ADU Costs in Los Altos, California

Planning-level pricing guide by ADUWizard.com
Updated for 2026 budgeting

Los Altos is not San Jose with nicer landscaping. It is its own animal: smaller in population, wealthier per household, and built almost entirely on large single-family lots where the design bar sits higher than nearly anywhere else in Santa Clara County. When a homeowner here asks me what an ADU costs, my honest answer is that the build itself is not that different from San Jose’s, but the labor, the finish level people expect, and the design fees stack on top in a way that pushes the total noticeably higher.

Quick disclaimer: This is a budgeting guide, not a quote or legal advice. Actual bids swing on lot access, slope, utility runs, and how custom your design gets. Use these ranges to sanity-check bids, not to price a contract.

What Changed in Los Altos for 2026

Start with the state, because that is where the real movement happened. Four bills, AB 462, AB 1154, SB 543, and a 2025 amendment to SB 9, took effect January 1, 2026, and all four touch how ADUs get processed and sized in every California city, Los Altos included.

SB 543 shifts how ADU square footage gets measured to interior livable space and requires cities to make a completeness determination on an application within 15 days or the application is deemed complete by default. AB 1154 narrows JADU owner-occupancy so it only applies when the JADU shares a bathroom with the main house. AB 462 adds a firm 60-day clock for Coastal Development Permits tied to an ADU, running alongside the ministerial review rather than after it. The 2025 SB 9 amendment gives California’s Department of Housing and Community Development real teeth: if a city’s ADU ordinance does not comply with state law and the city does not fix it after being notified, HCD can render the noncompliant parts void, and state standards apply in their place.

That last piece matters here specifically. HCD’s ADU ordinance technical assistance letter to the City of Los Altos, dated December 5, 2025, flags that the city’s ordinance needs updates to line up with current state law, including how third parties like homeowners associations get treated in the approval process. I want to be straight about what I could and could not confirm: I was not able to pull the full letter text directly (the PDF returned an access error when I tried to fetch it), so I am relying on HCD’s public letter listing and secondary reporting for the specifics. What I can confirm from the city’s own published ADU information guide is that its JADU owner-occupancy language still reads as a blanket requirement, without the narrower bathroom-sharing condition AB 1154 now sets. If that guide has not been updated by the time you read this, the state’s narrower rule controls, not the older city page. Confirm current status directly with the Los Altos Building Division before you assume either way.

Locally, I did not find a Los Altos-specific ADU ordinance rewrite that took effect in 2025 or 2026 beyond what the HCD letter is pushing toward. If the city adopts amendments in response to that letter, this page will need an update, and I will flag it then.

What an ADU Actually Costs in Los Altos

Here is the reality: Los Altos ADU construction runs from roughly $180,000 for a modest garage conversion or JADU to $750,000 or more for a large, fully custom detached unit on a demanding lot. That is meaningfully above the California statewide range, and it sits at or above what I quote in San Jose, the largest nearby market.

Three things drive that gap. First, labor here is inner-Peninsula pricing, not just Bay Area pricing, and good crews are booked months out. Second, the design expectation on a $4 million-plus lot is rarely “build it plain.” Third, even a straightforward detached ADU on a large parcel still has to run new utility lines, and trenching across a bigger lot costs more than trenching across a small one. Our Data Hub tracks these ranges as bids come in, and the spread within Los Altos itself is wide depending on how custom the job gets.

Component	Cost range	Notes
Site work	$35,000–$90,000	Utility trenching across a larger lot, drainage, grading
Foundation	$30,000–$60,000	Soil conditions and setback constraints on tighter infill parcels
Framing	$70,000–$140,000	Premium crews are the single biggest lever on this line
MEP	$60,000–$110,000	Full kitchen and bath required by code, separate utility runs for a detached unit
Finishes	$60,000–$140,000	Local finish expectations run well above the state average
Permits and fees	$2,000–$15,000	City waives its own ADU building permit, plan check, and utility permit fees; construction tax and outside agency fees still apply
Design	$15,000–$45,000	Custom architecture costs more here than in most of the state; a pre-approved or permit-ready plan cuts this dramatically

Source: ADU Wizard builder estimate

Size	Low finish	Mid finish	High finish
400 sq ft	$220,000	$290,000	$380,000
600 sq ft	$280,000	$370,000	$480,000
800 sq ft	$340,000	$450,000	$580,000
1,000 sq ft	$410,000	$540,000	$680,000
1,200 sq ft	$480,000	$630,000	$780,000

Source: ADU Wizard builder estimate

Fees and Exemptions in Los Altos

This is the part that actually surprised me when I dug into the city’s own published fee schedule, so let me be precise about what I found and where I got it.

The City of Los Altos’s own Development Services fee schedule for fiscal year 2025 to 2026, effective July 1, 2025, lists the ADU building permit fee, the ADU electrical, mechanical, and plumbing permit fee, and the ADU plan check fee at $0.00 each. Not a discount, not a percentage reduction: zero. That is a real, city-specific waiver on top of anything state law already requires, and it does not appear to be limited to units under 750 square feet the way some state minimums are, since the fee schedule line item does not carry that qualifier. Confirm your specific project against the current schedule at submittal, since fee schedules get revised annually, but as of this writing that is what the city’s own document says.

What is not waived: the city’s construction tax under Los Altos Municipal Code Chapter 3.24, currently $0.42 per square foot of residential construction, does not appear as a $0.00 ADU line item the way the permit and plan check fees do, so I am treating it as still applicable and telling you to confirm rather than assuming it is waived. Development impact fees under Chapter 14.14 are required to be proportional to unit size, which lines up with the state’s own under-750-square-foot exemption rather than adding a separate city-specific break beyond it.

