Hawaii Statewide ADU Authorization Act 39

Act 39, Session Laws of Hawaii 2024, codified at HRS Chapter 46, requires each county to adopt ordinances allowing a minimum of two ADUs on each residentially zoned lot, provided water and wastewater infrastructure is adequate. Counties may not impose owner-occupancy requirements, and minimum lot sizes for ADUs cannot exceed standards established by state law. Counties retain authority over design, setbacks, parking, and short-term rental prohibitions, but cannot prohibit ADUs outright in residential zones meeting baseline infrastructure tests. The law took effect July 1, 2024, with counties required to update codes.

Counties failing to comply face state administrative review; property owners denied permits in violation of Act 39 may seek mandamus or judicial review of denials.