California's 2026 ADU Laws Explained: Every New Rule That Changes What You Can Build

California's 2026 legislative session produced the most significant overhaul of ADU and housing law in years. More than a dozen bills that directly affect ADU permitting, processing, disaster recovery, and enforcement went into effect on January 1, 2026, with a few already active since October 2025.

SB 543: The 15-Day Completeness Clock Is Now Law. Effective January 1, 2026, SB 543 requires every local agency to issue a completeness determination within 15 days of receiving an ADU application. If they miss that deadline, the application is automatically deemed complete. Local agencies must also provide a written list of any missing items, and on resubmittal, they can only review items they previously flagged.

SB 543 also clarifies that the 800-square-foot size limit for 'state exempt' detached ADUs refers to interior livable space only. Exterior walls and stairs are excluded from the calculation. SB 543 also exempts ADUs and JADUs under 500 square feet from school impact fees.

AB 462: You Can Get a Certificate of Occupancy Before Your Primary Home Is Rebuilt. Under prior law, a local agency could not issue a Certificate of Occupancy for a detached ADU until the primary dwelling on the same lot received its own. AB 462 reverses that rule for any county subject to a state emergency proclamation issued on or after February 1, 2025.

SB 9 (2025) deals specifically with enforcement of state ADU law against noncompliant local ordinances. If a local agency fails to submit a new or amended ADU ordinance to HCD within 60 days of adoption, or fails to respond to HCD's deficiency findings within 30 days, that ordinance is null and void as a matter of law.

AB 1154 changes owner-occupancy requirements on Junior ADUs. Effective January 1, 2026, owner-occupancy requirements on JADUs only apply if the JADU shares sanitation facilities with the primary dwelling. If a JADU has its own private bathroom, local agencies can no longer require the property owner to reside on-site.

AB 818 requires cities and counties to approve or deny a complete application within 10 calendar days when the structure is a state-approved modular home intended to house a person until their damaged property is rebuilt. ADORE Homes units are HCD state-approved modular homes and qualify for this 10-day permit clock.

These new laws represent a dramatic shift in California's approach to ADU development. At ADORE Homes, our HCD-approved modular process is built to operate inside this new framework from day one, giving homeowners faster permitting, stronger protections, and more flexible housing options than ever before.