ADU housing laws and regulations in Hawaii: 2026 guide

Hawaii's counties changed accessory dwelling unit rules between October 2024 and July 2026 as they worked toward meeting Act 39, the 2024 state law that requires every county to allow at least two ADUs on residentially zoned lots by December 31, 2026.

With housing scarce and prices high, homeowners want the units to house an aging parent or an adult child priced out of buying, or to rent long-term against a mortgage; one Honolulu family profiled by the Star-Advertiser paid $250,000 for theirs. But whether you can build, how large, and who may live in it still depends on the island. On Oahu the planning department's own FAQ lags its own ordinance by a full size tier.

Hawaii ADU rules at a glance

The ADU Hawaii rules below are Oahu's under Ordinance 25-2, effective September 30, 2025, with the other counties noted where they differ. Confirm the current figures with your county planning department (DPP on Oahu, or Maui, Kauai or Hawaii County Planning) before you design:

• Rule | Oahu (Honolulu) | Maui, Kauai and Hawaii County
• Minimum lot size | No numeric minimum in the Ord. 25-2 text; DPP's FAQ still lists the legacy 3,500 sq ft floor | Maui and Kauai set none for one ADU; Hawaii County has no ADU-specific minimum, only district minimums (7,500 sq ft in RS)
• Maximum floor area | 500 sq ft on lots under 5,000 sq ft; 1,000 sq ft at 5,000 sq ft or more | Maui 500–1,200 sq ft by lot size; Hawaii County 1,250 sq ft; Kauai 800 sq ft
• Units per lot | One ADU, plus an ohana unit if otherwise permitted | Maui two; Hawaii County three; Kauai one guest house per dwelling
• Off-street parking | One added stall, waived near rail stations and some bus stops | Maui by bedroom count; Hawaii County two per dwelling unit; Kauai one added space per guest house
• Owner-occupancy | Required by DPP's covenant form; absent from the ordinance text | No general requirement in current Maui, Kauai or Hawaii County code
• Minimum lease | Six months, recorded in a covenant | Maui 180 days; no short-term use on Maui or Kauai
• Separate sale | Barred; no condominium property regime may split the ADU from the house | Kauai bars CPR splits of guest houses

What is an ADU in Hawaii?

An ADU is a second, self-contained dwelling on the same lot as your main house, with its own kitchen, bedroom and bathroom, and on Oahu it can be rented to anyone, not only relatives (DPP FAQ). Hawaii has two older categories that look similar from the street but carry different rules.

The ohana unit dates to 1981 state zoning law. The Legislative Reference Bureau notes that the state statute never limited occupancy to family; the family-only rule came from county regulations. A home addition is the third category. It is not a dwelling at all.

• Feature | ADU | Ohana unit (Oahu) | Home addition
• Who can live there | Anyone, on a lease of six months or longer | Family members only, unless the unit was permitted before September 10, 1992 | Occupants of the main house
• Kitchen | Its own full kitchen, one per dwelling | Its own kitchen, since it is a second dwelling | Shares the main kitchen
• Size cap | 500 or 1,000 sq ft by lot size | No size limitation under the DPP FAQ | Counted as part of the main house
• Electric meter | Capped at one per dwelling (one meter per legal dwelling) | Same rule | Counted with the main house
• Zoning districts | Country and R districts | AG-1, AG-2, Country and R districts except R-3.5 | Wherever the house is allowed

Sources: DPP FAQ, Hawaii Business, Honolulu Council record.

Before Ord. 25-2, an Oahu lot could hold one ADU or one ohana. It can now hold both, and the ohana does not count against the one-ADU limit (City document). Converting an existing ohana, accessory structure or interior room into an ADU takes a building permit, and a structure built before the ADU ordinance may exceed the floor-area cap or skip the parking stall through a zoning adjustment (DPP FAQ).

The other islands handle the categories differently. Hawaii County struck the "Ohana dwelling" definition from its code in Ord. 24-70 and replaced it with the ADU. Maui allows a single kitchen and bars kitchenettes in ADUs under Ord. 5832, effective July 9, 2025. Kauai calls its unit a guest house, caps it at 800 sq ft, and lets it be used by "guests, tenants, or owner(s)" (Kauai Planning).

