Essential Guide to Accessory Dwelling Units (ADUs) – FAQs and Regulations

A welcoming modern accessory dwelling unit (ADU) in a lush backyard setting, visible behind a main house under clear, sunny skies.


Accessory Dwelling Units (ADUs) are becoming increasingly popular as a solution to housing shortages and a way to utilize property space efficiently. Often referred to as granny flats, in-law units, or secondary houses, ADUs provide a unique opportunity for homeowners to maximize their property’s potential. In this guide, we will dive into the most frequently asked questions about ADUs, exploring their benefits, regulatory requirements, and practical considerations.

What You’ll Learn

On this page, you’ll discover:

  • What an ADU is and how it can benefit you.
  • Important considerations and requirements for building an ADU.
  • Answers to the most common questions homeowners have about ADUs.

What is an Accessory Dwelling Unit (ADU)?

An Accessory Dwelling Unit (ADU) is a secondary housing unit on the same lot as a primary residence. These units can be attached to the existing home or built as a standalone structure. ADUs include all the essential facilities needed for independent living, such as a kitchen, bedroom, and bathroom. They are designed to be smaller and are often located to the rear or side of the main house. Whether you’re looking to accommodate family members or generate rental income, ADUs offer a versatile solution.

Why are ADUs Important?

ADUs play a crucial role in addressing the affordable housing crisis in many urban areas. For example, on Oahu, as outlined in the Mayor’s Affordable Housing Strategy, ADUs provide much-needed rental housing options. They also allow homeowners, particularly seniors, the flexibility to “age in place” by moving into these smaller, manageable units while renting out their primary homes to supplement their income.

Eligibility and Requirements: Where Can I Build an ADU?

You can build an ADU on any residentially zoned lot of 3,500 square feet or larger, subject to specific land use ordinances. However, the lot must not already contain more than one dwelling unit. The construction of an ADU also depends on the availability of adequate infrastructure like sewer and water services, which must be confirmed through a pre-check with local authorities.

Size and Occupancy Limits: How Big Can an ADU Be?

The size of an ADU depends on the size of your lot:

  • Lots between 3,500 SF and 4,999 SF can accommodate an ADU up to 400 square feet.
  • Lots 5,000 SF and larger can have an ADU up to 800 square feet.

Only one ADU is allowed per lot, and it cannot be sold separately from the primary residence.

The Difference Between ADUs and Ohana Dwelling Units

While both ADUs and ohana dwelling units are designed for family living, they cater to different needs. Ohana dwellings are intended strictly for family members and have no size limitations, requiring two parking spaces. ADUs, on the other hand, can be rented to non-family members, require only one parking space, and are size-restricted.

Ownership and Legal Considerations

The owner of the property must live in either the primary house or the ADU. ADUs must be rented for a minimum of six months to prevent their use as short-term vacation rentals. This regulation helps maintain the integrity of residential neighborhoods and ensures that ADUs serve their purpose as long-term housing solutions.

Infrastructure and Building Permits

Before you build an ADU, it’s essential to ensure that your property has adequate sewer and water capacity to support the additional dwelling. Local authorities must approve all infrastructure capabilities to avoid overloading systems. Moreover, a building permit is required for both new constructions and conversions of existing structures into ADUs.

Parking and Accessibility

ADUs generally require one additional parking space on the lot. However, if the ADU is located within a half-mile of a rail transit station, the parking requirement may be waived to encourage the use of public transportation.

Building an ADU: Steps to Consider

  1. Planning: Determine the feasibility of an ADU on your lot by reviewing local zoning laws and consulting with the Department of Planning and Permitting.
  2. Designing: Choose a design that complies with local regulations and fits the character of your neighborhood.
  3. Permitting: Obtain the necessary permits, ensuring your plan meets all structural, safety, and zoning requirements.
  4. Building: Hire qualified contractors to build your ADU, adhering to all building codes and inspections.

Conclusion: Enhancing Your Property with an ADU

An architect's drawing table with blueprints of an ADU design spread out, featuring architectural tools like a ruler, pencils, and a calculator.
Architect’s drawing & model of an ADU.



Adding an ADU to your property can be a smart investment, enhancing your living space or providing additional income through rental. With careful planning and adherence to local regulations, homeowners can successfully navigate the process and enjoy the benefits of an additional dwelling unit.

Do you have any more questions on ADUs, or is there another topic related to real estate or home improvement you’re curious about?