What is an ADU under Hawaiʻi law?

An ADU is a secondary dwelling permitted on the same parcel as a primary residence. It is accessory in use and may be detached or attached, subject to zoning and building rules.

How many ADUs are allowed on one Big Island property?

Hawaiʻi County rules may allow up to three ADUs on a single lot, depending on zoning, lot size, and infrastructure capacity.

Can ADUs use catchment water?

Yes. Hawaiʻi County now clearly allows ADUs to be served by compliant rainwater catchment systems.

Do ADUs require a separate septic system?

Not necessarily. State Act 224 allows one septic or aerobic system to serve up to five bedrooms total across all dwellings.

Does Act 224 allow cesspools?

No. The law applies only to approved septic or aerobic wastewater systems, not cesspools.

Is the 1,250 sq. ft. ADU size guaranteed?

No. The 1,250 square foot limit is a state maximum. Actual ADU size depends on county zoning and site conditions.

Do CC&Rs override County ADU rules?

Yes. Private covenants and subdivision restrictions may prohibit or limit ADUs even if county rules allow them.

Are these ADU rules already in effect?

Yes. Both Hawaiʻi County Bill 123 and State Act 224 are currently in effect.

Can ADUs be used as long-term rentals?

In many cases, yes. Long-term residential use is generally allowed where zoning permits, though short-term rental rules are separate.

Does zoning still matter with the new laws?

Yes. Zoning remains the primary factor in determining what may be built. The new laws expand flexibility but do not replace zoning rules.

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Home » Archives for January 6, 2026

Archive for January 6, 2026

Big Island ADU Laws 2024: What Hawaiʻi County and State Rules Now Allow

By Emil Knysh and Kai IohKai and Emil are luxury real estate advisors based in Kona, Hawai‘i, specializing in resort and ultra-high-net-worth markets across the Big Island. Executive Summary This page explains how 2024 updates to Ha...

January 6, 2026

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