Who qualifies: lot size, zoning district and infrastructure

On Oahu, any residential or Country lot with adequate infrastructure and no restrictive covenants can hold an ADU. DPP states that "No building permit for ADUs will be issued unless there is adequate infrastructure to support it" (DPP FAQ).

Infrastructure includes water and sewer capacity, along with road access and first-responder capacity. Sewer capacity is a frequent obstacle; Civil Beat reports sewer capacity "has halted at least some proposed ADUs" (Civil Beat).

The 3,500 sq ft minimum you will see on the DPP FAQ is the legacy rule. Ord. 25-2 sets no numeric lot minimum. It bars ADUs on planned development housing and cluster housing, as well as landlocked lots (ordinance text).

Your zoning district also matters. Table 21-5.1 lists ADUs as an accessory use in Country, R-20, R-10, R-7.5, R-5 and R-3.5, and not in AG-1, AG-2, apartment or commercial districts (Council document).

Oahu allows one ADU per zoning lot. One ADU is also allowed on a lot that already has an ohana unit or two principal dwellings, provided the lot is not in a condominium property regime (Architect Honolulu).

Hawaii County permits ADUs in the RS, RD, RA, FA and A districts, up to three per site or one per authorized primary dwelling (HCC). The RA allowance applies only within the State Land Use Urban District under a December 2025 Planning Department interpretation (County record), and State conservation districts are excluded.

Maui sets no lot minimum for one ADU on an ordinary residential lot. Rural lots get one ADU under 7,500 sq ft and two at 7,500 sq ft or more (Ord. 6013). Agricultural districts and other nonresidential districts, including commercial and industrial districts, are generally excluded (County guide). Kauai's § 8-15.2 makes any residentially zoned lot eligible where only one single-family dwelling is otherwise permitted (KCC).

Act 39 draws the outer boundary for all four counties. Its "residentially zoned lot" definition excludes rural districts and districts intended for low-density residential development or open-space preservation, so the two-ADU mandate never reaches ag or rural land.

How big can your ADU be?

ADU Hawaii size caps run from 500 square feet on a small Oahu lot to 1,250 square feet on Hawaii Island, and Oahu and Maui tie the cap to lot size:

• Lot size | Oahu (Ord. 25-2) | Maui (Ord. 5966)
• Under 5,000 sq ft | 500 sq ft | 500 sq ft (lots to 7,499 sq ft)
• 5,000–7,499 sq ft | 1,000 sq ft | 500 sq ft
• 7,500–9,999 sq ft | 1,000 sq ft | 600 sq ft
• 10,000–21,779 sq ft | 1,000 sq ft | 720 sq ft
• 21,780–43,559 sq ft | 1,000 sq ft | 840 sq ft
• 43,560–87,119 sq ft | 1,000 sq ft | 960 sq ft
• 87,120 sq ft and up | 1,000 sq ft | 1,200 sq ft

Sources: Ord. 25-2 text, Ord. 5966.

Oahu's 400 and 800 sq ft figures are superseded. DPP's FAQ still displays them, but Ord. 25-2 raised the caps to 500 and 1,000 sq ft (City document). Design to 500 and 1,000 sq ft, and confirm with DPP at pre-check.

Maui's small-lot tiers differ across County documents. The finer table above comes from Ord. 5966, effective March 23, 2026, which carried forward the 2018 tiers. Other Maui documents read the first tier as 720 sq ft for every lot up to 21,779 sq ft, and Ord. 6013 contains a stray "600 square feet" phrase. Maui also caps decks: a lot up to 21,779 sq ft gets 280 sq ft uncovered and 280 sq ft covered.

Hawaii County allows 1,250 sq ft of living area, excluding lanai, garage and carport (HCC § 25-6-34). Kauai's R-6 planning guide gives 800 sq ft for a guest house, ADU or ARU (Kauai Planning).

