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

Kauai County's accessory dwelling regime is set out in the Comprehensive Zoning Ordinance (CZO), Kauai County Code Chapter 8, and is administered by the County of Kauai Planning Department. The County recognizes three different accessory unit types, each with its own rules. The Additional Dwelling Unit (ADU) is governed by CZO Article 15 and is allowed in Residential districts only, on a lot with one principal single-family dwelling. ADUs are not capped at a specific square footage; size is limited only by the underlying district's lot coverage, height, and setback rules. Each ADU must contain a complete kitchen, may be attached or detached, and requires two additional off-street parking spaces beyond the two required for the principal dwelling. The Additional Rental Unit (ARU) is governed by CZO Article 8 (additional rental units) and was created in 2017-2018 to expand long-term rental supply. ARUs are limited to a maximum gross floor area of 800 square feet (excluding lanais, porches, and breezeways), must include a kitchen, bedroom(s), and bathroom, and require one additional off-street parking space. ARUs are restricted to Residential zoning districts (R-1 through R-6 and R-10 through R-20), and a parcel may have either one Guest House or one ARU per dwelling unit, but not both. ARUs must be used only for long-term rental, defined as occupancy for at least six months per year, and may not be used as transient vacation rentals, homestays, or any short-term accommodation. ARUs are prohibited on any property located within a Kauai County Tsunami Evacuation Zone west of the Hanalei River. The Guest House provisions, recently amended by Council Bill 2933 (2024), now allow one Guest House per principal dwelling in Residential, Agricultural, Open, and certain other districts. The maximum size was raised from 500 to 800 square feet, and a kitchen is now permitted. Guest Houses cannot be sold separately from the principal dwelling and cannot be used as short-term rentals. Within the Special Management Area (SMA) under HRS Chapter 205A, only one Guest House is allowed per parcel. Before applying for a building permit, an applicant must submit an ADU/ARU/Guest House Clearance Form through the County's online portal (connect.kauai.gov) for sign-off by the Department of Water (water meter capacity), the Wastewater Division of Public Works or the State Department of Health (sewer or individual wastewater system / septic / cesspool review), the Fire Department (access and fire flow), and other reviewing agencies. The Department of Water typically requires that the existing 5/8-inch meter has sufficient fixture-unit capacity for the additional unit; otherwise a meter upgrade or additional Facilities Reserve Charge (FRC) is required. The County Housing Agency administers an Affordable Additional Rental Unit Program that provides FRC subsidies for ARU owners who deed-restrict the unit to local income-qualified renters. At the state level, Hawaii Revised Statutes Section 46-4 has long authorized counties to allow a second dwelling on residential lots ('ohana zoning'). Hawaii Act 39, Session Laws of Hawaii 2024 (SB 3202), enacted HRS Section 46-4.8, which requires every county to adopt or amend ordinances by December 31, 2026 to allow at least two accessory dwelling units on each residentially zoned lot, subject to limited exceptions where utility infrastructure is insufficient. Kauai County officials have stated that the County's existing ADU + ARU framework (one ADU plus up to one ARU or Guest House) already meets the Act 39 requirement, so no major Chapter 8 overhaul is anticipated. Kauai's Hawaii Revised Statutes Chapter 205 agricultural-district lots use a separate farm-dwelling regime tied to bona fide agricultural use rather than the CZO accessory-dwelling rules.