What Is an ADU?
An accessory dwelling unit (ADU) is a second, smaller home on the same lot as a primary residence, with its own kitchen, bathroom and living space. On Oahu, ADUs may be detached cottages, units above a garage or attached units within or added to an existing home.
Why Consider One?
- Create rental income from land you already own.
- Provide housing for family members, such as aging parents or adult children, which is common in multigenerational Hawaii households.
- Add long-term value to your property.
- Help address Oahu’s housing shortage.
Honolulu Rules Overview
The City and County of Honolulu adopted an ADU ordinance in 2015 to encourage more long-term rental housing. ADU rules address items such as minimum lot size, maximum unit size, parking, utility connections, owner requirements and rental terms. Short-term (vacation) rental of ADUs is generally not allowed.
Zoning and building rules change and depend on your specific lot, including sewer and water capacity, easements and any private covenants or HOA restrictions. Always confirm current requirements with the Honolulu Department of Planning and Permitting (DPP) before you design or build.
Planning & Permits
- Check eligibility. Review your zoning, lot size and any CC&Rs or HOA rules. DPP offers resources on ADU requirements.
- Confirm infrastructure. Sewer and water availability can determine whether an ADU is feasible.
- Work with professionals. A licensed architect or designer and contractor can prepare plans that meet code.
- Apply for permits. Submit plans to DPP for review and building permits.
- Build and inspect. Construction is inspected before the unit can be occupied.
Budgeting
Costs vary widely depending on size, design, site conditions and utility work. Budget for design, permitting fees, utility connections, construction, landscaping and furnishings, plus a contingency. Compare total cost against realistic long-term rent to estimate your return; our rental cash flow calculator can help.
Renting Your ADU
Once your ADU is complete and approved for occupancy, it can be rented like any other long-term rental, following the Hawaii Residential Landlord-Tenant Code. Consider how the units share parking, utilities, yard space and privacy, and put clear expectations in writing.
- Decide whether utilities will be separately metered or included in rent.
- Set clear rules for shared areas and parking.
- Screen applicants consistently and follow fair housing laws.
- Account for General Excise Tax (GET) on rental income.
Next Steps
Thinking about building or renting an ADU? We can help you estimate rent and manage the unit once it is ready. Tell us about your property.