Los Altos ADU Builder & 2026 Rules Guide
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Building ADUs in Los Altos, From Permit to Move-In
Axiom Renovation (CA Lic. #1105006) is building two ADUs in Los Altos in 2026: a 1,200 sq ft detached ADU with a new underground electrical service, and a detached backyard ADU now in interior finishes. This guide collects what we have learned working with the City of Los Altos, checked against the City’s current ADU ordinance (Chapter 14.14 of the Municipal Code, as replaced by Ordinance 2024-512 in April 2024).
Last reviewed: October 2026. State ADU law changes often, and where state law is more permissive than the City code, state law controls. We confirm the rules for your lot before design.
| Quick facts | Los Altos rule |
|---|---|
| Units per single-family lot | 1 attached ADU + 1 detached ADU + 2 JADUs |
| Detached ADU size | Up to 850 sq ft (one bedroom) or 1,200 sq ft (two or more) |
| Attached ADU size | 850 / 1,200 sq ft, up to 50% of the house; 850 sq ft always allowed |
| JADU size | Up to 500 sq ft, inside the house or attached garage |
| Counts toward floor area or lot coverage? | No |
| Height | Detached 18 ft; attached the greater of 16 ft or the zone limit |
| Side and rear setbacks | 4 ft |
| Approval | Ministerial building permit, no hearing or neighbor notice, decision within 60 days |
| Free City plans | Permit-Ready detached plans of 525, 854 and 1,190 sq ft |
Our Los Altos ADUs Under Construction
Photos from two ADUs we are building in Los Altos in 2026. See the 1,200 sq ft detached ADU and the backyard ADU at the finishes stage.
How Many Units You Can Build
On a single-family lot, Los Altos allows one attached ADU, one detached ADU and two junior ADUs (JADUs), a change made in the 2024 update. Under the City’s SB 9 rules, a lot that is not split can reach up to four units in total (primary units plus ADUs), with two JADUs on top of that; after an SB 9 lot split, each lot is limited to two units. On multifamily properties, ADUs can be added within existing non-livable space, and detached ADUs are allowed under state limits.
Two Detached ADUs from January 1, 2027 (AB 956)
AB 956 raises the number of detached ADUs the city must approve from one to two. The city may limit each of the two to 800 sq ft of living space and keep its height limits, and side and rear setbacks can be no more than 4 ft. If two detached ADUs are built, the city no longer has to approve a JADU on the same lot. Los Altos will need to update its ADU ordinance, so we will confirm exactly how the two-unit option combines with its attached-ADU and JADU rules as the city adopts its changes.
The new law matters most on larger Los Altos lots, where tree protection, the underground electrical service rule and sewer layout decide whether two backyard units fit. Read our full guide: Two Detached ADUs in Los Altos, or the AB 956 overview for every city we serve.
Size, Height and Setbacks
Los Altos sets ADU sizes by bedroom count and, importantly, excludes ADUs from floor area ratio and lot coverage, so an ADU does not eat into what you can build on the main house.
| Detached ADU | Attached ADU | JADU | |
|---|---|---|---|
| Maximum size | 850 sq ft (1 bedroom) / 1,200 sq ft (2+ bedrooms); basements excluded | 850 / 1,200 sq ft, capped at 50% of the house, but 850 sq ft or less cannot be denied | 500 sq ft, including up to 150 sq ft added for an entry |
| Height | 18 ft | Greater of 16 ft or the zone height limit | Within the house |
| Side and rear setback | 4 ft | 4 ft | Existing walls |
| Front setback | Underlying zone setback, unless that would physically preclude the ADU | Same | — |
Buildings must be at least 5 feet apart, eaves may project up to 2 feet into setbacks, and stairs and porches must meet ADU setbacks, with porches limited to 20 sq ft. The 2024 update removed the daylight plane that used to apply to ADUs.
Parking
A new ADU needs one uncovered 9 by 18 foot space, which can be tandem, in the driveway or in the front setback. No space is required when the ADU is a conversion, replaces a garage or carport in the same spot, is within half a mile walking distance of transit, is in a historic district, is in an area with on-street permits not offered to the occupant, or is within one block of a car-share site. Converting a garage does not require replacement parking, and JADUs need no parking.
Design Rules and Protected Trees
Los Altos ADUs need a separate exterior entrance with a side-hinged door, their own bathroom (a JADU may share the house bathroom) and a full kitchen. Exterior lights cannot face neighbors, mechanical equipment cannot go on the roof, and equipment must sit on interior sides outside the setbacks. Stairs and porches must be architecturally integrated. ADUs on historic properties are still reviewed ministerially, against the City’s historic standards.
Trees often decide where an ADU can go. Los Altos protects trees 12 inches or more in diameter and native trees 10 inches or more, measured 48 inches above grade, plus heritage trees. A tree removal permit must be issued before the building permit, an arborist report is required for work in the inner two-thirds of a protected tree’s dripline, and plans must show tree protection fencing. We site the ADU, trenches and utility routes around trees from the start.
