Honolulu County, HI Accessory Structures: ADU Rules (2026)

50-3 screens both the lot and the unit before an ADU can be approved on Oahu. Two kinds of lots are disqualified outright. ADUs are not permitted on zoning lots in planned development housing or cluster housing, and they are not permitted on zoning lots that are landlocked. No design change fixes either condition; an ineligible lot simply cannot host an ADU. On an eligible lot, the unit's size is tied to the lot's size by a two-step scale. A zoning lot of up to 4,999 square feet may add an accessory dwelling unit with total floor area of no more than 500 square feet.

A zoning lot of 5,000 square feet or more may go up to 1,000 square feet. The cap applies to total floor area, so the whole unit counts against the number, and crossing the 5,000 square foot lot threshold doubles the allowance. Size and lot eligibility are not the whole test. The ADU must also meet all development standards of the underlying zoning district, and approval depends on infrastructure adequate to serve the added unit: wastewater capacity, water supply, roadways, and first-responder access all have to be sufficient. A lot that clears the size math can still fail on infrastructure.