Building or Buying a Home with an Accessory Dwelli

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Real Estate

An ADU is a second dwelling unit that includes its own kitchen, bedroom, and bathroom facilities, and is attached or detached from the primary dwelling unit on the lot. ADUs are intended to be “accessory” to the main house and are typically smaller in size. ADU’s can be a new structure either separate or attached to the main home or they can be built from existing space, such as: a garage, recreation rooms, basements, or attic space.

Owners can convert an existing structure to an ADU, even if it was built without a building permit, by getting an after-the-fact permit. In addition to fulfilling the requirements of the after-the-fact permit, any alterations required to the structure must conform to ADU regulations.

How do residents benefit from an ADU? Depending on the situation, an ADU could be a solution for families with elderly parents or grandparents, adult children with disabilities, adult children returning home, live-in caregivers, a need for supplemental rental income or a need for flex space – such as guest quarters or a home office.

Some home buyers only want to look at homes that have ADU space already included in the home. Other buyers are willing to add an ADU to an existing home they purchase.

ADUs help address the shortage of rental properties by allowing existing homeowners to build a second unit on their property. Even when homeowners house family members, those are people that are not putting more pressure on the rental market.  In addition, senior homeowners can often “age in place,” often by moving into their ADU and renting the larger home for additional income or by having separate quarters for caregivers.

In Honolulu County, an ADU can be built on any lot zoned Residential (R-3.5, R-5, R-7.5, R-10, and R-20) or Country District with a lot area of 3,500 square feet (SF) or more provided that there is adequate infrastructure and subject to meeting all other Land Use Ordinance provisions for the zoning lot and where there are no restrictive covenants.

An ADU can be a maximum of 400 sf for lots ranging between 3,500 SF and 4,999 SF, and 800 sf in size for lots greater than 5,000 sf.

Only one additional dwelling unit is permitted to be built on a residential lot. ADUs are not permitted on lots that already have more than one dwelling unit, including but not necessarily limited to: more than one single-family dwelling, a two-family dwelling, accessory authorized Ohana Dwelling, or guest house. Properties with multi- family dwellings, or which are part of a planned development housing, cluster, or group living facility are also not eligible.

The Department of Planning and Permitting has an ADU Pre-Check Form available for homeowners or contractors to determine if there is adequate infrastructure and other requirements to permit an ADU.

If you would like to learn more about how to add an ADU to your property, visit

 By Cindy Siok, owner and Principal Broker of At Home Hawaii.