Building an ADU in Hawaii: what state law allows

What state law lets you build

Act 39 of 2024 (SB 3202) was approved on May 28, 2024, and took effect on approval. It added section 46-4.8 to the Hawaii Revised Statutes. Under it, each county "shall 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."

A "residentially zoned lot" is a lot in a county zoning district "principally reserved for single-family and two-family detached dwellings." Districts meant for rural, low-density residential development and open space preservation don't count.

A county that doesn't take that route must instead create districts that allow at least two ADUs, or the reasonable equivalent, per existing single-family home. Those districts must be spread around the county, including near transit stations and major roads. A county of 500,000 or more people must also add housing capacity in its apartment districts. [1][2]

The December 31, 2026 deadline

If a county hasn't adopted or amended its zoning ordinances under the law by December 31, 2026, it "shall not deny any permit application on the basis of exceeding the maximum number of housing units allowed" when the owner of a single-family home on a residentially zoned lot (or the owner's representative) applies to build up to two ADUs, or the reasonable equivalent. That lasts until the county adopts its ordinance.

The county can still deny the permit "on the basis of infrastructure, design, or development standards." The law also says it doesn't preempt a county's ability to accept, review, approve and deny permit applications, and that it creates no vested rights. [1]

What counties can still require

ADUs built under the law are "subject to all development standards adopted by the respective county." That means your county's rules on size, height, setbacks, parking and design still apply. A county may also deny a permit "if there is insufficient utility infrastructure" to serve the added ADUs.

The law doesn't apply to:

• Any area outside the state urban district
• County powers within special management areas (coastal zones)
• Urban district areas the county deems at high risk of a natural hazard such as flooding, lava or fire, based on current federal or state maps

[1]

Renting your ADU

No county may "adopt prohibitions on using any dwelling unit on a residentially zoned lot as separately leased long-term rentals." Each county defines what counts as a long-term rental. This protection covers long-term rentals only; the law says nothing about short-term vacation rentals, so check your county's rules before you list an ADU. [1]

HOA covenants and deed restrictions

Act 39 also added HRS 205-20. A private covenant on a residentially zoned lot in the urban district recorded after May 28, 2024, can't limit:

• The number of ADUs on the lot below the number allowed under HRS 46-4.8
• Long-term rental of homes on the lot

This doesn't apply to covenants recorded before May 28, 2024. Check your deed and any HOA rules if your covenants are older. [3]

Impact fees

Act 39 amended the impact fee law, HRS 46-143. When counties figure a development's share of public facility costs, they must now consider "the square footage of the development." If you convert an existing structure, its square footage is deducted when the impact fees are calculated. For water or sewer facilities, the board of water supply may instead base the fee on the number of fixtures. [4]

Bills to change the law in 2025 and 2026

Two House bills filed in 2025 would have changed Act 39:

• HB 516 would have repealed the requirement that counties allow at least two ADUs on residentially zoned lots, and set up an incentive program instead.
• HB 541 would have made the ADU and related requirements apply only to counties with fewer than 500,000 people.

The Legislature's records show both bills were referred to committees on January 21, 2025, and carried over to the 2026 session on December 8, 2025, with no later action listed. HRS 46-4.8 still shows only Act 39 of 2024 as its source. [1][5][6][7][8]

Where local rules differ

Each county writes its own ADU rules within the state law, and the details differ by island. For Oahu, see our page on building an ADU in Honolulu, which covers the City and County of Honolulu's size limits, parking and permit steps. Ask your county planning department how it is handling the December 31, 2026 deadline.

For the general process, see our guides to the ADU permit process, ADU costs and ADU financing. [1]

Hawaii laws to know

• Act 39 (2024), SB 3202, HRS 46-4.8

  Requires each county to allow at least two ADUs, or the reasonable equivalent, on residentially zoned lots in the urban district, with a December 31, 2026 deadline. [1][2][9]

• Act 39 (2024), HRS 205-20

  Bars private covenants recorded after May 28, 2024 from limiting ADUs below the state number or limiting long-term rentals on residentially zoned urban lots. [3]

• Act 39 (2024), HRS 46-143(d)

  Adds a development's square footage to the factors for county impact fees and deducts the existing floor area when you convert a structure. [4]

Questions people ask