Accessory Dwelling Units (ADUs) in Hawaii – A Homeowner’s Guide

Hawaii, renowned for its stunning natural beauty and vibrant culture, faces significant challenges when it comes to housing availability and affordability. In response, Accessory Dwelling Units (ADUs) have gained considerable traction as a versatile and practical solution.

These secondary residential units, built on the same property as a primary home, offer a multitude of benefits. They provide homeowners with opportunities to generate rental income, accommodate extended family members, or simply maximize the use of their property. Furthermore, ADUs contribute incrementally but significantly to the state’s overall housing inventory, offering more options for residents across the islands. As regulations continue to evolve, particularly on Oʻahu and neighboring islands, the prospect of adding an ADU is becoming increasingly accessible for many property owners.

Accessory Dwelling Units (ADUs) are increasingly recognized as a vital solution to Hawaii’s housing challenges. This summary provides homeowners, buyers, and real estate professionals with a concise overview of ADUs across the islands, covering regulations, benefits, and key considerations as of May 2025.  This blog article covers the essential definitions, county-specific regulations as of May 2025, practical tips for planning and construction, and valuable resources for homeowners considering this path. Whether you are a homeowner exploring options, a potential buyer evaluating property potential, or a real estate professional advising clients, this information aims to clarify the process and highlight the opportunities presented by ADUs in the Aloha State.

What is an ADU?

An ADU is a secondary, self-contained home (with kitchen, bathroom, sleeping area) on the same lot as a primary single-family residence. They can be detached, attached, or converted spaces. ADUs offer benefits like rental income, housing for family, and contribute to affordable housing options within existing neighborhoods.

An Accessory Dwelling Unit, often referred to simply as an ADU, is a distinct, self-contained living space situated on the same parcel of land as a primary single-family residence. These units are designed for independent living and typically include their own kitchen, bathroom, and sleeping area. ADUs can take various forms: they might be detached structures (like a small cottage in the backyard), attached additions to the main house (such as a unit built over a garage or as a separate wing), or even conversions of existing spaces like basements or garages, provided they meet building code requirements for living spaces.

The primary purpose of encouraging ADU development is multifaceted. For homeowners, it can be a strategic financial move, providing a steady stream of rental income to offset mortgage payments or living costs. For families, ADUs offer a way to support multi-generational living, allowing aging parents or adult children to live nearby while maintaining privacy and independence. From a community perspective, ADUs represent a form of gentle density, increasing housing options within established neighborhoods without drastically altering their character. They often provide more affordable rental choices compared to larger single-family homes or new apartment complexes, contributing to a more diverse housing market.

Why Consider an ADU in Hawaii?

• Financial Gain: Generate rental income to offset costs.
• Family Needs: Provide independent living space for relatives.
• Housing Solution: Contribute to easing Hawaii’s housing shortage.
• Property Value: Potentially increase your property’s market value.

The Bottom Line: 9 High-Level Requirements for ADU on Oahu

1. Your lot must be zoned in a residential or country district.
2. Your lot area must be at least 3,500 square feet.
3. Your lot cannot be landlocked.
4. Your lot must contain one single-family dwelling.
5. The property owner, family of the property owner, or designated representative must reside on the property.
6. The property owner will record covenants running with the land with the Bureau of Conveyances or the Land Court of the State of Hawaii, or both.
7. If you are in a private covenant prohibiting ADUs, you cannot build one.
8. You must provide a parking space for the ADU in addition to the required parking for your existing house.
9. You must abide by the ADU floor space limitations.

Hawaii ADU Regulations Overview (May 2025)

Here is a Summary of Regulations which vary by county. Always consult the local planning department for specifics.

• Oʻahu (Honolulu):
  • Allowed in most Residential (R-3.5 to R-20) and Country zones.
  • Requires 1 extra parking space (waiver possible near rail stations).
  • Fee waivers (e.g., wastewater fees from July 1, 2025) available.
• Maui County (Maui, Molokaʻi, Lānaʻi):
  • Currently: Up to 2 units (“ohana”) on lots >= 7,500 sq ft; 1 unit possible on smaller lots.
  • Upcoming (by Dec 31, 2026): State law requires allowing at least 2 ADUs on all residential lots, regardless of size.
• Hawaiʻi County (Big Island):
  • Up to 3 ADUs allowed on residential and agricultural properties (Bill 123).
  • Intended for long-term occupancy.
• Kauaʻi County:
  • Residential: 1 ADU and up to 2 Accessory Rental Units (ARUs) allowed.
  • Agricultural: 1 guest house per farm dwelling permitted (Bill 2933).