Off-street parking, utility and wastewater requirements

DPP can deny an Oahu application that lacks a required parking stall. Ord. 25-2 requires one additional off-street space for the ADU, except within one-half mile of a rail station. DPP also waives the stall within 800 feet of a city bus stop in the Primary Urban Center or Ewa development plan areas (ROH). Compact and tandem stalls count (ROH).

Maui sizes parking by bedrooms under Ord. 5983: one space for a studio or one-bedroom, two for two or three bedrooms. Hawaii County applies its general two-spaces-per-dwelling-unit rule. Kauai's guest-house ordinance requires one added space.

Sewer capacity is the second gate on Oahu. DPP's Wastewater Branch requires a Sewer Capacity Analysis for any new second unit, valid two years. Projects on cesspools or septic tanks need State Department of Health approval (Wastewater Branch).

The Wastewater System Facilities Charge is $7,539.00 per ESDU; Bill 60 assigned ADUs 0.7 ESDU (Council document). That works out to roughly $5,277 per unit, a derived figure rather than a published fee.

If your lot runs on a cesspool, the ADU may force a conversion. DOH has accepted no new cesspool since March 22, 2016. Its memo states that when a building permit shows a bedroom added to a dwelling on a cesspool, the owner "will be required" to upgrade to a septic tank or aerobic treatment unit (DOH memo).

Every cesspool in the state must be upgraded, converted or connected to sewer by January 1, 2050 under HRS §342D-72. Act 224 (2025) lets one individual wastewater system serve up to five bedrooms across the main house and ADU, but it excludes "any new requests to connect additional dwelling units to an existing cesspool" (Act 224).

Water and electric metering rules differ by island, so check your parcel against the list below before you size the service:

• On Oahu, a separate ADU water meter is optional, but the existing meter must be sufficient for both units (BWS Q&A). That guidance is dated 2016, so confirm current requirements with BWS.
• Hawaii County requires "each dwelling unit or lot shall use a separate 5/8-inch meter at a minimum" where water is available (DWS rules).
• Kauai's Department of Water told its board in December 2025 that an ADU with a kitchen "would require an upgrade to a ¾" meter" (DOW minutes). Kapaʻa Homesteads and Kīlauea lots sit under meter restrictions until new source and storage come online.
• Maui lets you request a separate meter for the ADU at half the water system development fee, and since January 1, 2026 it counts demand in 300-gallon-per-day Equivalent Single Dwellings instead of fixture units (Maui DWS). The Upcountry meter list has accepted no new applications since January 1, 2013.
• Honolulu DPP allows "no more than 1 electrical watt-hour meter" per legal dwelling unit, so DPP caps each legal dwelling at one (DPP FAQ). Hawaiian Electric prices service upgrades case by case through a cost proposal issued in 5–10 days (HECO brochure); with more than two dwellings on a lot you must consult the utility before wiring.

Owner-occupancy, covenants and the no-separate-sale rule

On Oahu you record a covenant before DPP will issue the permit, and that covenant follows the land to every future buyer. Ord. 25-2 § 21-5.50-3(a)(2)(A) requires covenants recorded with the Bureau of Conveyances or the Land Court, "or both, as appropriate," in a form approved by the DPP director (Ord. 25-2). The ordinance requires four covenant terms:

• Long-term residential occupancy only, with no B&B or transient use.
• Deed restrictions that lapse if the ADU is removed.
• Binding effect on heirs, successors and assigns.
• No condominium property regime separating ADU ownership (Council record).

Owner-occupancy is where the ordinance and DPP's paperwork part ways. The ordinance text does not list it. DPP's sample declaration does: the owner, a relative by blood, marriage or adoption, or an authorized representative must occupy either the main house or the ADU while the other is rented, with an exception for unforeseen hardship such as military deployment or serious illness (DPP covenant form).

A third-party reference, aduzoning.org, reports owner occupancy was "Dropped from the code in 2025" and that the posted form is the 2017 version (aduzoning.org). Ask DPP which version it will accept before you sign.

