In plain words

Hawaii has a state law that pushes counties to allow more ADUs. An ADU (accessory dwelling unit) is a second, smaller home on the same lot as a house. The main law is Act 39, signed in May 2024. It tells all four counties to update their zoning by December 31, 2026. Zoning means the local rules about what can be built where. The new rules must allow at least two ADUs, or something close to it, on residential lots. The law only covers the state's urban district, which is land the state has set aside for towns and cities. Within it, only lots zoned mainly for houses and duplexes count. It skips rural and low-density zones, farm land and protected coastal zones. It also skips places a county marks as high risk for floods, lava or fire. Counties still set size, height, parking and setbacks. A setback is the required distance between a building and the property line. Counties can also say no if water or sewer lines can't handle more homes. Approval is not 'by right,' which would mean automatic once you meet the written rules. If a county misses the 2026 deadline, it can't refuse up to two ADUs just because of the unit count. It can still refuse for infrastructure, design or building standards. Private covenants are rules written into a deed or set by an HOA. New ones recorded after May 28, 2024 can't cap ADUs below the state number or ban long-term renting. Older covenants still apply. Maui updated its rules in July 2026. Honolulu still lists one ADU per lot and requires the owner to live on site. You still need permits, and your lot must work for an ADU.

The rules at a glance

State ADU lawYes
Where the state rule appliesAll four counties (City and County of Honolulu, Hawaii County, Maui County, Kauai County). The two-ADU rule covers only lots in a county zoning district mainly reserved for single-family and two-family detached homes, inside the State Land Use Urban District. It does not apply to rural, low-density residential or open-space zones, land outside the urban district, county powers in coastal Special Management Areas, or urban areas a county deems at high risk of flooding, lava or fire based on current federal or state maps.
Approved without a hearing?No. Towns can still require hearings or special permits.
ADUs per lotAt least 2 ADUs (or the reasonable equivalent) per residentially zoned urban-district lot, through county ordinances due by 2026-12-31; counties may allow more
Owner living on siteTowns may require the owner to live on the property.
ParkingState law doesn’t set this. Counties set parking rules. Honolulu requires one extra space per ADU, but none within half a mile of a rail transit station.
SizeState law doesn’t set this. ADUs must meet county development standards. Local caps vary: Honolulu allows up to 400 sq ft on lots of 3,500-4,999 sq ft and 800 sq ft on lots of 5,000 sq ft or more; Hawaii County's 2024 ordinance is reported to allow up to 1,250 sq ft.
Lot size and setbacksState law doesn’t set this. ADUs built under Act 39 must follow all county development standards, and counties may deny permits where water, sewer or other utility capacity is not enough. Honolulu requires a lot of at least 3,500 sq ft for an ADU.
FeesAct 39 changed the impact-fee formula: fees must consider a project's square footage, the existing floor area is subtracted when a building is converted, and water or sewer boards may charge by number of fixtures instead. Honolulu extended its waiver of certain ADU sewer and building-permit fees to 2030 (Ordinance 25-12, per secondary reporting).
Short-term rentalsState law does not address short-term rentals of ADUs. It bars counties from adopting bans on renting each home on a covered lot separately as a long-term rental, and bars new private covenants from limiting long-term rentals. Counties regulate vacation rentals: Maui's July 2026 ordinance bars using accessory dwellings as short-term or vacation rentals or bed-and-breakfasts, and Honolulu's planning department says an ADU may not be rented for less than six months.
Can towns opt out?No

What to do next

  1. Look up your lot's county zoning and state land use district on your county's GIS map; Act 39 only covers residential lots in the urban district.
  2. Check whether your lot is in a coastal Special Management Area or a flood, lava or fire hazard area, which the law excludes.
  3. Read your county's current ADU rules and ask the planning department whether its Act 39 update has been adopted; check again after December 31, 2026.
  4. Confirm water, sewer or septic capacity early; one individual wastewater system can serve up to five bedrooms across all units, but no new hookups to a cesspool are allowed.
  5. Pull your deed and any HOA covenants and note the recording date: covenants recorded after May 28, 2024 cannot cap ADUs below the state number.
  6. If you plan to rent, check county rental rules first (Honolulu requires six-month minimum ADU rentals; Maui bars short-term use of accessory dwellings).

