Side by side · Hawaii
Compare ADU rules across Hawaii
Flexible on where you build? The same backyard cottage can be routine in one county and impossible in the next. This table puts 4 Hawaii counties side by side so you can shortlist the friendly ones before reading a single ordinance, or narrow it to just the counties you are deciding between. Every value is summarized from the county's own code; open a county's page for the full notes, citations, and fine print.
The short answers
Only comparing a few counties?
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Only show counties where:
| County | Detached ADU | Attached / internal | Max size | Owner occupancy | Extra parking | Approval |
|---|---|---|---|---|---|---|
| Hawai’i County | Allowed | Allowed | 1,250 sq ft of living area per ADU, excluding lanai, garage and carport; height capped at the district limit (35 ft in RS) | Not required | 2 off-street spaces per ADU (the single-family dwelling rate) | Permitted use |
| Honolulu | Allowed | Allowed | 1,000 sq ft on zoning lots of 5,000 sq ft or more; 500 sq ft on lots up to 4,999 sq ft | Not required | 1 space in addition to the primary dwelling's; none within 0.5 miles of a rail transit station | Permitted use |
| Kaua’i | Allowed | Allowed | No square-foot cap on the ADU itself (it is a full second single-family dwelling); sized instead by lot coverage (60% in R-1 to R-6, 80% in R-10, 90% in R-20) and height (2 stories, 20 ft to the wall plate, 30 ft to the roof peak). Guest house: 800 sq ft maximum. The separate 800 sq ft Additional Rental Unit lapsed 3-19-2023. | Not required | 2 off-street spaces for the ADU (Kaua'i requires 2 per dwelling unit and an ADU is a dwelling unit); a guest house adds 1 space | Permitted use |
| Maui | Allowed | Allowed | 720 sq ft on lots up to 21,779 sq ft; 840 sq ft (21,780 to 43,559); 960 sq ft (43,560 to 87,119); 1,200 sq ft (87,120 sq ft or more); carports, garages and parking areas excluded | Not required | 1 space for a studio or 1-bedroom ADU, 2 spaces for a 2- or 3-bedroom ADU; must be a carport, garage or other off-street space, and tandem parking counts | Permitted use |
Summarized from each county's ordinance text, most recently read September 2026. Tap a value marked with an info icon to see exactly what it depends on, straight from the ordinance. Nearly every value has zone, lot-size, or process caveats beyond that; the per-county pages carry the full notes and code citations. Treat this as orientation, not a permit determination.
These rules change. Be first to know.
Counties amend ADU ordinances all the time. Get one short email when any Hawaii county in this table changes its rules; no newsletter, no noise.
Reading the table
- Detached ADU is the make-or-break column for backyard cottages: minimum lot sizes (often 8,000 to 14,500 sq ft) hide behind many of the "Allowed" values, so check your lot before celebrating.
- Attached / internal (basement apartments, additions) is legal almost everywhere and is usually the path of least resistance.
- Approval: "Permitted use" means staff-level sign-off; "Conditional use" means a public hearing, which adds time, cost, and uncertainty.
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