ADU rules in Honolulu County, HI — also called accessory dwelling unit regulations or granny flat ordinances — cover setbacks, owner-occupancy, parking, and permit requirements.
Honolulu County allows one ADU per zoning lot in Country, R-3.5, R-5, R-7.5, R-10, and R-20 residential districts, subject to density, parking, occupancy, and rental-duration requirements under the Land Use Ordinance.
ROH Sec. 21-5.720 governs accessory dwelling units islandwide on Oahu. A single ADU is permitted per qualifying zoning lot in Country and most low-density residential districts. The ordinance sets minimum lot size, building-separation, off-street parking, and owner-occupancy standards, and restricts short-term rentals of less than 30 days. Ordinance 25-2 (2025) amended the section to refine permit and advertising rules. After-the-fact permits are available for qualifying pre-existing units built before September 14, 2015. Separate building permits under ROH Sec. 18-3.2 are required for construction, conversion, or addition of habitable space.
Unpermitted ADUs, illegal short-term rental under 30 days, advertising a non-permitted unit, or exceeding one ADU per lot can trigger notices of violation and daily fines under Chapter 21.
Honolulu County, HI
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See how Honolulu County's adu rules rules stack up against other locations.
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