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Hawaii County, HI Accessory Structures: ADU Rules (2026)

Some Restrictions

Compiled from the official code textEditor Martyn O'NeillLast verified September 2026

ADU rules in Hawaii County, HI, also called accessory dwelling unit regulations or granny flat ordinances, cover setbacks, owner-occupancy, parking, and permit requirements.

Key Facts

Permitted districts: RS, RA, FA, A
Units per lot: One ohana dwelling (base rule)
Minimum combined lot area: 10,000 square feet
Detached yard setback: District minimum plus 5 feet
Variances: Prohibited (HCC 25-6-38)

ADU Eligibility Quick-Check

An Accessory Dwelling Unit (ADU) is a smaller, independent living unit on the same lot as a single-family home: converted garages, detached cottages, basement or attic conversions, and attached additions all qualify. Most states have streamlined ADU permitting since 2020, but local rules still control size, height, and setback.

Is your lot zoned single-family or multi-family residential?
Does your lot have at least 1,200 sq ft of buildable space?
Is your existing home owner-occupied (in most states this is not required, but some require it for ADU permits)?
Will the ADU stay under your state's size cap (commonly 850-1,200 sq ft for detached)?
Can you provide one off-street parking space (waived in many states within 0.5 mi of transit)?

If you answered 'yes' to most of these, you likely qualify in Hawaii under recent ADU streamlining laws. The Hawaii County, HI rules below cover local size, height, and setback requirements.

Summary

Hawai'i County allows one ohana dwelling (accessory dwelling unit) on RS, RA, FA and A zoned lots. The lot must be a legal lot of record, meet height, yard and parking rules, and have adequate sewage, water, fire and street access.

These county ordinances apply to unincorporated areas of Hawaii County. Incorporated cities within the county may have their own rules that take precedence over county-level regulations.

Regardless of the size of a building site, not more than one ohana dwelling unit shall be permitted on the same building site with the first single-family dwelling unit. The minimum building site area for a building site containing both the first dwelling and the ohana dwelling unit shall be ten thousand square feet.

Source: Hawaii County Code - Office of the County ClerkView official code

Full Breakdown

Under HCC Chapter 25 (Zoning), Section 25-6-30, an ohana dwelling is permitted on building sites in the RS, RA, FA and A districts. Section 25-6-33 allows not more than one ohana unit per lot with the first single-family dwelling. Combined lot area must be at least 10,000 sq ft (25-6-35(a)), and the detached ohana unit's yards must equal the district minimums plus 5 extra feet. Ohana units are prohibited in the State conservation district and on lots already permitting multiple dwellings (25-6-32). No variances are allowed (25-6-38). 2024's Bill 123 renamed ohana units as 'accessory dwelling units' and eased limits.

Violations & Fines

Illegally constructed ohana dwellings are subject to enforcement, permit revocation and code-compliance action by the planning director (HCC 25-6-39.5, 25-6-39.6).

Frequently Asked Questions

How many ohana units can I build?

The base code allows one ohana dwelling per lot. Bill 123 (2024) now permits up to three ADUs on lots in districts where ohana units are allowed.

Do I need public water and sewer?

The site needs an approved sewage system and an approved public or private water system; a catchment system may be allowed where the director finds sufficient rainfall (HCC 25-6-30).

Sources & Official References

Other rules in Hawaii County

Parking Rules: Street Parking LimitsSome Restrictions Shed & Carport RulesSome Restrictions Parking Rules: RV & Boat ParkingSome Restrictions Tiny HomesSome Restrictions ADU Rental RestrictionsSignificant Restrictions Noise Ordinances: Amplified Music & Outdoor SoundSome Restrictions Parking Rules: EV ChargingSome Restrictions Fence Regulations: Material RestrictionsSome Restrictions Fence Regulations: Fence Permit RequirementsSome Restrictions

All Hawaii County rules

How Hawaii County compares: Easiest Cities to Build an ADU·Compare Hawaii County to another location·View the Hawaii accessory structures overview

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