ADU Rules: Honolulu vs Mililani Town
How do adu rules rules compare between Honolulu, HI and Mililani Town, HI?
Honolulu has fewer restrictions than Mililani Town.
Honolulu, HI
Honolulu County
Honolulu allows one accessory dwelling unit on residential lots of 3,500 sq ft or more. Size is capped at 400–800 sq ft depending on lot size. Owner must live on the property and ADUs may only be rented long-term (six-month minimum).
View full Honolulu rules →Mililani Town, HI
Honolulu County
Mililani Town homeowners may build one ADU per zoning lot under ROH Sec. 21-5.720, but HOA covenants and association design rules often impose additional restrictions on top of city code.
View full Mililani Town rules →Key Facts Comparison
| Fact | Honolulu | Mililani Town |
|---|---|---|
| Minimum lot size | 3,500 sq ft | - |
| Max ADU size (small lot) | 400 sq ft (3,500–4,999 sq ft lot) | - |
| Max ADU size (large lot) | 800 sq ft (5,000+ sq ft lot) | - |
| Minimum rental term | 6 months (180 days) | - |
| Owner occupancy required | Yes — main house or ADU | - |
| - | - |
Highlighted rows indicate differences between cities.
Honolulu FAQ
Can I rent my ADU on Airbnb or use it as a vacation rental?
No. ADUs are expressly prohibited from being used as bed and breakfast homes or transient vacation units. All rentals must be for a minimum of six months under a written lease.
Do I need a permit to build an ADU or convert an existing structure?
Yes. A building permit is required. Structures built without a permit before September 14, 2015 may obtain an after-the-fact permit to be converted to an ADU.
What happens if I no longer live on the property?
The ordinance allows limited hardship exceptions (e.g., military deployment or serious illness). All other absences require the director's confirmation; failing to maintain owner-occupancy is a code violation.
Mililani Town FAQ
Does Mililani Town allow ADUs?
City zoning allows one ADU per eligible lot, but the Mililani Town Association and your neighborhood association may restrict or prohibit them. Always check your CC&Rs first.
Can I rent my Mililani ADU short-term?
No. City rules require 30-day minimum rentals under ROH Sec. 21-5.720, and most Mililani HOAs additionally restrict short-term rentals.
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