What Hawaii can learn from California’s long and bumpy road to ADU stardom

California's decade-long effort to expand Accessory Dwelling Unit (ADU) development has produced remarkable results that offer lessons for other states grappling with housing shortages.

The state issued nearly 28,000 ADU permits in 2023, representing 21% of all housing permits for the year, demonstrating how streamlined regulations can unlock significant housing supply even when overall construction remains sluggish.

California's success began with 2016 legislation requiring local governments to allow ADUs in all single-family zones, but the real breakthrough came through subsequent reforms that systematically eliminated local restrictions.

State lawmakers have spent years creating exemptions from burdensome local development standards that initially limited ADU construction, resulting in an explosion of production that shows no signs of slowing.

For California property owners, ADUs represent an increasingly viable option to generate rental income or house family members, while developers are finding new opportunities in the accessory unit market.

The California model is now being studied by other states, including Hawaii, which recently passed legislation requiring counties to allow at least two ADUs on single-family lots.

However, Hawaii's law still permits local governments to impose restrictive development standards, potentially limiting its effectiveness.

California's experience suggests that initial ADU legislation often produces disappointing results until states follow up with stronger protections against local obstruction, making ongoing regulatory refinement crucial for success.