Building an ADU or Ohana Unit in Hawaii: Rules, Costs, and Design

An ADU or ohana unit is one of the most flexible things you can add to an Oahu property: rental income, room for family, a home office, or a guest house. Here is what it takes to build one well, what it costs, and what we learned delivering a luxury ADU in Kailua.

What qualifies as an ADU or ohana on Oahu

Oahu allows accessory dwelling units and ohana units on many residential lots, each with its own eligibility rules. The first step is confirming your lot qualifies and which type fits, because the requirements differ and they shape everything that follows.

Lot and zoning basics

Eligibility hinges on lot size, zoning, parking, and whether utilities can support a second unit. This is a determination worth making before you fall in love with a design, and it is the first thing we check.

What it costs

A small ADU can be modest, but a high-end one is a real build. Our Kailua Modern project is a two-story, 1,000-square-foot luxury ADU we designed and delivered for around $750,000. Small footprint, high finish. The number reflects the quality, not the square footage, which is the thing most people underestimate about a luxury ADU. For the bigger picture, see our guide to what a renovation costs.

Design lessons from a small footprint

A great ADU is a design problem before it is a construction one. Every square foot has to work, storage has to be engineered in, and the unit has to feel generous despite its size. That is exactly where a design-led process earns its keep.

Rental versus multigenerational use

How you will use it shapes the design. A rental wants durability and a layout that lives independently. A multigenerational unit wants connection to the main home and aging-in-place thinking. Decide the use first, then design to it. If you are weighing renovating versus building new on the same lot, we cover that too.

When you are ready, tell us about your project and we will start with whether your lot qualifies.