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All ADU Regulations

Los Altos Hills, California

Los Altos Hills has updated its ADU building codes for new state laws. We’ll help you build a custom ADU to fit your property characteristics & local regulations!
Updated Jul 11, 2023
Los Altos Hills has adopted an ADU ordinance that is in line with state laws, making it easier to get your ADU project up and running! Keep reading for a quick look at the requirements for building an ADU in Los Altos Hills.

General ADU requirements

The smallest ADU you can build in California is 150 sq.ft.
There is no minimum lot size requirement to build an ADU in Los Altos Hill. Single-story ADUs (detached, attached, conversion) and Junior ADUs (JADUs) can be built wherever permitted. However, JADUs must be converted out of existing living space.
ADUs must be equipped for independent living and have permanent facilities for:
  • Living
  • Eating
  • Sleeping
  • Sanitation
  • Cooking
  • Exterior access that is separate from the primary dwelling
ADU kitchens must also have:
  • A sink with running water and a maximum waste line diameter of 1.5 inches
  • Countertops and cabinetry of a reasonable size
  • A refrigerator
  • A permanent range/cooktop that doesn’t need more than 120V or natural/propane gas
JADUs may share bathroom/sanitation facilities with the primary dwelling, but they must have their own entrance that is separate from the primary dwelling.


All properties zoned in the R-A (Residential Agricultural) District with an existing single-family house are eligible to build an ADU.


Building your Los Altos Hills ADU comes with the following fees:
  • Planning fees - $1-$2 per sq. ft.
  • Building fees - $4,000-$5,000 per sq. ft.
  • School fees - $4 per sq. ft. of accessible space
  • Impact fees
Fee amounts depend on several factors, such as the scope/complexity of your project and the exact location, but our team can speed up the permitting process and get you discounted specialist rates!

Number of units

Generally, properties with existing single-family homes are allowed to have one ADU and/or one JADU on them. With multi-family properties, this number depends on the types of ADUs being constructed and the number of existing dwelling units.

Building size

The maximum square footage allowed for your ADU is determined by your lot size, zoning regulations, and ADU type:
  • JADUs without on-site parking may be up to 500 sq. ft.
  • JADUs with an on-site parking space can be up to 650 sq. ft.
  • All eligible properties are allowed to build an 800 sq. ft ADU
  • Attached ADUs may be up to 1,000 sq. ft. or 50% of the primary home’s living area (whichever is less) but no less than 800 sq. ft.
  • Studio and one-bedroom ADUs may be up to 850 sq. ft. and two-bedroom ADUs may be up to 1,000 sq. ft.
  • If an ADU can accommodate 30 ft. side and rear yard setbacks and one on-site parking space, it may be up to 1,200 sq. ft.

Design requirements

Detached ADUs must share the following similarities with the primary dwelling:
  • Exterior wall materials
  • Roof forms
  • Window types
  • Trims
An attached ADU and its primary dwelling must look like one cohesive structure and should share the following similarities:
  • Building forms
  • Colors
  • Window styles and trims
  • Door trims
All exterior lighting should be downward facing and/or shrouded. Exterior wall finishes and roofing should have low to mid-light reflectivity values (maximum LRV of 50 and 40, respectively).

Building height

New detached ADUs may be up to 16 ft tall (one-story). If it provides side and rear setbacks of 30 ft and an on-site parking space, they may be up to 19 ft.
Attached ADUs can reach the same height as the main dwelling, assuming they comply with municipal code (including daylight plane).


New detached ADUs need to maintain a minimum of 40 ft from the street frontage, setbacks of 4 ft from the side and rear lot lines, and 6 ft from any existing structures. Attached ADUs should also maintain rear and side setbacks of 4 ft.
ADUs with side and rear setbacks of 30 ft may be eligible for additional floor area and building height.
While there are no specific setback requirements for JADUs and conversion ADUs, they should be sufficiently setback for fire safety and comply with zoning codes and site development standards.


New ADUs need their own on-site parking space (in addition to what is required for the primary residence’s parking) according to the following conditions:
  • The parking space may be covered or uncovered and shall measure 10 ft by 20 ft
  • The space may be provided as tandem parking (one in front of the other) on a driveway or in a setback area unless the planning director can prove it is not feasible or impedes fire and/or life safety
  • The parking space shall not encroach into a required fire truck turnaround or obstruct the driveway access to the primary dwelling
New ADUs may use the same driveway as the primary dwelling and need not build a new driveway on the property. Additional parking is not needed when:
  • The ADU is in a historically or architecturally significant area
  • The ADU was converted from the existing floor area of the primary dwelling or existing accessory structure
  • The ADU is located within walking distance (one-half mile) of a public transit stop
No additional parking is required for JADUs, conversion ADUs, or the replacement of an existing garage or carport. ADUs are exempt from the parking requirement if they are within half a mile of public transportation or a car-share vehicle.

Scenic Views and Traffic

The inclusion of an ADU should not block prominent scenery from the view of primary dwellings, or result in the removal or damage of a Heritage Oak. They also may not interrupt the flow and safety of street traffic, or be within a recorded easement.
New ADUs must also not be within 25 ft of the top of a creekbank, or be located on a slope that exceeds 30%.

Fire safety

ADUs can meet the same fire safety requirements as the primary dwelling, or implement alternative measures if exact requirements aren’t specified.

Utility Connections

All ADUs should have at least one bathroom. Lots should be connected to either the public sanitary sewer system, or a private sewage disposal system. Private systems should have an adequate water supply and be approved by the Santa Clara County Division of Health Services.


Owner occupancy is not a requirement for permitting an ADU from January 1, 2020, until January 1, 2025. This is a great opportunity for rental property owners to increase their yields in a cost-effective way.
After the stated period, owners may be required to live on the property only if a JADU is built on it.
ADUs cannot be sold separately from the primary dwelling.


While long-term rentals are permitted for ADUs, homeowners may be required to sign a deed restriction preventing short-term rentals. The minimum rental period for ADUs is 31 days.


Do you have a utility easement over your property? Do you have special setbacks? Reach out to us and we’ll help you build a custom ADU to fit your unique property characteristics and local regulations!


How much does an ADU increase property taxes in Los Altos Hills?

You can expect the value of your property to increase after you build your ADU. Since property taxes are often based on property values, this can increase your property taxes. However, any increases will only be based on the value of the ADU itself. Building an ADU will not trigger a reassessment of the value of your main home.

How many bedrooms can an ADU have in Los Altos Hills?

If your ADU is under 850 sq. ft., it can have up to one bedroom. If your ADU is larger than 850 sq. ft., it can have up to two bedrooms.

Can I build 2 ADUs on my property in Los Altos Hills?

Yes, single-family homes may have up to one ADU and/or one JADU on the property. This number of ADUs allowed for multi-family homes may vary depending on a number of factors.

How long does it take to build an ADU in Los Altos Hills?

The entire process, from start to finish, would depend on the type of ADU you’re building, the scope of your project, and the area you’re building in. New ADU projects may take 4-6 months to design and permit, and an additional 4-6 months to build.

Can I rent my ADU in Los Altos Hills?

Yes, renting out your ADU can be a great way to earn an additional income. While short-term rentals of less than 30 days are not allowed, you may rent out your ADU for a minimum period of 31 days at a time.

How many square feet can an ADU be in Los Altos Hills?

Depending on zoning conditions, setbacks and the number of bedrooms you are hoping to include, you may be allowed to build an ADU that has a maximum area of 1,200 sq. ft.

Want to learn more about ADU regulations in your area?

Book a free consultation and estimate with a Cottage ADU Expert to find out what’s possible on your property!