What your town still decides

Counties cannot opt out, but Act 39 gives them two ways to comply. The default is allowing at least two ADUs, or the reasonable equivalent, on all covered residential lots. A county that does not do that must instead create districts allowing two ADUs per existing house that fit its general plan, are spread across its regions, and add room for about half of its five-year housing need from the 2019 Hawaii Housing Planning Study, plus districts near planned transit stations and major urban roads. A county of 500,000 or more people (only Honolulu) must also add apartment-district capacity equal to its five-year need. The law creates no vested rights and leaves counties free to review, approve and deny permits. Pushback has been real: the Honolulu City Council introduced Resolution 25-271 in September 2025 asking for a repeal proposal in the 2026 county legislative package, and bills to repeal or narrow Act 39 (HB516, HB541) were filed in 2025, but none passed, no bill introduced in the 2026 session mentions ADUs in its title or description on the Legislature's bill report (so the Honolulu repeal request does not appear to have become a bill), and no 2026 Act changed HRS §46-4.8. Maui County adopted a compliance ordinance effective July 8, 2026. Honolulu's planning department still lists one ADU per lot and an owner-occupancy rule as of this check, and Kauai's compliance status was not confirmed.

Examples of local rules

  • City and County of Honolulu (Oahu): ADUs are allowed on residential and country-zoned lots of at least 3,500 sq ft: up to 400 sq ft on lots of 3,500-4,999 sq ft and up to 800 sq ft on larger lots. One ADU per lot, one extra parking space (none near rail stations), and the owner or a relative must live in one of the units. A covenant must block selling the ADU separately, and ADU rentals must be at least six months. Separate 'ohana' units are family-only with no size cap. Honolulu had not posted its Act 39 update as of this check; the state deadline is December 31, 2026. Source
  • Maui County (Maui, Molokai, Lanai): A new ordinance effective July 8, 2026, adopted to comply with Act 39, allows up to two accessory dwellings plus the main home on qualifying lots, mainly in Residential and Rural zones. Accessory dwellings must be long-term housing and may not be used as short-term or vacation rentals or bed-and-breakfasts. Setbacks, height, parking, water, wastewater and emergency access rules still apply. The county's Department of Housing also runs an 'Ohana Assistance Program offering grants of up to $100,000 toward building an ADU (a county program, not a state one). Source
  • Hawaii County (Big Island): Ordinance 2024-070 (Bill 123), signed in October 2024, replaced 'ohana dwelling' rules with accessory dwelling unit rules. Reports say it allows up to three ADUs in residential and several agricultural zones, each up to 1,250 sq ft, with only one unit usable as a vacation rental. Lots without sewer must meet septic and water supply rules. The exact text was not read for this record. Source
  • Kauai County: Kauai's Planning Department processes Additional Dwelling Unit (ADU), Additional Rental Unit (ARU) and Guest House clearances through an online system. Whether Kauai has adopted its Act 39 update was not confirmed; check with the Planning Department. Source

What we’re still confirming

  • Honolulu's planning FAQ still shows one ADU per lot and an owner-occupancy rule; whether the city has adopted its Act 39 update, and whether that owner rule conflicts with Act 39, isn't settled.
  • Kauai's Act 39 update and the details of Maui's July 2026 ordinance aren't confirmed; check size, lot and parking rules with your county.

