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 detached ADU in Los Altos typically runs $90,000-$220,000, while a room addition or second story falls between $45,000-$130,000, with most projects taking 4-8 months from permit to final inspection. We also handle Bathroom Remodeling - Los Altos projects alongside these larger builds. We’re Rowan Construction Group, and our crews work Los Altos regularly - from the ranch homes off Springer Road to the rebuilds in the hills above El Monte Road. Call (650) 405-3637 to schedule your free estimate. Every quote is a flat written price with a written scope before anyone picks up a tool.
Why Rowan Construction Group Sunnyvale Is Los Altos’s Preferred Home Additions & ADU Company
We’ve delivered 4,000-plus projects since 2007, and a meaningful share of them sit within Los Altos city limits. Our Home Additions & ADU in Los Altos customers come back because we put the number in writing first - no exceptions, no “call for a quote.”
Our San Francisco Bay Area office coordinates dispatch and scheduling for Los Altos, which means crews are on-site within a day or two of estimate approval, not weeks. The 365-Day Done Right Promise backs every job: if it is not done right, we make it right, with terms printed on every page, not an asterisk. Background-checked, uniformed technicians arrive with a documented photo record as standard - included, not optional - so you leave with evidence of what was found and what was done.
Los Altos homeowners tend to read the three-star reviews before they call. We encourage that. Our record is documented: signed change orders on every modification, projects finishing within a few percent of the agreed price, and a Free Second Opinion on any competitor’s written estimate - we’ll review it line by line at no charge.
Our Home Additions & ADU Services in Los Altos
ADU Design & Permitting
Los Altos operates an Architectural and Site Review process - a city design review layer on top of standard Santa Clara County building permits - that scrutinizes new construction for neighborhood compatibility in massing, materials, and roofline. Contractors who don’t build working relationships with architects familiar with the committee’s expectations routinely face revision cycles that push project timelines out by three to six months before a single shovel hits the ground. We front-load this: our permit package includes preliminary massing studies that have passed the committee before, and we don’t submit until the architect and our project manager agree the design language matches what the committee has approved on comparable lots.
Garage Conversion ADU
Garage conversions in Los Altos run $30,000-$80,000, and they’re most common in the flatlands near San Antonio Road where 1950s ranch homes sit on 8,000-10,000 sq ft lots with detached or attached two-car garages. The catch is the foundation: many of these garages were poured as slabs without footings adequate for habitable space, and the city will require a new footing or a demonstration that the existing slab meets current CBC standards for live loads. We test before we quote. No written price until we know what the slab is.
New Detached ADU
This is where Los Altos gets interesting. Because land values alone often exceed $2M, most owners adding square footage here are either building a detached ADU on a lot that already has a teardown-scale house, or they’re weighing an addition against a full ground-up rebuild - so the honest first conversation is about whether the existing cripple-wall or slab foundation is worth tying new structure into at all. On a 1950s ranch off Springer Road, we framed a new 900 sq ft detached ADU in the side yard of an existing single-story house, and the whole job hinged on the rough-in and foundation detail: the original house sat on a cripple-wall foundation with no modern seismic anchorage, so our engineer designed the ADU on its own slab with a code-compliant separation, and we used KraftMaid cabinetry and a Cambria island cap so the unit matched the main house finish. We ran the utility trench before the winter rains hit the clay, scheduled every inspection sign-off ahead of close-in, and handed over a punch list of only eleven items.
Junior ADU & Interior Conversion
Junior ADUs - conversions within the existing footprint, typically 500 sq ft or less - work best in Los Altos when the original house has already been rebuilt or substantially remodeled and the owner wants rental income without a separate structure. The city’s Architectural and Site Review still applies if the conversion changes the exterior envelope or roofline, but interior-only work can sometimes bypass the full committee review. We verify that with the planning desk before we quote.
Room Additions
A single-room addition on a Los Altos ranch home runs toward the lower end of our $45,000-$130,000 range if we’re tying into an existing slab with verified seismic capacity. Most aren’t. The 1950-1975 foundations in this city - cripple-wall or minimal slab - predate modern seismic standards, and new load-bearing walls can’t legally bear on a footing that doesn’t meet current CBC requirements. We bring a structural engineer to the first site visit. The price reflects what we find, not what we hope.
Second Story Additions
Second stories are the most scrutinized project type in Los Altos. The Architectural and Site Review committee weighs roofline continuity, massing relative to neighboring homes, and material compatibility heavily - a flat-roofed modern second story on a gabled ranch will draw revisions. We design with the committee’s precedent in mind, not against it. Structurally, the existing cripple-wall foundation almost always requires retrofit or replacement to carry second-floor loads in this high seismic design category, wedged as Los Altos is between the San Andreas Fault to the west and the Hayward Fault to the east.
Sunrooms & Enclosed Patios
These fall under the same review process if they’re conditioned space with permanent foundations. A three-season room on piers may bypass some requirements, but Los Altos’s winter rainfall concentration between November and April means any structure with a slab needs proper grading and French drains - the clay-heavy Santa Clara Valley soils expand and shift seasonally, and we’ve seen unwaterproofed slabs crack in their first winter.
