Colourful hats on sandy beach by the sea, Guest house or ADU, short for Accessory Dwelling Unit.

The 9 Basic Requirements for Building an ADU in Hawaii

Since the dawn of property ownership, people have been building smaller dwellings near their pre-existing homes. In Australia, they call them Granny Flats, in certain parts of the continental US, they’re called Mother-in-Law houses and here in Hawaii, the official term is ADU, short for Accessory Dwelling Unit. Like the acronym says, these are little homes that act as complete residential unit accessories to a main house on a shared property. Theoretically, anyone can build an ADU simply by making a nice addition or building with a kitchen, bedroom, and bathroom but officially there are certain legal guidelines for an approved ADU construction. Before you break out your or call a contractor, make sure your home and plans conform to the 9 basic requirements for building an ADU.

1) Proper Zoning

Zoning in Hawaii and knowing your exact zone can be a little tricky and ADUs can only be built in six specific zones. Before even thinking about building one of these tiny separate houses, make sure to check your lot’s zone by address and ensure that it matches one of the following:

  • R-3.5
  • R-5
  • R-7.5
  • R10
  • R20
  • Country District

2) Lot is At Least 3,5000 Square Feet

To ensure that there is plenty of room for your ADU, anyone who wants to build one needs to have a house lot that reaches at least 3,500 square feet of space. Fortunately, your original house is allowed to sit on some of it. Rather than breaking out the incredibly long measuring tape, you can check your lot’s size online to figure out if you qualify. Bigger is always better, and you’ll find out why with point nine.

3) Lot is Not Landlocked

To build an ADU, your lot will need to actually make contact with the road. Due to certain access requirements, a landlocked lot cannot hold an ADU. However, you can have an ADU if your lot is connected via driveway to the road by way of an easement through another closer lot. Flat lots are also perfectly fine.

4) Lot Has One Current Dwelling

You cannot build an ADU unless there is only one complete dwelling on your lot. This means that duplexes, apartment buildings, and lots that already have two or more homes on them cannot build an ADU. However, renovated apartments above the garage or in the basement are just fine as long as they’re not legally a second dwelling.

5) Owner or Family Lives On the Lot

To qualify for ADU construction, the homeowner or at least one member of their family needs to live on the lot when the ADU is complete. This means they can either live in the main house or the ADU based on preference, as long as the owner or a family member is in residence. You are allowed to rent out whichever building you’re not going to live in.

6) Record Your Covenants

When you do build an ADU, you are required to record covenants with the land with either the Bureau of Conveyances or the Land Court of the State of Hawaii or both. This ensures that you will never sell the ADU separately from the rest of the property and the lot cannot be subdivided into two separate properties. There are also other related rules in the Declaration of Restrictive Covenants.

7) No ADU Covenant Restrictions

Check any private covenant signed for your land to ensure that it doesn’t prohibit an ADU. If you have already agreed not to build one, starting now would be both illegal and quite silly. However, you may not be aware that the prohibition exists to make sure to check with your HOA to be sure.

8) Room For One More Parking Space

An ADU is officially an entirely separate second dwelling and because of this, it’s expected that the resident is likely to have their own car. Therefore to build a legal ADU, you’ll need room within the lot’s parking areas for one more parking space than your lot’s minimum spaces unless the edge of your property is within a half mile of a rail station.

9) ADU Plans Meet Maximum Size

The maximum square foot area of your ADU is limited by the square footage of the lot itself. There is also a minimum lot size for ADU’s which is 3,500 SF. On lots of this size and up too 4,999 SF your ADU can be 400 square feet maximum.

For lots 5,000 SF and larger, your ADU can be built up to 800 SF maximum.

By the end of this list, you should have a fairly good idea of whether or not you’re legally allowed to build an ADU on your property in addition to your single-family home. If you do decide that an ADU is right for you, your lot, and your family contact us today! We’ll be happy to help you figure out the next steps.

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4 thoughts on “The 9 Basic Requirements for Building an ADU in Hawaii

  1. Hi. You mention that for lots above 5K, an ADU greater than 800 sq ft is possible. All other information sources I read state that 800 sq ft is the largest for lots above 5K. Can you clarify? thanks.

  2. Our existing home has a 2 car covered garage and 2 uncovered parking spaces behind the 2 covered spaces. My husband and I are retired and we each have our own car. We plan to build an ADU for my 88 year old mother who doesn’t drive. Do we still need to create another parking space even though we don’t need it. The ADU we plan to build is 430 sqft. Are we still required to create another parking space on our property?

    1. Hi Bryan, the only provision I am aware of that would eliminate the parking requirement is if it is located within a certain distance to a proposed rail station. Unfortunately they look at the parking requirement as part of the property rather than the tenant need. The parking you have may be enough though, it depends on the square footage of the main dwelling. If it is less than 2,500 SF you only need two for that dwelling, so you could use one of the other stalls you have for the ADU.

      Call me if you need more info.

      (808) 383-4632

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