Laws, bills and citations

The laws behind this (3)
  • Individual wastewater systems may serve ADUs (Act 224, Session Laws of Hawaii 2025 (HB735 HD2 SD1 CD1); new section in HRS chapter 342D, signed June 25, 2025, in effect June 25, 2025). A wastewater law, not a zoning law. One individual wastewater system (such as a septic system) may serve up to five bedrooms total, no matter how many homes or accessory units share it, attached or detached. It does not allow new homes to be hooked up to an existing cesspool. Read it
  • Two ADUs on residential lots; covenant limits; impact fees by size (Act 39, Session Laws of Hawaii 2024 (SB3202 SD2 HD1 CD1); HRS §46-4.8 and §205-20; amends HRS §46-4 and §46-143, signed May 28, 2024, in effect May 28, 2024). Requires each county to adopt rules by December 31, 2026 that allow at least two ADUs, or the reasonable equivalent, on residentially zoned lots in the state urban district. If a county misses the deadline, it cannot deny up to two ADUs just for exceeding the unit count, though it can still deny for infrastructure, design or development standards. It bars private covenants recorded after May 28, 2024 from capping ADUs below that number or banning long-term rentals, and makes impact fees account for a project's square footage. Read it
  • Counties may allow two homes on a residential lot (HRS §46-4(c) (county zoning statute; amended many times, most recently by Act 39 of 2024), signed Invalid Date, in effect Invalid Date). Lets, but does not require, each county set reasonable standards allowing two single-family homes on any lot where a home is allowed. This older provision is the basis for Hawaii's long-standing 'ohana' second units. Since 2024 it yields to the stronger rule in HRS §46-4.8. Read it
Proposed and failed bills (4). None of these is law.
  • HB740 (2025-2026): died as of October 3, 2026. Would have created a state ADU Financing and Deed Restriction Program, in which the state buys equity in a homeowner's new ADU in exchange for deed restrictions. It reached conference committee in April 2025 but stalled and was carried over. In 2026 the Senate conferees were discharged on March 20 and the House conferees on April 1, with no further action before the 2026 session ended. Bill page
  • SB491 (2025-2026): died as of October 3, 2026. Senate companion to HB740 (ADU equity and deed-restriction program, plus a conveyance tax exemption). It was last referred to the Ways and Means Committee in February 2025, carried over to 2026 with no further action, and died when the 2026 session ended. Bill page
  • HB516 (2025-2026): died as of October 3, 2026. Would have repealed the Act 39 two-ADU requirement and replaced it with a state subsidy program for people who build ADUs, plus a three-year general excise tax break on ADU rent. It never left committee. Bill page
  • HB541 (2025-2026): died as of October 3, 2026. Would have limited the Act 39 ADU, subdivision, impact-fee and covenant rules to counties under 500,000 people, which would have exempted Honolulu. It never left committee. Bill page

Sources

  1. Act 224, Session Laws of Hawaii 2025 (HB735 HD2 SD1 CD1); new section in HRS chapter 342D · official source
  2. Act 39, Session Laws of Hawaii 2024 (SB3202 SD2 HD1 CD1); HRS §46-4.8 and §205-20; amends HRS §46-4 and §46-143 · official source
  3. HRS §46-4(c) (county zoning statute; amended many times, most recently by Act 39 of 2024) · official source
  4. HRS §205-20 Private covenants; residentially zoned lots; urban district · official source · checked 2026-10-03
  5. SB3202 SD2 HD1 CD1 (2024) final bill text · official source · checked 2026-10-03
  6. SB3202 (2024) measure status: Act 039, approved 05/28/2024 · official source · checked 2026-10-03
  7. Office of the Governor: Gov. Green signs bills to unlock housing across the state (May 28, 2024) · official source · checked 2026-10-03
  8. HB735 (2025) measure status: Act 224, approved 06/25/2025 · official source · checked 2026-10-03
  9. Hawaii Legislature report: 2025 Acts · official source · checked 2026-10-03
  10. Hawaii Legislature report: 2026 Acts (no Act amends HRS §46-4.8) · official source · checked 2026-10-03
  11. Hawaii Legislature report: 2025 and 2026 House Bills (status of HB516, HB541, HB740) · official source · checked 2026-10-03
  12. HB740 (2025-2026) measure status · official source · checked 2026-10-03
  13. SB491 (2025-2026) measure status · official source · checked 2026-10-03
  14. HB516 (2025-2026) measure status · official source · checked 2026-10-03
  15. HB541 (2025-2026) measure status · official source · checked 2026-10-03
  16. Honolulu City Council Resolution 25-271 (introduced September 2025), repeal proposal for the 2026 HSAC package · official source · checked 2026-10-03
  17. Honolulu Department of Planning and Permitting FAQ (ADU and ohana sections) · official source · checked 2026-10-03
  18. County of Maui: County adopts new accessory dwelling rules to expand housing opportunities (July 16, 2026) · official source · checked 2026-10-03
  19. Hawaii County records: ORD 2024-070 (Bill 123, relating to ohana dwelling units) · official source · checked 2026-10-03
  20. Kauai Planning: Additional Dwelling Unit, Additional Rental Unit, and Guest House · official source · checked 2026-10-03
  21. Hawaii Life: Big Island Mayor signs ADU bill (Bill 123 details) · checked 2026-10-03
  22. Grassroot Institute of Hawaii: Repealing Act 39 would be step backward (October 2025; cites Honolulu Ordinance 25-12 fee waiver) · checked 2026-10-03
  23. Grassroot Institute of Hawaii: Support drafting bill that would allow two ADUs per lot (Maui Resolution 25-203, November 2025) · checked 2026-10-03

This is general information, not legal advice. Your town, village, city or county decides what can be built on your property. If something here looks out of date, email the team.