For an investor who will never live on the parcel, that gap is the whole decision. If DPP still requires the form as written, a non-resident owner would need a relative or authorized representative living on the lot to satisfy it. If you plan to buy an Oahu house, build an ADU and rent both units, the recorded covenant blocks that plan unless DPP has retired the form.

The mechanics of recording are fixed. Fee-simple owners sign with notarized signatures, and trustees must be identified for trust-owned property. The first page reserves the top 3½ inches for the Registrar.

A certified recorded copy goes to DPP before permit issuance (DPP covenant form). Bureau of Conveyances fees, last set in 2017, are $41.00 for a Regular System document up to 50 pages and $36.00 for Land Court (BOC fees).

You cannot sell the ADU on its own, and you cannot undo the covenant without the DPP Director's approval (DPP FAQ). DPP may enforce violations under § 21-2.150. The general civil fine schedule runs up to $5,000 plus $5,000 per day (ROH).

The neighbor islands are looser. Kauai's § 8-15.2 imposes no owner-occupancy rule, though guest houses may not be split off by CPR (Bill 2933). Hawaii County's current code contains no owner-occupancy rule or recorded ADU covenant, and § 25-6-39 voids any private restriction recorded after October 7, 2024 that prohibits ADUs (HCC). Maui ties owner-occupancy only to its ʻOhana Assistance Program grants, which must be repaid in full if the property is sold or placed in a CPR (Ord. 5601).

Renting your ADU: six-month minimum lease and the STVR ban

An Oahu ADU "may not be rented for less than a six month period," and that term is written into the recorded covenant (DPP FAQ). Airbnb-style use is out, and the fine for an unregistered short-term rental on Oahu runs up to $10,000 per day under Ord. 22-7 (court order).

The federal injunction that protects some pre-existing 30- to 89-day rentals from that ordinance does nothing for an ADU, because the six-month floor comes from the covenant, not from the STR rules. Renting does not open a path to selling: the covenant bars selling the ADU separately from the main house.

Maui bars the use outright. Ord. 6013's § 19.35.010(E) says accessory dwellings "must not be used as a bed and breakfast home, short-term rental home, or transient vacation rental," and the County guide adds that an ADU "may not be rented for less than 180 days" (County guide). Maui's legacy STRH permit rule allowed short-term rental use in one single-family unit plus one ADU per lot; it predates the Ord. 6013 ban.

Kauai does not permit rentals under 180 days outside the Visitor Destination Area. Ord. 1166 also bars guest houses from TVR or homestay use (Kauai Planning). Kauai may impose a $10,000 fine for operating a TVR without a Nonconforming Use Certificate.

Hawaii County has conflicting County records. The Tribune-Herald reported that one ADU per site may be used as a short-term vacation rental (Tribune-Herald). The code excerpt at § 25-6-38 caps transient use at one ADU, while a separate County record states ADUs "shall not be permitted for use as transient accommodation rentals" (County record). Any STVR there also needs the County's $500 registration and sits outside the owner-on-site definition.

Act 39 protects long-term rental statewide. It provides that no county may prohibit using any dwelling unit on a residentially zoned lot as a separately leased long-term rental, and it voids private covenants recorded after May 28, 2024 that try to (HRS §205-20). Owners may also qualify for favorable property tax treatment in three counties:

• Maui exempts up to $200,000 of value for a 12-month lease to the same tenant (Maui FAQ).
• Hawaii County reclassifies a principal residence with units rented six months or longer as homeowner (Hawaii County ARH).
• Kauai treats 180 days as long-term for classification.

Permits step by step: precheck form to final approval

The Oahu sequence starts with an infrastructure sign-off and ends, at the median, about two and a half months after DPP accepts a complete application. The steps below follow DPP's published process, with current timing from news reports:

