ADU rules in Honolulu

A detached accessory dwelling unit is allowed as an accessory use subject to standards ("Ac*" in Table 21-5.1) in all five residential zoning districts (R-20, R-10, R-7.5, R-5, R-3.5) and in the Country district, and ROH § 21-5.50-3(a)(1)(D)(i) expressly lets you create one by "building a new structure ... detached from the principal dwelling unit but located on the same zoning lot." Only one ADU is permitted per zoning lot, but an otherwise-permitted ohana unit does not count against that limit, so a lot can carry a principal dwelling, an ADU, and an ohana unit. Ordinance 25-2 (approved 1-3-2025, ADU provisions effective 9-30-2025) deleted the old 3,500 sq ft minimum lot area and the old bar on lots that already held more than one dwelling unit: an ADU is now allowed even on a zoning lot with existing multiple dwelling units. ADUs are still barred on landlocked lots and on lots in planned development housing or cluster housing, are not allowed at all in the agricultural (AG-1, AG-2), preservation (P-2), apartment, apartment mixed-use, resort, business, or industrial districts, and are permitted only where wastewater, water, roadway, and first-responder capacity is sufficient.

The same section treats attached and internal units identically: ROH § 21-5.50-3(a)(1)(D) allows an ADU created either by building a new structure "attached to, such as a two-unit dwelling, or detached from the principal dwelling unit," or by "converting a legally established structure ..., attic, or basement." The LUO's definition of accessory dwelling unit (§ 21-10.1) likewise covers a dwelling unit with its own kitchen, bedroom, and bathroom "attached or detached from the principal dwelling unit on the zoning lot." Attached and internal ADUs carry the same floor-area caps, the same recorded-covenant condition, and the same one-per-lot limit as detached ones. Converting an existing accessory structure built before 9-14-2015 that exceeds the floor-area cap or cannot meet parking requires a zoning adjustment under ROH § 21-2.140-1(n).

ROH § 21-5.50-3(a)(1)(B) sets the total floor area of an ADU at no more than 500 sq ft for zoning lots up to 4,999 sq ft and 1,000 sq ft for lots of 5,000 sq ft or more. These caps replaced the 400 and 800 sq ft caps of Ordinance 15-41 when Ord. 25-2 took effect on 9-30-2025, and unlike the old rule there is no minimum lot area, so a lot smaller than the former 3,500 sq ft floor can now host a 500 sq ft ADU. The cap is not the only size constraint: § 21-5.50-3(a)(1)(C) requires the ADU to meet all development standards for the principal use in the underlying district, which in the residential districts means the whole lot stays within a 0.7 floor area ratio, 50 percent maximum building area, 5-foot side and rear yards, a 10-foot front yard, and the 25-to-30-foot height envelope of Table 21-3.2 and § 21-3.70-1(c). A zoning adjustment under § 21-2.140-1(n) can let a legally established accessory structure built before 9-14-2015 exceed the floor-area cap when the director finds the floor area cannot practicably be reduced.

Honolulu dropped its ADU owner-occupancy rule. ROH § 21-5.50-3(a) as enacted by Ordinance 25-2 (effective 9-30-2025) contains no requirement that the owner live on the property, and the mandatory recorded covenant in § 21-5.50-3(a)(2)(A) lists only four minimum terms: long-term residential occupancy only, deed restrictions lapse on removal of the unit, the covenants bind heirs and successors, and no condominium property regime may separate ownership of the ADU from the principal dwelling. Nothing about occupancy by the owner appears there. Under the prior Ordinance 15-41 regime, DPP's ADU Quick Guide listed as a basic requirement that "either the main dwelling or the ADU will be occupied by the property owner(s), the owner's family, or a designated authorized representative"; the LUO now keeps only a vestigial § 21-10.1 definition of "Designated Authorized Representative" with no operative section using it. Two cautions: the covenant "may contain such terms as the director deems necessary to ensure its enforceability," so confirm the form DPP is currently approving, and several DPP handouts still describe the repealed rules.

ROH § 21-6.30(b) requires one off-street parking space for an ADU on top of the parking required for the primary dwelling, except for ADUs within 0.5 miles of a rail station, where none is required. There is a second waiver: on lots in the Primary Urban Center or Ewa development plan areas, the ADU space is waived if the lot is within 800 feet of a city bus stop, but that waiver is unavailable if it has already been used for an ohana unit on the same lot. The broader parking exemptions in § 21-6.20(a) (no off-street parking in the Primary Urban Center and Ewa plan areas, or within 0.5 miles of an existing or future rail station, or in a transit-oriented development special district) expressly do not reach residential, agricultural, and preservation zoning districts inside those plan areas, which is why the ADU-specific rule in § 21-6.30(b) governs most house lots. A zoning adjustment under § 21-2.140-1(n) can waive the space for conversion of a pre-9-14-2015 accessory structure when no feasible alternative parking site exists.

An accessory dwelling unit is an accessory use subject to standards, shown as "Ac*" in Table 21-5.1 for the Country and residential districts with a pointer to § 21-5.50-3(a); there is no conditional use permit, planning commission review, or public hearing. Review is a building permit, and ROH § 18-5.8 gives the building official 60 calendar days to notify the applicant whether the application is complete and to approve or deny it, after which the permit is deemed approved (time while an incomplete application sits with the applicant is excluded). Before the permit issues, the owner must record the ADU covenant with the Bureau of Conveyances or Land Court. Applications go through DPP's HNL Build portal; a discretionary step appears only in two situations, a zoning adjustment under § 21-2.140-1(n) for converting a pre-9-14-2015 accessory structure that busts the size cap or the parking rule, and a special management area permit if the lot is in the coastal SMA under ROH Chapter 25.

The recorded ADU covenant required by ROH § 21-5.50-3(a)(2)(A) must state that the unit "may only be used for long-term residential occupancy and may not be used as a bed and breakfast home or transient vacation unit," and that the owner may not submit the lot to a condominium property regime to separate ownership of the ADU from the principal dwelling, so the ADU can be rented long term but cannot be short-term rented or sold off separately. The LUO defines transient occupancy as less than 90 consecutive days (§ 21-10.1), but DPP's Short-Term Rentals page, updated 7-29-2026, states the department "will continue to enforce STRs as less than 30 consecutive days across the board"; either way, short-term rental is confined to resort-zoned areas and a few mapped apartment-zoned areas, and no new nonconforming use certificates are being issued. Long-term rental is also protected from the other direction by HRS § 46-4.8(h), which bars any county from prohibiting the use of a dwelling unit on a residentially zoned lot as a separately leased long-term rental. Renting rooms rather than the whole unit is separately regulated: § 21-5.50-3(d) allows no more than three roomers, for 90 consecutive days or more, in the same dwelling unit as the owner or operator.