Los Altos Permit-Ready ADU Plans
Since February 2025, the City offers free Permit-Ready plans for detached ADUs in three sizes, 525, 854 and 1,190 sq ft, each in Mediterranean, Traditional or Contemporary style. The plans cannot be modified, and they are not available in flood zones other than X or D or on properties on the Historic Inventory. Several privately designed plans are also pre-approved by the City.
Permit-Ready plans save design time, but you still need a site plan, survey, utility design, Title 24 energy compliance and a contractor to build it. We build Permit-Ready plans as well as custom ADU designs, and help you decide which fits your lot.
The Los Altos ADU Permit Process, Step by Step
- Site assessment: lot survey, setbacks, protected trees, sewer route, electrical service and fire access.
- Design: Permit-Ready plan or custom design, with structural, Title 24 (CF-1R and QII) and CALGreen documentation.
- Civil items: ADUs over 750 sq ft need a civil and grading plan with erosion control and stormwater measures; projects within 50 feet of a creek follow the City’s stream guidelines.
- Submittal: the complete application is emailed to the Building Division. The City aims to check completeness within about 5 business days.
- Approval: ADUs are approved ministerially through a building permit only, with no planning permit, notice or hearing, and the City must act within 60 days of a complete application. Only solar and fire sprinklers may be deferred submittals.
- Construction and inspections: inspections are scheduled in eTRAKiT, and Los Altos requires all rough inspections to be ready together rather than in phases.
- Final: the ADU’s certificate of occupancy cannot issue before the main house’s.
City Hall, 1 N. San Antonio Road, Los Altos, CA 94022 · Building (650) 947-2752 · Planning (650) 947-2750
ADU permit applications are emailed to bl*******@********ca.gov (no hard copies). Inspections are scheduled online through eTRAKiT.
Utilities: Electrical, Sewer and Fire
Electrical: Los Altos allows the ADU to share the main house meter or have its own. Either way the ADU needs its own subpanel and a load calculation, and any new or relocated electrical service must be underground, with no waiver. On our 1,200 sq ft detached ADU, we upgraded to a 200-amp underground service and fed the ADU from the main panel. We explain the options in electrical service for a detached ADU in Los Altos.
Sewer: a detached ADU usually connects to the main sewer lateral downstream of the house; if gravity flow does not work, an ejector pump is designed as a deferred item.
Fire sprinklers: an ADU does not need sprinklers if they are not required for the main house. Fire review is triggered when the ADU is more than 200 feet from an access road or 600 feet from a hydrant, and the site must show adequate fire flow with a letter from Cal Water.
Fees: state law prohibits impact fees on ADUs of 750 sq ft or less; school district fees depend on size, and we confirm them with the district.
Renting, Ownership and Construction Rules
- Owner occupancy is not required for an ADU. Los Altos code requires owner occupancy and a recorded deed restriction for JADUs; state law has since narrowed these rules, so we confirm current requirements.
- Rentals must be at least 30 days; short-term rentals are prohibited.
- An ADU cannot be sold separately from the main house in Los Altos.
- Construction hours in single-family zones: 7 a.m. to 5:30 p.m. weekdays and 9 a.m. to 3 p.m. Saturdays, with no work on Sundays or City holidays.
- Wood-burning fireplaces are not allowed, air conditioner locations and noise ratings must be shown, and the ADU is addressed as unit “A”.
Our Los Altos ADU Projects
More ADU Resources
Nearby ADU guides: Los Altos Hills · Mountain View · Palo Alto · Cupertino · Sunnyvale
Plan your ADU: How to build an ADU · Garage conversion ADUs · ADU vs. home addition · Los Altos remodeling
Los Altos ADU FAQ
How big can an ADU be in Los Altos?
A detached ADU can be up to 850 sq ft with one bedroom or 1,200 sq ft with two or more. Attached ADUs follow the same sizes, capped at 50% of the house, though 850 sq ft is always allowed. JADUs are up to 500 sq ft.
Does an ADU count toward my floor area in Los Altos?
No. ADUs are excluded from floor area ratio and lot coverage.
How tall can a detached ADU be in Los Altos?
18 feet. Attached ADUs can be the greater of 16 feet or the zone height limit.
Can I have more than one ADU in Los Altos?
Yes. A single-family lot can have one attached ADU, one detached ADU and two JADUs.
Do neighbors get notice of my ADU?
No. ADUs are approved ministerially through a building permit, with no notice or hearing.
How long does ADU approval take in Los Altos?
The City must act within 60 days of a complete application.
Does Los Altos have pre-approved ADU plans?
Yes. Free Permit-Ready detached plans of 525, 854 and 1,190 sq ft, plus several privately designed pre-approved plans.
Do I need to replace parking if I convert my garage?
No. Replacement parking is not required for a garage conversion.
Does my ADU need fire sprinklers?
Not if sprinklers are not required for the main house, subject to fire access and water supply.
Can the ADU have its own electric meter?
Yes, or it can share the main meter. New or relocated services must be underground.