1. Submit the Public Facilities Pre-Check Form, listed under "ADU/Ohana" on the DPP forms page. It checks "infrastructure adequacy (water, sewer, road access)" and needs signatures from the DPP Customer Service Division, Wastewater Branch, Traffic Review Branch, State Department of Health and Board of Water Supply (DPP FAQ). A construction-industry source put the pre-check at about a month with no guaranteed turnaround (Homeworks Hawaii), and in December 2025 DPP reported running about six weeks behind on prescreening with 830 projects in the queue (Star-Advertiser).
2. Clear wastewater. Request the Sewer Capacity Analysis through HNL:BUILD, or, on a cesspool or septic lot, hire a Hawaii-licensed civil engineer to prepare the IWS plan for DOH, which must approve or deny a complete application within 180 days (DOH IWS). DOH approvals lapse if no building permit issues within one year.
3. Prepare plans to the residential code DPP currently enforces with Honolulu amendments, including Appendix W wind design where the basic design wind speed reaches 130 mph; confirm the code edition with DPP.
4. Record the restrictive covenant and obtain a certified copy. DPP says the returned document must carry three stamps.
5. Apply for the building permit through ePlans with the approved pre-check form and the recorded covenant (DPP FAQ). The permit fee on a $100,000.01–$500,000 valuation is $700 plus $10 per $1,000, with plan review at 20% of that (Table 18-A), though the City's ADU fee waiver on building permit, plan review and wastewater facility charges has been extended to June 30, 2030 (Civil Beat).
6. Respond to review comments. Civil Beat's June 2026 analysis put the median residential review at 2.5 months, down from 4 months, and DPP's CivCheck fast track cut average review from 73 to 32.5 days for eligible projects including ADUs and ohana units (Star-Advertiser). The 2024 median was about 288 days, and the FY2024 average was 364. Time spent in CivCheck is not counted in the official figure.
7. Pull the permit, build, pass inspections and close out. Under Act 295, effective July 1, 2026, you may request expedited handling once a permit has sat for sixty business days (Act 295).

Building it yourself changes the rental math. The owner-builder exemption in HRS §444-2.5 covers owners building for their own use or for grandparents, parents, siblings or children who "do not offer the buildings or structures for sale or lease." Proof of a lease within one year of completion is "prima facie evidence" of a violation, and using the exemption more than once in two years creates a rebuttable presumption of one.

First-offense penalties are "$5,000 or forty per cent of the appraised value," whichever is greater. You must also hire licensed electricians and plumbers, treat unlicensed workers as employees, and keep records for three years. An investor who self-builds and leases in year one takes on that risk.

County differences: Oahu, Maui, Kauai and Hawaii County

The standards below come from each county's adopted code:

• County | Minimum lot | Max floor area | Units per lot | Off-street parking | Wastewater | Ordinance and effective date
• Honolulu (Oahu) | None stated in Ord. 25-2; legacy 3,500 sq ft on DPP FAQ | 500 sq ft under 5,000 sq ft lots; 1,000 sq ft at 5,000+ | One ADU plus one ohana | One added stall; waived near rail and qualifying bus stops | Sewer Capacity Analysis; $7,539/ESDU charge at 0.7 ESDU, waived through 2030 | Ord. 25-2, effective Sept 30, 2025
• Maui | None for one ADU; rural lots need 7,500 sq ft for a second | 500–1,200 sq ft by lot tier | Two | By bedroom count; tandem allowed | Written adequacy from DEM or DOH; infrastructure denial allowed | Ord. 5966 (Mar 23, 2026), Ord. 5983 (Apr 20, 2026), Ord. 6013 (Jul 8, 2026)
• Kauai | None stated; residentially zoned lot with one permitted dwelling | 800 sq ft guest house | One guest house per dwelling unit | One space per guest house | Public sewer, DOH-approved IWS or cesspool; Facilities Clearance Form before application | Ord. 1166, approved Oct 25, 2024
• Hawaii County | No ADU-specific minimum; district minimums apply | 1,250 sq ft living area | Three, or one per primary dwelling | Two per dwelling unit | Approved disposal system under § 25-6-30; sewer connection within 180 days of notice | Ord. 24-70, signed Oct 8, 2024

Sources: Ord. 25-2 text, Ord. 6013, Bill 2933, HCC.

