California ADU rules · Santa Clara County
Can I Build an ADU in Los Altos Hills?
Yes — you can build an ADU in Los Altos Hills. California's statewide ADU law requires every city, including Los Altos Hills, to approve a qualifying accessory dwelling unit by right, plus a junior ADU (JADU) on a single-family lot. What varies city to city are the local specifics below.
Can you build one?
ADUs allowed
CA state lawYes — by right
ADUs and JADUs are allowed by right in all residential and mixed-use zones; ministerial approval, no hearing.
Units on a single-family lot
Local rule1 ADU (attached or detached) + 1 JADU, subject to water/sewer/septic adequacy
Units on a multifamily lot
CA state lawConversion ADUs (≥1, up to 25% of units) + up to 8 detached
Conversions of non-livable space (at least 1, or up to 25% of existing units) plus up to 8 detached ADUs on a lot with existing multifamily — never exceeding the number of existing units. JADUs are single-family only.
Junior ADU (JADU)
CA state lawAllowed
Up to 500 sq ft, within the walls of an existing single-family home (single-family lots only).
Size & height
Max detached ADU size
Local rule1,000 sq ft
Standard max: 850 sq ft (studio/1-bed), 1,000 sq ft (2+ beds); up to 1,200 sq ft if side/rear setbacks ≥30 ft and extra parking is provided
Max attached ADU size
Local rule1,000 sq ft or 50% of primary, whichever is less; at least 800 sq ft must be allowed; up to 1,200 sq ft with extra parking
Incentive max: 1,200 sq ft if extra parking is provided
Guaranteed minimum size
CA state law800 sq ft
An 800 sq ft ADU must be allowed regardless of FAR/lot-coverage caps; a city also cannot cap an attached ADU below 850 sq ft (1,000 for 2+ bedrooms).
Max JADU size
CA state law500 sq ft
Max height
Local rule16 ft
Detached: 16 ft standard; 18 ft near transit; 20 ft to match primary roof; up to 19 ft with 30 ft setbacks and extra parking; attached may match primary dwelling height
Setbacks
Side setback
CA state law4 ft
A city cannot require more than 4 ft side or rear setback; conversions of existing structures are exempt.
Rear setback
CA state law4 ft
Front setback
Local rule40 ft from property line adjacent to street/right-of-way
Separation from main house
CA state lawNone required
No statewide separation requirement between the ADU and the primary dwelling.
Parking
Parking required
CA state lawUp to 1 space
At most 1 space per ADU (or per bedroom, whichever is less) — and waived in the common cases below.
Transit parking exemption
CA state lawYes — no parking required near transit & for conversions
No parking may be required within ½ mi walking distance of transit, in a historic district, for an ADU within an existing structure, within one block of car-share, or in certain permit-parking areas.
Garage-conversion replacement parking
CA state lawNot required
When a garage/carport is demolished or converted to build an ADU, replacement parking cannot be required.
Approval, timeline & cost
Approval path
CA state lawMinisterial (by right)
No public hearing or discretionary design review for a qualifying ADU.
Approval timeline
CA state law60 days
The agency must act within 60 days of a complete application — and the ADU is deemed approved if it does not.
Owner-occupancy
CA state lawOnly for a JADU
No owner-occupancy requirement for ADUs (the prior sunset was removed). A JADU may require owner-occupancy, principally where it shares sanitation with the main house.
Impact-fee waiver
CA state lawWaived under 750 sq ft
An ADU of 750 sq ft or less pays no impact fees (the 750 sq ft threshold itself is exempt under § 66311.5); larger ADUs pay fees proportional to the primary dwelling. JADUs (≤ 500 sq ft) are exempt.
Permits required
Local ruleBuilding permit, Demolition permit (if demolishing existing structure)
Los Altos Hills-specific factors that can change the answer
State law sets the floor, but these local conditions often decide whether a specific parcel works.
Very High Fire Hazard Severity Zone (VHFHSZ)
Sprinklers required for ADU only if required for primary dwelling
Hillside/geotechnical hazard zones
Site development permit required for ADUs in geotechnical/seismic hazard zones, steep slopes (≥30%), or within 25 ft of creek bank
Heritage Oak critical root zone
Arborist report required if ADU is within critical root zone of a heritage oak
Los Altos Hills sets a lower standard max size for detached ADUs (1,000 sq ft for 2+ bedrooms) and requires a 40 ft front setback. Larger ADUs (up to 1,200 sq ft or 19 ft tall) are allowed only with increased setbacks and extra parking. Site development permits are required for ADUs in hillside or hazard areas.
Frequently asked questions
Can I build an ADU in Los Altos Hills?
Yes. California's statewide ADU law requires Los Altos Hills to approve a qualifying accessory dwelling unit ministerial (by right), plus a junior ADU (JADU) on a single-family lot.
How big can an ADU be in Los Altos Hills?
A detached ADU of at least 1,000 sq ft must be allowed, and an ADU of 800 sq ft can never be zoned out by floor-area or lot-coverage limits. A JADU can be up to 500 sq ft.
What are the setback requirements for an ADU in Los Altos Hills?
Side and rear setbacks are limited to 4 ft. 40 ft from property line adjacent to street/right-of-way.
Is parking required for an ADU in Los Altos Hills?
Up to 1 space. No parking can be required within ½ mile of transit, for conversions, or for units inside the primary home. When a garage is converted, replacement parking is not required.
How long does ADU approval take in Los Altos Hills?
A complete application must be acted on within 60 days, with ministerial (by right) — no public hearing or discretionary design review for a qualifying ADU.
Do I have to live on the property to build an ADU in Los Altos Hills?
Owner-occupancy: only for a jadu. ADUs under 750 sq ft are also exempt from impact fees.
Sources
- Los Altos Hills Municipal Code, Article 14 of Title 10 (Zoning and Site Development), last amended by Ord. 612, effective July 20, 2024
- § 10-1.1403(a)(1)
- § 10-1.1403(a)(2), § 10-1.1407(a)
- § 10-1.1403(a)(3), § 10-1.1407(b)
- § 10-1.1403(f), § 10-1.1407(a)(2), § 10-1.504
- § 10-1.1403(e)
- § 10-1.1408
- § 10-1.1403(b), § 10-1.1405
- § 10-1.1403(b)
- Cal. Gov. Code §§ 66310–66342 (ADU Law)
- HCD 2025 Accessory Dwelling Unit Handbook
- Cal. Gov. Code § 66317 (ministerial approval, 60-day deadline)
- Cal. Gov. Code § 66323 (by-right ADU/JADU, 800 sq ft, 4-ft setbacks)
- Cal. Gov. Code § 66333 (junior ADUs)
- Cal. Gov. Code § 66321 (size minimums & height)
- Cal. Gov. Code § 66322 (ADU parking; § 66334 covers JADU parking)
- Cal. Gov. Code § 66314 (local ordinance standards)
- Cal. Gov. Code § 66315 (owner-occupancy)
- Cal. Gov. Code § 66311.5 (impact fees: none for an ADU ≤ 750 sq ft or JADU ≤ 500 sq ft)
Last updated 2026-06-25. This is an AI-assisted summary of Los Altos Hills's ADU rules, drawn from its published ordinance and California ADU law — general information, not legal advice, and it may be incomplete or out of date. Verify the controlling rule for your specific parcel with Los Altos Hills Planning before relying on it.
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