ADU rules in Hawai’i County

A detached backyard unit is allowed. Ordinance 24-70 (Bill 123), passed by the County Council 5 to 3 on 9-17-2024 and filed with the County Clerk on 10-7-2024, retired the old ohana dwelling permit and rewrote HCC chapter 25, article 6, division 3 as 'Accessory Dwelling Units'. Section 25-6-31(a) permits ADUs on a building site in the RS (single-family residential), RD (double-family residential), RA (residential and agricultural), FA (family agricultural) and A (agricultural) districts, and section 25-1-5 defines an ADU as a unit that 'can be detached from or attached to an existing residence', so nothing turns on the form. There is no minimum lot size, no family-relationship test and no hearing. Two limits bite: section 25-6-32 bars ADUs outright on building sites in the State land use conservation district, on sites developed under an affordable housing project granted Code preemptions, and where a change of zone ordinance specifically prohibits a second unit; and section 25-6-31(b) subjects any building site inside the State land use agricultural district (a very large share of Big Island house lots) to the county's farm dwelling requirements.

Attached and internal units are explicitly allowed on the same terms as detached ones. The section 25-1-5 definition covers 'a structure or portion thereof' that 'can be detached from or attached to an existing residence', which reaches a basement or interior conversion as well as an addition, and section 25-6-35(b) says in so many words that 'An accessory dwelling unit and a single-family dwelling unit may be constructed as a duplex (i.e., there is a common wall or floor/ceiling).' The only defining feature is that the unit contain one kitchen. Setbacks are the ordinary district yards, except that a lot that is nonconforming as to size uses the yards of the zoning appropriate to its size (section 25-6-35(a)).

Section 25-6-34 sets a flat cap: an ADU 'shall not exceed one thousand two hundred and fifty square feet in living area, exclusive of any lanai, garage, or carport', and may not exceed the height limit of the zoning district it sits in (35 feet in the RS district under section 25-5-4). There is no percent-of-primary-dwelling formula and no per-zone size split, so a 1,250 sq ft unit is available on a small RS-7.5 lot and on a five-acre A-5a lot alike. The cap is per unit, not per site, so a building site with the maximum three ADUs can hold 3,750 sq ft of accessory living area. Note that lanai, garage and carport area is excluded from the count, which is unusually generous.

Nothing in HCC chapter 25, article 6, division 3 requires the owner to live on the property, either at application or afterward. Ordinance 24-70 repealed sections 25-6-39.1 through 25-6-39.7 outright, which is where the old ohana dwelling permit apparatus lived, and the surviving sections 25-6-30 through 25-6-39 speak only to utilities, districts, size, yards, density, parking, transient use and covenants. HRS 46-4.8(h) independently bars any county from prohibiting the use of a dwelling unit on a residentially zoned lot as a separately leased long-term rental. Three occupancy rules do exist elsewhere and are easy to confuse with owner occupancy, but all three attach to renting to visitors rather than to owning the ADU: a bed and breakfast operator must reside on the same building site (section 25-4-7(b)(2)); a short-term vacation rental owner or reachable person must reside in the County of Hawai'i, which is a countywide and not an on-site test (section 25-4-16(c)(1)); and since 9-1-2026 a transient vacation rental only counts as 'hosted' under Ordinance 25-50, at the lower registration fee, if the host's principal home is on the same property and the host lives there while the unit is rented.

Section 25-6-37 provides only that 'The number of parking spaces for an accessory dwelling unit shall be as provided under section 25-4-51.' Section 25-4-51 has no line item for accessory dwelling units; its alphabetical use table jumps from 'Agricultural tourism' to 'Bed and breakfast establishments', so an ADU falls under 25-4-51(a)(7), 'Dwellings, single-family and double-family or duplex: two for each dwelling unit'. That works out to two spaces for each ADU, on top of the two required for the primary dwelling. There is no transit-proximity or small-unit reduction, and section 25-4-51(a)(8) sets the same two-space figure for a dwelling unit rented as a whole for periods under 180 days. The absence of an ADU line in 25-4-51 is a drafting gap left by Ord. 24-70 and Ord. 25-66; confirm the count with the Planning Department before designing to it.

There is no ADU permit any more. Ordinance 24-70 abolished the ohana dwelling permit, and the only procedural requirement left in the ADU division is section 25-6-31(c), 'Accessory dwelling units shall have a final inspection before use as a dwelling', so the path is an ordinary building permit from the Department of Public Works Building Division plus the section 25-6-30 utility showings (an approved sewage disposal system and a public or private water system, catchment system or well). Section 25-2-71(a) exempts the RS, RA, FA, A and IA districts from plan approval entirely. The one exception is the RD district, where section 25-5-27(c) requires plan approval for all new buildings except one single-family dwelling and accessory buildings per lot; plan approval is still an administrative director decision with no hearing, and section 25-2-76(d) deems it approved if the director does not act within thirty days. Section 25-2-71(h) also imposes plan approval inside any special district that has adopted design guidelines, which today means the Kailua Village, Downtown Hilo (CDH), University (UNV), Pahoa Village (PVD) and Downtown Pahoa Commercial (DPC) districts of chapter 25, article 7, though ADUs are not a permitted use in the CDH, UNV or DPC districts in the first place.

Long-term renting an ADU is unrestricted: no owner occupancy, no family requirement and no lease minimum in chapter 25, and HRS 46-4.8(h) forbids the county from banning separately leased long-term rentals on residentially zoned lots. Renting for short stays now clears two separate hurdles. The zoning hurdle is unchanged: section 25-6-38 allows 'No more than one accessory dwelling unit ... for use as a transient accommodation rental, provided there are no other transient accommodation rentals on the building site', and that permission only matters where the district itself allows the use, section 25-4-16(a)(1) permitting short-term vacation rentals in the V, CG and CV districts, in residential and commercial districts inside a General Plan Resort or Resort Node area, and in RM condominium units, while the RA, FA and A permitted-use lists (sections 25-5-52, 25-5-62 and 25-5-72) omit them entirely; outside those areas a vacation rental can operate only under a nonconforming use certificate for a use established before 4-1-2019 (section 25-4-16.1), and every short-term vacation rental must register with the planning director for a one-time $500 fee (section 25-4-16(b)). The registration hurdle is new: Ordinance 25-50 took effect on 9-1-2026 after two postponements (Ord. 25-92 moved it to 7-1-2026, Ord. 26-51 to 9-1-2026), and no transient vacation rental, hosted or un-hosted, may now operate until the owner registers it and pays $250 hosted or $500 un-hosted, renewed annually at $100 or $250, with a separate $1,000 fee for booking platforms. Existing short-term vacation rental registrants, nonconforming use certificate holders and bed and breakfasts operating under a county use permit or special permit are deemed registered; the group newly swept in is the hosted rental, which is exactly the case of renting out an ADU on the lot where the owner lives. Registration is not a permit and legalizes nothing: it does not change zoning, so a unit outside the districts above still cannot be rented to visitors.