Kauai's enacted Ord. 1166 governs guest houses rather than establishing a separate two-ADU framework. The county's separate Article 30 ARU (additional rental unit) provisions expired under a sunset on March 19, 2023. The Grassroot Institute, an advocacy group, asserts that Kauai and Hawaii counties "have already met or exceeded the law's requirements" (Grassroot), but Kauai's enacted ordinance addresses guest houses rather than two ADUs.

Hawaii County's records give its effective date as June 30, September 30 and October 7, 2024. Maui's Ord. 6013 reached full effect only on July 8, 2026, which Grassroot describes as "the last county to comply."

Before you design, check four points with the county planning department: ask Maui Planning which size tier applies to your lot, ask Hawaii County Planning for the effective date and whether an ADU may operate as an STVR, ask Kauai Planning whether a second ADU is allowed beyond the guest house, and ask DPP whether its covenant form still requires owner-occupancy.

What an ADU costs in Hawaii and how to pay for it

Stick-built ADUs in Hawaii run roughly $250 to $400 per square foot before land and soft costs such as permits and utilities. Owner Built Design estimated $250–$300 per square foot in February 2025, or $100,000–$120,000 for 400 sq ft and $200,000–$240,000 for 800 sq ft, with permits adding $5,000–$10,000 (Owner Built Design). H1 Construction quotes $250–$400 and adds 10–15% for professional fees (H1 Construction). Senate committee testimony in March 2025 used $250–$350 (Star-Advertiser). Those vendor figures line up with Rider Levett Bucknall's Q4 2025 Honolulu benchmark of $275–$325 per square foot for single-family construction, with Hawaii costs rising 5.69% year over year (RLB).

A Maui resident reported a 464 sq ft ohana unit at "about $130,000 to $140,000" (Maui Now).

Prefab shifts where the money goes rather than how much. PrefabMarket lists kits at $45–$89 per square foot and turnkey installation at $150–$300 (PrefabMarket); DC Structures puts a finished Hawaii prefab ADU at $200–$400 (DC Structures). Affordable Hawaiian Homes prices a 430 sq ft model at $59,079 delivered "to the nearest port of your island" (Affordable Hawaiian Homes).

The utility line items vary more by county than the structure does:

• Line item | Oahu | Maui | Hawaii County | Kauai
• Water meter or facilities charge | 3/4" meter $1,797 (2016 figure); WSFC minimum $3,706.60 (2012–2017 table) | 5/8" development fee $12,060, ADU pays half ($6,030) | $6,095 per additional unit plus $1,319 first connection | Facility Reserve Charge $14,115; $4,940 under the affordable ARU program
• Sewer connection | $7,539/ESDU at 0.7 ESDU, waived through 2030; market connection cost $8,500–$10,000 | $4.65 per gallon of flow in Kīhei | No charge beyond the plumbing permit | Connection fee and capacity assessment, with amounts requiring confirmation
• Cesspool to septic conversion | $25,000–$50,000 per HPR, June 2026 | Same statewide range | Task force median $40,000, upper range $70,000 | $20,000 county grants, 50 available in 2026
• Electrical heavy-up (100A to 200A) | $4,500–$7,500+ per Knoles Electric; underground $8,000–$15,000+ | Not quoted | Not quoted | Not quoted

Sources: BWS, Maui DWS, Hawaii DWS, Kauai DOW, Civil Beat, HPR, Knoles Electric. Knoles also reports Hawaiian Electric engineering approval of a service upgrade can take 3–6 months.

School impact fees apply in designated districts and ADUs are not listed as exempt: $3,363 per single-family unit in Leeward Oahu and $5,373 in Central Maui, while West Maui's fee is suspended under the wildfire proclamation (SFA).