Here is a Summary of Key Details – County-by-County for ADU Regulations (May 2025 Update)

Navigating the regulatory landscape is crucial when considering an ADU project, as rules vary significantly across Hawaii’s counties. It’s essential to consult the latest ordinances and guidelines from the respective county planning departments. Here’s a summary of the regulations as of May 2025:

Oʻahu (City & County of Honolulu)

On the most populous island, Oʻahu, the City and County of Honolulu has specific guidelines for ADU development aimed at balancing increased housing with neighborhood character.

• Zoning Eligibility: ADUs are generally permitted in Residential zones designated R-3.5, R-5, R-7.5, R-10, and R-20. They are also allowed in Country Districts (zoned AG-1 and AG-2), provided certain conditions are met. It’s vital to verify the specific zoning of your property through the Department of Planning and Permitting (DPP).
• Unit Size and Setbacks: Size limitations and setback requirements (the distance the structure must be from property lines) apply and can vary based on the zone and lot size. These regulations ensure adequate space, light, and privacy.
• Parking Requirements: Generally, one additional off-street parking space must be provided for each ADU. However, a significant incentive exists for properties located within a half-mile radius of a planned or existing rail transit station; in these transit-oriented development (TOD) areas, the additional parking requirement may be waived, encouraging density near public transportation.
• Fee Waivers: To further incentivize ADU construction, the City and County has implemented fee waivers. Notably, starting from July 1, 2025, certain impact fees, such as wastewater system facility charges, are waived for qualifying ADU projects. This can represent substantial cost savings for homeowners. (Source: Steadily | Landlord Insurance, Honolulu DPP)

Maui County (Maui, Molokaʻi, Lānaʻi)

Maui County has traditionally referred to ADUs as “ohana units,” reflecting their common use for extended family. Recent state legislation is prompting changes to expand ADU allowances.

• Current Allowances: Under existing rules, properties with a lot size of 7,500 square feet or more are typically allowed to have up to two accessory dwellings (ohana units). Smaller lots might be permitted one unit, contingent upon meeting specific requirements for parking, setbacks, and utility connections.
• Upcoming Changes (Act 39): State legislation (Act 39) mandates significant changes to promote ADU development statewide. By December 31, 2026, Maui County must amend its ordinances to allow at least two accessory dwellings on all residentially zoned properties, irrespective of the lot size or specific island location (Maui, Molokaʻi, or Lānaʻi). This change is expected to open up ADU possibilities for many more homeowners.
• Considerations: Prospective builders should monitor the Maui County Planning Department for updates on how Act 39 will be implemented locally, including any new design standards or requirements. (Source: Mauiproperty.com, Maui Now, AllSides)

Hawaiʻi County (Big Island)

The County of Hawaiʻi has taken proactive steps to increase housing density through ADUs, particularly on larger residential and agricultural lots.

• Expanded Capacity (Bill 123): Recent legislation, specifically Bill 123, significantly expands ADU potential. Homeowners on properties zoned for residential (RS) or agricultural (A) use are now permitted to build up to three ADUs. This applies to both attached and detached units.
• Occupancy: These ADUs are intended for long-term occupancy, aiming to increase the stable housing supply rather than short-term vacation rentals.
• Infrastructure: As with other counties, property owners must ensure that existing or planned infrastructure (water, wastewater, etc.) can support the additional units. (Source: Steadily | Landlord Insurance, Hawaii Public Radio, Honolulu Civil Beat)

Kauaʻi County

Kauaʻi County allows for ADUs and has also introduced provisions for Accessory Rental Units (ARUs) and guest houses, providing flexibility for property owners.

• Residential Zones: On properties zoned for single-family residential use, homeowners are generally allowed to have one ADU. In addition, they may be permitted up to two Accessory Rental Units (ARUs), which might have different size or occupancy requirements compared to standard ADUs. Checking specific zoning definitions is key.
• Agricultural Zones: Recognizing the potential for housing on larger agricultural parcels, recent legislation (Bill 2933) now permits one guest house per existing farm dwelling on agriculturally zoned (AG) land. While distinct from an ADU, this increases the potential for additional housing units on these properties.
• Consultation: Given the nuances between ADUs, ARUs, and guest houses, consulting directly with the Kauaʻi County Planning Department is highly recommended to understand the specific entitlements for a given property. (Source: Hawaii Life, Hawaii Public Radio)