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.
Before
AfterTrusted Brands We Service in Los Altos
Our Home Additions & ADU team sources across fifty-plus vetted brands, and we stock local inventory for Los Altos projects on the brands that move fastest here: PGT and CGI windows for the large-format openings common in new detached ADUs, CWS siding and trim for exterior compatibility with existing ranch homes, and ES Windows for projects where the Architectural and Site Review has specified particular glazing performance. KraftMaid and Wellborn cabinets, Cambria and Caesarstone countertops, Kohler and Toto fixtures - these are in our standard quote library, not special-order guesses, which means the written price you receive reflects actual vendor pricing, not inflated placeholders.
Common Home Additions & ADU Problems We See in Los Altos Homes
- Building without confirming the foundation type. The 1950-1975 ranch homes that dominate Los Altos sit on cripple-wall or minimal slab foundations that predate current CBC seismic standards. New load-bearing walls can’t legally bear on these footings without engineering analysis and likely retrofit. We test first, quote second.
- Learning about Architectural and Site Review after design is complete. Massing, materials, and roofline compatibility get scrutinized by committee, and a mismatch pushes the permit timeline out three to six months before a shovel hits the ground. We engage the committee’s precedent before we draw.
- Constructing a detached ADU without accounting for clay soil movement. The Santa Clara Valley’s clay-heavy soils expand and shift with winter rainfall concentrated November through April. No French drain or proper waterproofing means seasonal soil movement under the new slab. We design drainage before we pour.
- Assuming a garage slab is adequate for habitable conversion. Many Los Altos garage slabs were poured without footings sized for live loads. The city requires demonstration of compliance - often meaning new footings or a replacement slab. We investigate before we quote.
Pricing for Home Additions & ADU in Los Altos, CA
Every project is quoted with a flat written price and written scope before work begins. No change proceeds without a signed change order. This is Clause 1 of the Haven Standard, and it’s why our projects finish within a few percent of the agreed cost.
| Service | Price Range in Los Altos |
|---|---|
| ADU Construction | $90,000-$220,000 |
| Home Addition | $45,000-$130,000 |
| Garage Conversion | $30,000-$80,000 |
What moves the number: foundation condition and required seismic work, Architectural and Site Review cycle count, utility trench length and soil conditions, finish level (KraftMaid vs. custom millwork, Cambria vs. natural stone), and whether the project is tied to an existing structure or stands alone. We don’t guess. The site visit produces the data; the written quote produces the number. Call (650) 405-3637 for your free estimate.
We Also Serve Cities Near Los Altos
Our crews work throughout the San Francisco Bay Area, with regular projects in Los Altos and neighboring Mountain View, Palo Alto, Menlo Park, and Sunnyvale. The same written-price-first discipline, the same documented photo record, the same 365-Day Done Right Promise applies in every city.
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, provided the lot meets minimum size requirements and the ADU complies with setback, height, and parking rules - but the critical question is whether to tie the ADU foundation to the existing house or separate it. Most 1950s ranch homes in Los Altos sit on cripple-wall foundations without modern seismic anchorage, so our engineers typically design the ADU on its own code-compliant slab with a legal separation. Call (650) 405-3637 and we’ll assess your specific lot and foundation.
Los Altos operates its own design review layer that scrutinizes neighborhood compatibility in massing, materials, and roofline, and this committee’s approval is mandatory before Santa Clara County will issue a building permit. Contractors unfamiliar with the committee’s precedent often face three to six months of revision cycles. We front-load this by researching comparable approvals before we submit. For a timeline estimate on your specific project, call (650) 405-3637 for a free consultation.
At Los Altos land values exceeding $2M, teardown and custom rebuild is frequently more economical than tying new structure to a dated foundation - the math flips around $1.5M-$2M in total project cost depending on finish level and foundation condition. We run both scenarios in our estimate meeting so you have the numbers in writing before deciding. Call (650) 405-3637 to schedule that conversation.
Document the existing condition with photos before any work begins, use licensed contractors who pull permits for all modifications, and keep all warranties and change documentation in a single file - most builder warranties cover defects in original work, not damage caused by unpermitted modifications or unlicensed labor. Our crews document every visit with a photo record as standard, and we pull permits on every trade. For warranty-safe upgrade planning, call (650) 405-3637.
Most Los Altos ADU projects use a five-draw schedule: 10% at permit approval, 25% at foundation completion, 30% at framing and rough-in inspection, 25% at final inspection, and 10% retained until punch list completion - typically 8-15 items on a well-managed job, fewer on projects where we’ve worked with the architect through Architectural and Site Review. The punch list is documented with photos and signed off by both parties. For a written quote with draw schedule on your project, call (650) 405-3637.
Ready to put a written price on your Los Altos addition or ADU? Call (650) 405-3637 to schedule your free estimate. We’ll review your foundation, your lot conditions, and the Architectural and Site Review path - then deliver a flat written price with a written scope before anyone picks up a tool. Under Clause 1 of the Haven Standard, that’s the only way we work.
Written by Grant Rowan, Owner at Rowan Construction Group, serving Los Altos since 2007.