On design review specifically: Los Altos Municipal Code Chapter 14.14 requires that a complete ADU application be approved or denied ministerially, without discretionary review or a public hearing, within 60 days. That means the city’s discretionary Design Review process and its associated fees (which do show up on the general fee schedule for other single-family projects) do not apply to a standard ADU application. I could not find anything in the city’s ADU-specific materials suggesting otherwise, and it matches how state ADU law is supposed to work everywhere. If your project also needs a separate discretionary approval for some other reason (a variance, a historic permit), that is a different track, not part of the base ADU review.

For comparison, San Jose, Riverside, and Long Beach have no city-specific ADU fee waiver beyond the state’s own under-750-square-foot break. Los Altos’s zeroed-out permit and plan check line items put it ahead of those markets on this specific point, which is not something I expected going in.

Los Altos-Specific Factors That Move the Number

The pre-approved and permit-ready plan system is a genuine two-tier program, and most homeowners only know about one tier. The city offers its own permit-ready detached ADU plans, developed by the city and available free of charge, in three sizes (roughly 525, 854, and 1,190 square feet) and three architectural styles (Mediterranean, Traditional, and Contemporary), with full structural, mechanical, electrical, and plumbing drawings included. Separately, the city also maintains a roster of privately designed pre-approved plans you contract for directly, including options I found actively on file as of this research: a 330-square-foot and a 434-square-foot design from Wellmade, and an 800-square-foot design from ADORE Homes. Between the two tiers, a homeowner on a standard lot has real options that skip a meaningful chunk of design cost entirely. Honestly, I’m still not sure why more people here don’t use the free city plans first before commissioning custom work.

Large lots do not buy extra ADU square footage. Much of Los Altos sits in R1-10, R1-20, or R1-40 zoning under Los Altos Municipal Code Chapter 14.06, with minimum site areas running from 10,000 up to 40,000 square feet. That is a genuinely large-lot city by California standards. But state ADU law caps detached units at 1,200 square feet regardless of how big the underlying parcel is, so a 40,000-square-foot lot does not get you a bigger ADU than a 10,000-square-foot one. What the extra land does buy is siting flexibility: more room to keep the unit away from the main house, more room to avoid a tight side yard, and generally an easier time hitting the 4-foot side and rear setbacks and 5-foot building separation the code requires for detached units.

Contractor availability is tighter than the dollar figures alone suggest. This is a small city surrounded by even more expensive neighbors, and the same pool of high-end residential crews works Los Altos, Los Altos Hills, and Atherton on rotation. That said, that’s not unique to ADUs, it’s just how Peninsula construction scheduling works right now.

Terrain is mostly gentle, not hillside. Unlike parts of San Jose’s Almaden or Evergreen districts, most of Los Altos sits on relatively flat to gently rolling ground rather than steep hillside lots. That keeps geotechnical and access surprises less common here than in some neighboring hill communities, though foothill-adjacent parcels on the west side of the city are the exception worth checking individually.

What I’d Budget If I Were Building Here

If I were building an ADU in Los Altos today, I would budget $450,000 for a detached, roughly 800-square-foot unit with a mid-to-upper finish level, and I would start by pulling the city’s free permit-ready plans before commissioning anything custom. On a standard flat lot, that plan set alone can save tens of thousands in design fees and real weeks in review, on top of the fee waiver the city already gives you on the building permit and plan check.

That said, if your lot is irregular, has a slope, or you want something the catalog plans do not offer, custom design still wins on a site like that, and the number can push toward $600,000 to $700,000 once you add premium finishes. Either way, get your actual bid checked against the California state cost page and the Data Hub before you sign anything.

FAQs

How much does an ADU cost in Los Altos in 2026?

Budget roughly $180,000 for a modest conversion or JADU and up to $750,000 or more for a large, fully custom detached unit. A typical mid-size detached ADU with solid finishes lands closer to $400,000 to $500,000.

Does Los Altos waive ADU permit fees?

Yes, based on the city’s own FY2025/2026 fee schedule. The ADU building permit, ADU plan check, and ADU electrical, mechanical, and plumbing permit fees are all listed at $0.00. The city’s construction tax and any outside agency fees are not part of that waiver, so confirm those separately at submittal.

Does Los Altos require design review for ADUs?

No, not for a standard ADU application. Los Altos Municipal Code Chapter 14.14 requires ministerial approval, without discretionary review or a public hearing, within 60 days of a complete application. The city’s discretionary Design Review process applies to other residential projects, not to ADUs processed under the state’s ministerial pathway.

Are there pre-approved or permit-ready ADU plans in Los Altos?

Yes, and it is a two-tier system. The city offers its own permit-ready detached ADU plans free of charge in three sizes and three styles, plus a separate roster of privately designed pre-approved plans available for purchase from outside designers. Either route skips a chunk of the design and plan check timeline compared to a fully custom design.

Does a large lot let me build a bigger ADU in Los Altos?

Not under current state law. Detached ADUs are capped at 1,200 square feet regardless of lot size, so a large parcel in an R1-20 or R1-40 zone does not get you extra square footage. It does make siting easier and gives you more room to work with on setbacks and building separation.

For the wider picture, see our California ADU cost guide and our California permit overview for how the process works statewide. If you are just starting to plan, the permits pillar and the Data Hub are the right next stops.