Financing is thin, and the state's flagship mortgage program excludes you. The Hale Kamaʻāina Mortgage Program offers 30-year fixed rates at least 50 basis points below market, but "Properties with ADUs are not eligible" (HHFDC FAQ). The state ADU financing bills (HB 740, HB 516) were carried over to 2026 without enactment. What is open:

• Home equity lines are the workhorse. Bank of Hawaii markets its HELOC for "ohana unit" use at a fully indexed 7.50% as of September 21, 2026 (BOH); HawaiiUSA FCU offers lines from $10,000 to $1 million, 4.99% for a two-year intro, 7.25% variable after, with investment properties allowed (HawaiiUSA FCU).
• Construction-to-permanent loans exist at the big four banks. American Savings Bank allows up to 18 months of construction and requires a licensed general contractor (ASB), which rules out the owner-builder route.
• FHA counts ADU rent toward qualifying, using 75% of the lesser of appraiser market rent or lease rent, capped at 30% of effective income (HUD ML 2023-17). The Standard 203(k) covers adding an ADU, but Hawaii Business found zero Hawaii lenders in a HUD participation search in October 2023.
• Fannie Mae's HomeStyle allows one ADU with a one-unit primary dwelling and caps ADU rental income at 30% of qualifying income; Freddie Mac's CHOICERenovation finances ADUs but, for applications from May 4, 2026, bars using rent from a mortgage-funded unit to qualify (Freddie Mac).
• County money is small and conditional. Maui's ʻOhana Assistance Program grants up to $100,000 to owner-occupants who keep rented units affordable for 10 years, and its second round is "not open yet" (County guide). Kauai's Affordable ARU program saves about $20,000 in fees against a five-year pledge at 90% of Kauai median income. The state cesspool grant now reaches $30,000 per project under Act 188 for priority-area cesspools.

What changed in the new Hawaii ADU law

Act 39 (SLH 2024), signed and effective May 28, 2024, is the law that forced every county to rewrite its ADU code. Codified at HRS §46-4.8, it requires each county to "adopt or amend ordinances defining reasonable standards that allow for the construction of at least two accessory dwelling units, or the reasonable equivalent, for residential use on all residentially zoned lots" by December 31, 2026. After that date a non-compliant county may not deny a permit for up to two ADUs for exceeding a unit cap, though it may still deny on infrastructure, design or development standards.

State leaders framed Act 39 as a way to add homes on lots that are already zoned and served, and the Star-Advertiser covered it as one of the new laws aimed at turning the tide on the housing crisis. Oahu's record shows the limits: Civil Beat reported about 1,320 ADUs permitted on Oahu since 2015, as of August 2025. It cited sewer capacity limits, small lots, slow permitting and construction costs as the barriers.

Two units per property is now the statewide floor, with the count varying by island. Oahu reaches it through one ADU plus one ohana, so a one-family lot can hold three units in total (Council record). Maui allows two ADUs outright as of July 8, 2026, including on Lānaʻi and Molokaʻi. Hawaii County went to three in October 2024. Kauai's position rests on its guest-house ordinance and an advocacy group's assertion.

Act 39 also contains limits that matter as much as the mandate. It sets no numeric lot minimum and does not preempt parking; counties keep their development standards. Subsection (f) preserves denial "if there is insufficient utility infrastructure." It does not apply outside the state urban district, inside special management areas, or in urban areas a county deems at high risk of natural hazards, and it creates no vested rights. Civil Beat commentary notes the original bill "prohibited counties from requiring owner-occupancy," but that clause was stripped before passage (Civil Beat).

Three other state measures touch ADU owners. Act 224 (2025), effective June 25, 2025, allows one wastewater system to serve five bedrooms across units. Act 17 (2024) declares transient accommodations a non-residential use that counties may phase out in any district; Maui has since moved to phase out TVRs in apartment districts under Ord. 5909. Act 96 (2025) raised the transient accommodations tax to 11.00% on January 1, 2026, a cost that only applies if your county lets the ADU operate short-term at all.

The 2025 bills you may have read about did not pass:

• Bill | Proposed change | Status
• HB 516 | Would repeal §46-4.8 and exempt ADU rent from GET for three years | Carried over on December 8, 2025 (HB 516 status); not enacted
• HB 541 | Would limit the mandate to counties under 500,000 people | Carried over on December 8, 2025; not enacted
• HB 1743 (2026) | Would lift the owner-builder leasing restriction | Its HD1 version carries a placeholder effective date of "7/1/3000," meaning no enactment

Insuring your Hawaii ADU

The day a tenant moves into the ADU, the homeowners' policy on the main house stops being the right contract for that unit. A homeowners' policy is written for a residence you occupy.