Key Steps & Tips for Building an ADU

1. Step: Verify Eligibility: Confirm your property’s zoning and lot size meet county requirements.
   1. Tip:  Verify Zoning and Lot Compliance: The first step is always confirmation. Use county resources (online maps, planning department consultations) to verify your property’s zoning designation and ensure it meets the minimum lot size and other prerequisites for ADU eligibility in your specific county.
2. Step: Understand Rules: Research specific county regulations on size, setbacks, parking, etc.
   1. Tip:  Deep Dive into Local Regulations: Beyond basic eligibility, understand the detailed requirements: maximum unit size, height restrictions, setback distances from property lines and existing structures, parking mandates (and potential waivers), and any specific design guidelines.
3. Step:  Consult Experts: Hire architects/builders familiar with local ADU rules.
   1. Tip: Consult Professionals: Engage with architects, designers, or builders who have experience with ADU projects in your specific county. Their familiarity with local codes, permitting processes, and common challenges can be invaluable, saving time and preventing costly mistakes.
4. Step:  Plan Design: Focus on efficiency, durability, and aesthetic fit.
   1. Tip: Design for Efficiency and Context: Thoughtful design is key. Consider space-saving layouts, energy-efficient features (like solar panels or good insulation, crucial in Hawaii’s climate), and materials that are durable and suitable for the local environment. The design should also complement the main house and fit within the neighborhood context.
5. Step:  Check Utilities: Ensure water, sewer/septic, and electrical systems can handle the load.
   1. Tip: Assess Infrastructure Capacity: Adding a dwelling unit places additional demands on utilities. Assess whether your existing water meter, sewer connection (or septic system), and electrical panel have sufficient capacity. Upgrades may be necessary and should be factored into the budget.
6. Step:  Budget Carefully: Account for all costs, including permits, construction, and contingency.
   1. Tip:  Budget Realistically: Develop a comprehensive budget that includes design fees, permit costs, construction labor and materials, utility connection or upgrade fees, landscaping, and a contingency fund (typically 10-15%) for unexpected issues.
7. Step:  Seek Incentives: Look for fee waivers or financing programs.
   1. Tip: Explore Financial Incentives and Financing: Research available financial assistance. Look into county-specific fee waivers (like Honolulu’s wastewater fee waiver), potential state or federal grants (if available), and financing options such as home equity loans, lines of credit, or construction loans specifically tailored for ADUs.
8. Step: Know Occupancy Rules: Be aware of owner-occupancy requirements and rental term restrictions (often no short-term rentals).
   1. Tip: Understand Occupancy Restrictions: Be aware of any county rules regarding occupancy. Some regulations may require the property owner to reside in either the primary dwelling or the ADU, and there are often restrictions against using ADUs for short-term vacation rentals (less than 30 days, or sometimes longer minimums like 180 days).

Financial Aspects

Building an ADU involves costs but offers potential returns through increased property value and rental income. Research local construction costs and rental rates. Utilize available county fee waivers to reduce expenses.  Building an ADU is a significant investment, but it can also offer substantial financial returns and benefits.

• Construction Costs: Costs can vary widely based on size, design complexity, materials, location, and whether it’s a new build, conversion, or prefabricated unit. Obtaining multiple quotes from qualified builders is essential.
• Increased Property Value: An ADU generally increases a property’s market value, although the exact amount depends on the quality of the unit, the local real estate market, and the potential rental income.
• Rental Income Potential: ADUs can generate significant rental income, helping to cover mortgage payments, property taxes, or other expenses. Research local rental rates for comparable units to estimate potential income.
• Fee Waivers and Incentives: As mentioned, take full advantage of any available fee waivers (e.g., impact fees, utility connection fees) offered by the county, as these can reduce the upfront cost considerably.

The Future is Evolving

Essential Resources

For the most accurate and up-to-date information, consult the official county planning departments and other reliable sources:

• City & County of Honolulu Department of Planning and Permitting (DPP): honolulu.gov/dpp
• Maui County Planning Department: mauicounty.gov/121/Planning-Department
• Hawaiʻi County Planning Department: hawaiicounty.gov/departments/planning
• Kauaʻi County Planning Department: kauai.gov/Planning
• Owner Built Design – ADU FAQs: ownerbuiltdesign.com/accessory-dwelling-unit-in-hawaii-a-deep-dive-faq-with-island-soul
• Steadily – ADU Laws and Regulations in Hawaii: steadily.com/blog/adu-laws-and-regulations-in-hawaii

Conclusion

• ADUs present a valuable opportunity for Hawaii property owners. By understanding the regulations, planning diligently, and utilizing available resources, you can successfully navigate the process and unlock the potential of an ADU on your property.
• Accessory Dwelling Units offer a compelling opportunity for Hawaii homeowners to enhance their property’s utility and value while contributing positively to the state’s housing landscape. By understanding the definition, navigating the specific county regulations, planning carefully, and utilizing available resources, property owners can successfully add an ADU. Whether for family, rental income, or flexible living space, ADUs represent a smart, adaptable housing solution well-suited to the unique context of the Hawaiian Islands. Staying informed and engaging with local planning departments and experienced professionals are key steps on the path to realizing your ADU project.

Thanks for reading this blog post and just let me know if I can be of further service……Aloha, Jon.