Free written scope consultation - allowances, timeline, and change-order rules on paper before a single trade is scheduled
Before
After
Before
After
Before
AfterFast, Reliable Home Additions & ADU Across Los Altos
A typical home addition in Los Altos runs $45,000-$130,000, while a full detached ADU runs $90,000-$220,000. Most projects require 4-8 months from permit to final, with long-lead designer materials, not framing labor, setting the actual calendar. Call (831) 704-1211 to schedule your free estimate.
We serve Los Altos from our San Francisco Bay Area dispatch office, with crews that know the difference between a Rancho San Antonio ranch and a South Los Altos split-level. Every project we quote in Los Altos gets a flat written price and written scope before anyone orders a stick of lumber, because Clause 1 of The Haven Standard is the whole point. Our Home Additions & ADU team has delivered 4,000-plus projects since 2007, and every one started with a number in writing.
Why Rowan Construction Group Campbell Is Los Altos’s Preferred Home Additions & ADU Company
Los Altos homeowners research contractors the way they research appliances. They read the three-star reviews. They want the price range on the page before they make contact. They prefer a text confirmation to a phone promise. We built our entire process around that customer.
Our crews are background-checked and uniformed. A live person answers calls 24/7. Every visit includes a documented photo record as standard, not an upgrade. The 365-Day Done Right Promise is printed on every page and backed by a terms page, not an asterisk. And our Free Second Opinion on any written estimate means you can bring a competitor’s quote and we’ll review it line by line at no charge.
We don’t sell enthusiasm. We sell a documented record. That’s why projects finish within a few percent of the agreed price.
Our Home Additions & ADU Services in Los Altos
ADU Design & Permitting
Every ADU in Los Altos runs through city design review and a separate Heritage Tree Ordinance clearance. We pull the tree permit and have an ISA-certified arborist walk the lot before any foundation excavation or utility trenching begins. An unpermitted grade change inside a coast live oak’s root zone can halt a project mid-pour. Our permitting package includes the arborist report, the tree permit application, and the full structural drawings for Santa Clara County’s seismic review, so the schedule doesn’t slip on administrative surprises.
Garage Conversion ADU
Garage conversions in Los Altos range $30,000-$80,000. Most 1950s-1970s garages in the city sit on slabs that weren’t engineered for habitable loads, and the Santa Clara Valley’s adobe clay soils have often cracked or shifted them over decades. We assess the existing slab and foundation before quoting, because treating a failing slab as reusable turns into an unplanned structural upgrade with a large change order. Our written price includes the structural assessment, so you know before we start whether the slab stays or goes.
New Detached ADU
A new detached ADU in Los Altos is the most complex residential project most homeowners undertake. The city is essentially built out, so “new construction” almost always means navigating strict floor-area-ratio limits and design review on a lot already occupied by a mature home and, frequently, mature trees protected by ordinance. We coordinate the arborist walk, the tree permit, the design review submittal, and the long-lead material orders - architectural glass, custom cabinetry, specialty finishes - as a single sequenced package. The price is fixed in writing before ordering begins.
Junior ADU & Interior Conversion
Junior ADUs and interior conversions in Los Altos work within the existing envelope, which can sidestep some design review complexity but still requires seismic evaluation on these older ranch and split-level homes. Post-and-pier foundations that fall short of current CBC standards are common. We identify the structural upgrade scope during the initial walk-through and include it in the written quote, not as a later surprise.
Second Story Additions
Second story additions in Los Altos demand the most rigorous structural analysis of any project type. The existing 1950s-1970s foundation and first-floor framing were never designed for a second floor’s live and dead loads, and Santa Clara County’s proximity to the San Andreas Fault means moment frames, hold-downs, and shear-wall schedules are non-negotiable. We handled a second-story addition and detached ADU on a 1955 ranch in the Rancho San Antonio-adjacent area, where the existing post-and-pier foundation fell short of current CBC seismic standards and adobe clay had visibly shifted the subfloor. Our crew worked with an ISA-certified arborist to route the utility trench outside the protected root zone of a mature coast live oak, then upgraded the foundation with hold-downs and a shear-wall schedule to satisfy Santa Clara County detailing. We sequenced the fast rough-in and framing to hit inspection sign-off before the rainy season, and the project cleared its punch list with the custom millwork and Cambria counters installed exactly as drawn.
What happens when you call
- 1
A real person answersNo phone trees - you reach a local pro.
- 2
You get an upfront price rangeHonest numbers before anyone is dispatched.
- 3
A background-checked tech heads outLicensed & insured, dispatched right away.
- 4
You approve before work beginsNothing starts until you say go.
