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

Hawaii County Code Chapter 25 (Zoning) is published by the Office of the County Clerk at hawaiicounty.gov/our-county/legislative/office-of-the-county-clerk/county-code. Article 1 (General Provisions) sets the framework, Article 4 (Use Districts) establishes the residential and agricultural districts where ADUs are allowed, and the ADU/ohana provisions live in Chapter 25 Article 6 around Sections 25-6-34 (Height and size limit) and 25-6-35 (Yard, parking and density standards). On September 19-20, 2024, the County Council adopted Bill 123 (Draft 5) by a 6-3 vote and Mayor Roth signed it into law shortly after. The ordinance: (1) eliminates the term 'ohana dwelling' from the Code and replaces it with 'Accessory Dwelling Unit'; (2) abolishes the historic two-step ohana-permit-then-building-permit process so an applicant now files only a building permit through the Department of Public Works (Building Division) and the Planning Department (planning.hawaiicounty.gov); (3) raises the maximum number of ADUs from one to three per lot in the eligible zones, while limiting lots that already host multiple primary dwellings to one ADU per primary unit; (4) caps each ADU at 1,250 square feet of living area, expressly excluding lanais, garages and carports from the calculation; (5) requires the lot to have a potable water source (county water or an approved private system) and an approved wastewater disposal system - county sewer where available, otherwise an Individual Wastewater System (IWS) under Hawaii Administrative Rules Title 11 Chapter 62, which generally requires at least 10,000 sq ft per septic system in unsewered areas; (6) requires the ADU to comply with the yard setbacks, height and lot-coverage standards of the underlying zoning district (RS, RD, RA, A or FA); and (7) allows guest houses (lodging without a kitchen) and ADUs to coexist on the same lot. Short-term vacation rental use is restricted - only one ADU per lot may be operated as a Short-Term Vacation Rental (STVR) and only where STVRs are otherwise permitted by Chapter 25 Article 6 Division 5 (the County's STVR ordinance). Bill 123 also implements Hawaii Revised Statutes Sec. 46-4.8 (enacted by Act 39, Session Laws of Hawaii 2024, effective May 28, 2024), which mandates that every county adopt or amend ordinances to allow at least two ADUs on each residentially-zoned urban lot and which voids private covenants recorded after May 28, 2024 that would limit ADU counts below the state floor. The state law excludes lots in zoning districts intended for rural, low-density residential development or open-space preservation. Hawaii County's RS, RD and RA districts qualify as 'residentially zoned' for purposes of HRS Sec. 46-4.8; the RG (General Residential), RM (Multiple-Family Residential) and the open and conservation districts use different rules. ADU applicants should also confirm Special Management Area (SMA) status under Chapter 25-A (the SMA chapter implements the Coastal Zone Management Act, HRS Ch. 205A) because lots within the SMA may require an SMA permit in addition to the building permit. Properties on lava-zone 1 or 2, in the flood hazard area, or on Department of Hawaiian Home Lands (DHHL) trust land have additional requirements - DHHL's separate Supplemental Dwelling Unit (SDU) program, not the County ADU rules, governs trust homestead lots.