A rental unit on the same lot exposes the owner to tenant-caused damage and liability for a tenant's guests. Repairs can also interrupt rent.

A landlord can address those exposures with landlord insurance. A DP3 policy covers the structure on an open-peril basis. It offers liability limits from $100K to $1M or more and can pay up to 12 months of fair rental value under loss-of-rental-income coverage.

Steadily writes ADUs and six-month-plus long-term rentals in all 50 states through its own carrier and A-rated partners. The national average premium is about $1,478 per year; a unit insured for $100K of dwelling coverage typically runs $500–$1K a year.

You can review how the main house and unit are written through Landlord insurance for an ADU rental. The guide to landlord insurance in Hawaii covers state-specific terms, and there is a page for Honolulu too.

Get a quote in minutes at quote.steadily.com; no phone call required.

FAQ

These answers cover permit requirements, county size and occupancy rules, rental restrictions, tiny homes, and typical ADU costs. They also summarize how owners finance and insure a legal rental unit.

Are tiny homes legal as ADUs in Hawaii?

Yes where the county has adopted IRC Appendix Q, and only on a permanent foundation. Hawaii County adopted Appendix Q (Tiny Homes) with its 2018 residential code effective September 1, 2021, and Kauai adopted it under Ordinance 2761 with a local definition of 500 sq ft or less excluding lofts (Kauai Council). Honolulu and Maui have not confirmed adoption, though DPP treats modular, prefab, container and mobile units as construction methods that can be ADUs if they meet ADU rules and all building codes. Kauai and Hawaii County both count a wheeled structure parked for 24 hours as a building, and Kauai requires wheels and axles removed.

Can I build an ADU without a permit?

No. No Hawaii county exempts a dwelling unit from building permits, and Honolulu requires a permit even to convert an existing ohana, accessory structure or interior room into an ADU. The owner-builder statute has an exception for work valued under $10,000, but that exception concerns the lease-within-a-year presumption. It does not exempt an ADU from permitting.

What is the difference between an ADU and an ohana unit?

On Oahu an ohana unit is limited to family occupancy and has no size cap, while an ADU can be rented to anyone on a six-month lease and is capped at 500 or 1,000 sq ft. Hawaii County retired the ohana category in 2024 and now calls everything an ADU. Since Ord. 25-2 an Oahu lot can hold one of each.

Can I list my Hawaii ADU on Airbnb?

Oahu, Maui and Kauai prohibit that use. Oahu's covenant sets a six-month minimum, Maui bars B&B, short-term and TVR use of ADUs, and Kauai forbids rentals under 180 days outside the Visitor Destination Area. Hawaii County's records conflict, with one reading allowing a single ADU per site as an STVR and another barring transient use.

Do I have to live on the property?

On Oahu, DPP's covenant form requires you or a relative to occupy one of the two units while the other is rented, though the ordinance text omits that term and a third-party source reports it was dropped. Maui, Kauai and Hawaii County impose no general owner-occupancy rule, and Act 39 contains none.

How much does an ADU cost in Hawaii and how do people pay for it?

Budget $250–$400 per square foot for stick-built construction, plus county water and sewer charges. Kauai's Facility Reserve Charge is $14,115, and a required cesspool conversion can add $25,000–$50,000. On Oahu, the sewer facility charge is waived through 2030, but Board of Water Supply charges still apply.

Most owners use a HELOC or a construction-to-permanent loan from a Hawaii bank or credit union. The state's Hale Kamaʻāina mortgage excludes ADU properties, and the second round of Maui's $100,000 grant has not opened.

How do the counties differ?

Oahu allows one ADU of 500 or 1,000 sq ft with a recorded covenant and a six-month lease floor. Maui allows two, sized 500–1,200 sq ft by lot, with no short-term use. Hawaii County allows three at 1,250 sq ft each, with no owner-occupancy covenant. Kauai's enacted rule covers one 800 sq ft guest house per dwelling rather than a separate two-ADU framework.