Newsom signs SB 543, making ADU reforms law in California

Governor Gavin Newsom formally signed Senate Bill 543 into law on October 10, 2025, giving the state’s ADU and Junior ADU reforms legal force. The law codifies key rules from the Department of Housing and Community Development (HCD), such as requiring local governments to conduct an initial review of ADU and JADU permit applications within 15 business days and applying the same standards to both types of units (California Legislature, 2025)

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“California needs to build housing to help make our state more affordable. And one of the most effective ways to address our housing affordability crisis is to accelerate the construction of low-cost housing, like ADUs and junior ADUs,” Senator McNerney said at the signing ceremony (Senate press release, 2025).

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The new law takes effect on January 1, 2026, and is expected to bring consistency to local ADU permitting practices across California.

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Why this matters

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California faces a massive housing shortfall. According to HCD data, HCD has estimated the state needs 180,000 new homes a year, far above the average of roughly 80,000 produced annually in recent years. SB 543 is designed to reduce regulatory ambiguity and accelerate permit approvals so more ADUs/JADUs can be built, especially in regions where local agencies have struggled to interpret prior guidance.

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By codifying ADU rules, including the 15-day review requirement and consistent treatment of JADUs, the new law reduces room for cities to delay or complicate the permitting process. In doing so, it aims to expand housing options for homeowners, renters, and those seeking multigenerational units.

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Timeline & trends since signing

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• February 2025: SB 543 is introduced in the Senate.  
• August 29, 2025: The Assembly approves the bill unanimously.  
• October 10, 2025: Governor Newsom signs SB 543 into law.  
• January 1, 2026: The law becomes effective statewide, replacing prior guidance with enforceable requirements.
• Trend to watch: Local jurisdictions that were previously inconsistent in applying ADU rules will now need to align their ordinances.

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Quick tip: If you’re planning to build an ADU or JADU in 2026 or later, check with your city’s planning department to confirm how the 15-day review rule and new processing standards will affect your timeline. Under SB 543, cities must now follow state-mandated timeframes—helping homeowners move from application to construction with fewer delays.

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Xavier Rodriguez is the CEO of ADU Geeks, San Diego’s leading ADU consulting and project management firm. In 2024, the company was ranked No. 1 on the San Diego Business Journal’s Fastest Growing Private Companies list in the Small Business category, achieving 594.3% revenue growth.