, and the city’s design-review submittal. We’ve sat through enough Los Altos planning counter conversations to know what gets flagged and what sails through. In Los Altos, the tree ordinance is the one that catches people.
Garage Conversion ADU
Converting an attached or detached garage into an ADU in Los Altos usually runs $30,000-$80,000 depending on whether we’re adding a kitchenette, upgrading the panel, or underpinning a slab that sits on Adobe clay. Most Los Altos garages from the 1950s and 60s were built on shallow perimeter footings, and they move with the seasons. We check for that before we quote, because a conversion that ignores foundation movement is a call-back waiting to happen.
New Detached ADU
A new detached ADU on a Los Altos lot over 10,000 square feet is the project we get called about most. These are the ones where septic and well setbacks, tree protection zones, and grading for drainage become the real scope. A 900 square foot detached ADU behind a 1950s ranch in this city can carry $90,000-$220,000 depending on what the county requires before we pour. We pull the permit, we pull the arborist report, we pull the septic review. Then we build.
Junior ADU & Interior Conversion
Junior ADUs, attached additions, and interior conversions inside the existing footprint are the fastest path to extra space in Los Altos. Attic build-outs, bonus-room conversions, a master suite over the garage, or Windows & Doors - Los Altos upgrades to improve light and access. These projects still touch the same Los Altos realities: structural upgrades for seismic, panel upgrades for the new loads, and city review whenever the footprint changes. Interior conversions that don’t change the envelope sometimes avoid the design-review step entirely, which can cut weeks off the schedule.
What happens when you call
- 1
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
We install materials you can look up. On Los Altos ADUs and additions, that often means Schluter waterproofing systems in the kitchens and baths we build into your new space during Bathroom Remodeling in Los Altos, Grohe fixtures where plumbing is specified, and GAF roofing on the new roof lines a second-story addition or detached ADU creates. We’re not pulling mystery brands off a van. If you want to look up the spec sheet before you sign, you can. We keep the brands we trust in consistent use across every Los Altos project, because the warranty we give you has to stand on what we installed.
Common Home Additions & ADU Problems We See in Los Altos Homes
- Heritage oak conflicts nobody caught at quote time. Los Altos’s tree-preservation ordinance protects native and heritage oaks with required arborist reports, canopy protection fencing, and city sign-off before any ground disturbance. A single missed report on a half-acre lot will stop the whole ADU or addition project until it’s resolved.
- County septic and well setback reviews that re-scope the foundation. Acreage parcels in Los Altos often fall under county review for septic and well clearances. If the existing field doesn’t meet current setback requirements for a detached ADU, we’re redesigning the system before we stake the slab. That changes both timeline and price.
- Adobe clay foundation movement. The Santa Clara Valley floor under Los Altos is expansive Adobe clay that swells in winter and shrinks in summer. Slab-on-grade rooms and shallow perimeter footings move with it. Any addition that ties into an existing 1950s or 60s ranch home needs a foundation plan that accounts for that differential movement.
- Quoting a city-style addition on a county-style lot. We see estimates from other contractors that treat a Los Altos acreage parcel like a Sunnyvale infill lot. Then the grading plan, erosion control, and long material runs show up as a five-figure change order after the contract is signed. We scope that before we quote, not after.
Pricing for Home Additions & ADU in Los Altos, CA
Here’s the honest Los Altos pricing table, the same ranges we give on the phone before we’ve walked your property. What can move these numbers: tree protection requirements, septic redesign, foundation upgrades on Adobe clay, and whether your ranch home’s existing panel can carry the new loads.
| Project Type | Los Altos Price Range |
|---|---|
| Home Addition | $45,000 - $130,000 |
| ADU Construction | $90,000 - $220,000 |
| Garage Conversion | $30,000 - $80,000 |
A second-story addition over an existing ranch in Los Altos runs toward the top of the home addition range because of structural shoring and the foundation work the Adobe clay demands. A single-story bump-out on a slab that’s already stable lands lower. Every quote we write for Los Altos carries a line-item breakdown, so you can see exactly where the tree protection, drainage, and material haul costs live. Call (877) 610-1318 for a free estimate and we’ll walk your lot together.
We Also Serve Cities Near Los Altos
Barrett & Sons Construction works across Los Altos Hills, Mountain View, Stanford, and Sunnyvale from our San Francisco Bay Area office. If your property sits near the Los Altos Hills line or in the unincorporated pockets around O’Keefe Lane, the county reviews change. We know the difference. Our Home Additions & ADU in Los Altos experience carries directly into those nearby projects.
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
The heritage oak ordinance typically adds four to eight weeks to a detached ADU permit in Los Altos, because the city requires an arborist report, canopy protection fencing, and written sign-off before any ground disturbance begins. If the oak sits within or near the ADU footprint, the tree protection zone can also shrink your buildable area, which may mean redesigning the structure. We file the arborist report as part of the original permit package, so the city sees the full site picture on day one. Call (877) 610-1318 if you’ve got an oak near your proposed ADU site, and we’ll tell you what the report will likely cost before you commit.
Sometimes, but it depends on where the existing septic field and well sit relative to the proposed ADU footprint and any new drain lines. County setback rules in Los Altos require minimum clearances between a septic field, a well, and any new dwelling unit. If the current field can’t meet those clearances, a redesign is mandatory, not optional. We pull the county review early, before final drawings, so we’re not pouring a foundation and then finding out the field has to move. Call (877) 610-1318 and we’ll pull the parcel records for your lot and give you a straight answer.
Expect a $10,000 to $35,000 premium for grading, drainage, and erosion control on a Los Altos acreage addition, depending on slope, soil type, and how far material has to move across the lot. The wet winter months from November through March make this work more expensive and more critical, because Santa Clara County stormwater rules require erosion control on any excavation. We include grading and drainage as a line item in every Los Altos quote, never as a post-contract surprise.
No. A 60-amp panel from a 1950s or 60s Los Altos ranch will not carry a garage conversion ADU with a kitchenette, mini-split, and lighting loads. You’ll need a panel upgrade or a subpanel run from the main service, which typically adds $2,500 to $6,000 to the project depending on the distance from the meter and whether the service entrance needs replacing too. We check the panel in the first walkthrough, so the upgrade is in the original quote, not a change order later. Call (877) 610-1318 and we’ll take a look at yours.
We plan the excavation and foundation work around Los Altos’s wet season by sequencing the permit, tree protection, and site prep to hit the ground before November or after March, and we build erosion-control measures into the original scope so rain doesn’t turn an open foundation into a re-grade. When rain does stop work, we protect the excavation with cover and move our crew to interior or off-site tasks that don’t require open ground. On a Los Altos Hills-adjacent lot near O’Keefe Lane, we scoped a 900 sq ft detached ADU and carried $1,500 in hardscape allowances for root-barrier panels and a 40-foot material haul from the street to keep the draw schedule on track through a wet December. The job stayed on budget because the weather was already priced in.
Call (877) 610-1318 to schedule your free estimate in Los Altos. Tom or one of his sons will answer the phone, and we’ll set a time to walk your lot and talk through what’s actually possible on your parcel. Written line-item quote, permits pulled, 90-Day Done Right Promise in writing. No charge to come out and look.
Written by Tom Barrett, Owner at Barrett & Sons Construction, serving Los Altos since 1998.