ADU Guide · Verified 2026
Building an ADU in Hawaii
Hawaii requires every county to allow ADUs on any residentially zoned lot, with ministerial approval and no size-based lot exclusion. Honolulu goes further, allowing up to three units on a single lot, but statewide, a private HOA covenant can still block an ADU even where county zoning approves it.
Quick Facts
Statewide floors set by Act 39/Act 28 (2023); Act 39 (2024, HB 2090), HRS 46-4.8. Your city may be more generous but cannot go below them.
The Three Types of ADU
Two different laws share the same name, a real trap
Hawaii reuses act numbers each legislative session, so "Act 39 of 2023" (SB 3202) and "Act 39 of 2024" (HB 2090) are genuinely different laws that happen to share a number. Always check the year alongside the act number when researching Hawaii ADU law.
Honolulu allows a real third unit
Under Ordinance 25-2, a single-family lot in Honolulu can host up to three units total, the primary home, an ADU rentable to anyone, and a separate Ohana unit restricted to family members only, a genuinely distinctive three-tier structure not found elsewhere in this research.
A statewide right that a private HOA can still block
Even though the state and counties cannot prohibit an ADU on a qualifying lot, Hawaii does not preempt private HOA covenants or deed restrictions, meaning a community association can still legally block an ADU that the county itself would approve, a real, documented gap other states like Colorado close.
Maui's deadline hasn't arrived yet
Maui County's requirement to allow at least two ADUs per lot regardless of size doesn't take effect until December 31, 2026, meaning current Maui rules may still be more restrictive than what the law will eventually require.
What people get wrong
The most common mistake is assuming Hawaii's statewide ADU mandate means no private restriction can stop a project. HOA covenants remain fully enforceable, a real, separate layer from the zoning question.
From Idea to Move-In: The Timeline
Who Administers It
Counties With Their Own Rules
These Hawaii counties have verified, county-specific ADU rules that add to the state baseline:
Building It Yourself? Here's What You'll Need
Planning an ADU? Tools homeowners use in the design and layout phase:
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Check Your Local Rules
Your city may allow more than the state floor
State law sets the minimum; many Hawaii cities and counties are more generous. Browse the full directory of 5 counties and every city in Hawaii for local rules, fees, and building department contact details.
Browse the full Hawaii directory →Not sure if your project needs a building permit, a zoning approval, or both? See our guide to building permits vs. zoning approval.
Official Sources
ADU law changes frequently and local ordinances add detail. This guide is general information, not legal advice, always confirm with your local building department. Reviewed by the StateDataIndex Editorial Team · Updated September 2026.