California ADU Law: A Homeowner’s Guide

Explore California’s recent ADU law changes, what is already in effect, and what starts in 2027. Understand the potential benefits before discussing a property-specific plan for your Los Altos or South Bay ADU.

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By Breakthrough Builders · Sources checked October 15, 2026

California’s ADU rules can open up options for a separate home for family or potential rental use. The practical question is which rules apply to your property, your design, and the date you apply. This guide focuses on recent changes relevant to homeowners considering an ADU in Los Altos and the surrounding South Bay.

In effect since January 1, 2026

What changed for ADUs and JADUs in 2026?

Earlier feedback on whether an application is complete

SB 543 requires a written completeness determination within 15 business days. An incomplete application must be accompanied by an explanation of what is missing and how to complete it. This helps identify missing documents earlier; it is not a promise of a building permit in 15 days.

A clearer impact-fee threshold

ADUs with 750 square feet of interior livable space or less are exempt from impact fees covered by the ADU statute. JADUs with 500 square feet or less receive that exemption too. Interior livable space matters when comparing designs. Permit fees, utility charges, and school fees have separate rules, so an impact-fee exemption does not make the entire permit process free.

Source: SB 543, Chapter 520 (2025) and HCD’s ADU Handbook.

More flexibility for a JADU with its own bathroom

A junior accessory dwelling unit is a smaller unit created within a proposed or existing single-family residence. Under AB 1154, the JADU owner-occupancy requirement applies when sanitation facilities are shared with the main home. A JADU with separate sanitation facilities is exempt from that requirement. JADU rentals must be for terms longer than 30 days.

Why it matters: Bathroom configuration can affect how the property may be used. Compare that choice during design, rather than assuming every JADU has the same occupancy conditions.

Source: AB 1154, Chapter 507 (2025).

Signed in 2026 · Effective January 1, 2027

California ADU law changes coming in 2027

AB 956 and SB 1117 were signed on September 29, 2026. These changes are scheduled to take effect January 1, 2027 under the state’s regular-session effective-date rule. They are future provisions as of this guide’s October 2026 review. See the Secretary of State’s effective-date explanation.

AB 956: A pathway for two detached ADUs

AB 956 expands the state-exempt approval pathway to two detached, newly constructed ADUs on a lot with a proposed or existing single-family dwelling, subject to the statute’s conditions. The local agency may impose an 800-square-foot interior livable-space cap per unit and the specified height limits. Side and rear setback requirements under this pathway cannot exceed four feet.

The agency is not required to approve a JADU ministerially on the same lot when those two detached new-construction ADUs are built under this provision. Two ADUs plus a JADU is therefore not a blanket entitlement.

Potential benefit: A qualifying homeowner may be able to explore two separate living spaces. Access, utilities, code compliance, layout, and construction cost still need a property-specific assessment.

Source: AB 956, Chapter 791 (2026), Government Code section 66323.

SB 1117: A smaller impact-fee calculation for larger ADUs

For up to two ADUs on a lot, SB 1117 requires impact fees on an ADU larger than 750 square feet to be calculated proportionately to the primary dwelling and only on the interior livable area above 750 square feet. The exemption for units at or below 750 square feet remains.

For example, a 1,000-square-foot ADU would use the 250 square feet above the threshold in that proportional calculation. That is an illustration of the area used, not a dollar estimate or an exemption from every charge. Properties with more than two ADUs retain a different proportional rule for units above the threshold.

Potential benefit: A design that needs an additional bedroom or more accessible circulation may have a lower covered impact-fee calculation than under the current rule. Request a written fee breakdown before finalizing a budget.

Source: SB 1117, Chapter 809 (2026), Government Code section 66311.5.

What could these changes mean for your property?

The best ADU plan starts with how the space will be used. A family home, a long-term rental, and a compact JADU can lead to different decisions about privacy, bathrooms, access, and size. Rental income and property-value gains are possibilities, not guaranteed outcomes.

Before committing to a layout, work through these questions:

  • Which jurisdiction handles the permit? Los Altos and Los Altos Hills are separate permitting authorities. Confirm the address and current local application requirements.
  • What already exists? Identify existing dwellings, ADUs, garages, recorded restrictions, and previous permits.
  • What can the site support? Review a site plan, setbacks, access, utility routes, drainage, and any slope, fire, or other site constraints with the appropriate professionals.
  • Which costs are included? Separate design, permits, impact fees, utilities, site work, and construction in the budget.
  • Which rules apply when you submit? Ask the permitting authority how the effective dates relate to your application. Do not delay a viable project solely on an assumed fee saving.

Discuss your ADU options with Breakthrough Builders

Tell us your property location, who the ADU is for, and whether you are considering a new detached unit, an attached ADU, or a garage conversion. We can discuss planning and construction steps around your goals.

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Official ADU resources

This is general planning information, not a parcel-specific legal opinion. Confirm current requirements with your permitting authority and obtain legal or tax advice for decisions that require it. This guide covers selected recent changes rather than every California ADU provision.

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