Before
AfterTrusted Brands We Service in Los Altos
We source across fifty-plus vetted brands, which gives our written quotes honest numbers rather than inflated ones. For Los Altos additions and ADUs, that means KraftMaid and Wellborn cabinets, Cambria and Caesarstone countertops, and PGT and CGI architectural windows and doors. We don’t spec a brand we can’t deliver on schedule, because a fixed price depends on a fixed delivery date. Our relationships with these suppliers mean we can confirm lead times before the quote is signed, not after.
Common Home Additions & ADU Problems We See in Los Altos Homes
- Heritage Tree Ordinance violations. Contractors who skip the separate tree permit and arborist walk before breaking ground can trigger a stop-work order, a remediation bond, and city fines that compress already-tight construction windows and push projects past the rainy season.
- Adobe clay foundation failure. The Santa Clara Valley’s expansive clay soils swell in the wet season and shrink in summer, cracking slabs and shifting post-and-pier foundations over decades. Underestimating this turns an addition into an unplanned structural upgrade with a large change order.
- Long-lead material delays. Treating architectural glass, custom cabinetry, or specialty finishes as if they run on normal submittal cycles means the rough-in finishes before materials arrive and the schedule slips by weeks. We confirm lead times before quoting and order on contract signature.
- Seismic detailing surprises. Santa Clara County’s rigorous seismic requirements - moment frames, hold-downs, shear-wall schedules - add cost and complexity compared to less fault-adjacent Bay Area cities. We include this scope in the initial written quote, not as a later add-on.
Pricing for Home Additions & ADU in Los Altos, CA
These are the ranges we see in the Los Altos market for permitted, design-reviewed work with structural upgrades included where needed:
| Project Type | Price Range in Los Altos |
|---|---|
| Home Addition | $45,000-$130,000 |
| ADU Construction | $90,000-$220,000 |
| Garage Conversion | $30,000-$80,000 |
What moves a project within these ranges: foundation condition, seismic upgrade scope, Heritage Tree Ordinance complexity, and long-lead material selection. A garage conversion reusing sound slab and framing sits at the lower end. A new detached ADU with custom millwork, architectural glass, and full foundation work sits at the upper end. Every quote we deliver is a flat price with a written scope and written warranty. No change proceeds without a signed change order. Call (831) 704-1211 for your free estimate.
We Also Serve Cities Near Los Altos
Our crews work across Santa Clara County, including Home Additions & ADU in Los Altos and neighboring Mountain View, Palo Alto, Sunnyvale, and Cupertino. Each city has its own permitting rhythm and soil conditions, and we adjust our sequencing accordingly.
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. Los Altos requires a separate tree permit under the Heritage Tree Ordinance before any grading, excavation, or trenching within a protected tree’s root zone. We pull this permit and coordinate the ISA-certified arborist walk as part of our standard pre-construction package, because an unpermitted auger hole inside a coast live oak’s dripline can trigger a stop-work order and remediation bond that halts the project mid-pour. Call (831) 704-1211 and we’ll walk your lot.
Long-lead designer materials, not labor, set the calendar. Architectural glass, custom cabinetry, and specialty finishes routinely run 8-14 weeks from submittal to delivery in the Los Altos market. We confirm lead times before the quote is signed and order on contract execution, so the rough-in doesn’t finish before the materials arrive. Call (831) 704-1211 to review the sequencing for your specific selections.
The Santa Clara Valley’s expansive adobe clay swells in the wet season and shrinks in summer dry periods, cracking slabs and shifting post-and-pier foundations over decades. Proximity to the San Andreas Fault means Santa Clara County also enforces rigorous seismic detailing. We assess soil conditions and foundation capacity during the initial walk-through and include any required upgrades in the written quote, because treating a failing foundation as sound turns into an unplanned structural upgrade with a large change order. Call (831) 704-1211 to schedule your assessment.
Sometimes, but not always. Most 1950s-1970s Los Altos garages sit on slabs engineered for vehicle loads, not habitable occupancy, and adobe clay soils have often cracked or shifted them. We test the slab and assess the framing against current CBC standards before quoting. If the slab and framing are sound, we say so and price accordingly. If not, we include the replacement scope in the written quote. Call (831) 704-1211 for an honest assessment.
Los Altos design review evaluates exterior massing, materials, and compatibility with the neighborhood character, plus strict floor-area-ratio compliance. The review runs concurrent with, but separate from, building permit and tree permit processes. We prepare the design review submittal package - including materials boards, elevation drawings, and neighborhood context photos - as part of our standard service, because a rejected design review submittal adds weeks to a schedule already constrained by long-lead materials. Call (831) 704-1211 to review the full permitting timeline for your project.
Ready to price your Los Altos addition or ADU? Call (831) 704-1211 to schedule your free estimate. We’ll deliver a flat written price with a written scope and written warranty before anyone picks up a tool.
Written by Grant Rowan, Owner at Rowan Construction Group, serving Los Altos since 2007.