An accessory dwelling unit (ADU) is typically an additional living area independent of the primary dwelling unit, and includes a fully functioning kitchen and bathroom.

Types of ADUs

There are three main types of ADUs:

  • Attached ADU (AADU)
  • An addition that adjoins the primary dwelling, either to the side or rear of the home, or built on top of an attached garage.

  • Detached ADU (DADU)
  • A stand-alone structure apart from the primary dwelling.  It can be built as an entirely separate unit or over an existing accessory structure, such as a detached garage. A DADU may be called a backyard cottage. Tiny houses, with foundations, are considered DADUs.

  • Interior ADU
  • Located within the primary dwelling unit, it is built from existing converted space, usually an attic or basement.  May be called basement apartments, mother-in-law units, in-law units, secondary suites, English basements, accessory apartments, and other.

Category Determinants

Whether a property is defined as a one-unit property with an accessory unit or a two-unit property will be based on the characteristics of the property, which may include, but are not limited to, the existence of separate utilities, a unique postal address, and whether the unit is rented.

Note: Tiny houses on wheels are treated like camper trailers. In many communities, you cannot live in a tiny house on wheels (or similar equipment such as RVs and boats) on a lot. If your tiny house has wheels you need to follow parking rules for large vehicles.

ADUs Offer Privacy, But Proximity to Assistance When Needed

An ADU may be a preferred living arrangement for a senior who wants to age in place, allowing them to live next to and receive needed support and from residents in the primary dwelling unit, while retaining as much independence as possible. Building an accessory dwelling unit can involve a significant investment of both time and money. But after the initial build has been completed, ADUs are less costly to operate and maintain per square foot.

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In-laws, Outlaws, and Granny Flats: Your Guide to Turning One House into Two Homes Paperback – March 15, 2011

In response to tight times, a remarkably upbeat and widespread change is taking place in households across America. Homeowners are creating second dwelling units often called in-law suites, mother-in-law apartments, or granny flats. Second units make a lot of sense. They're perfect for families who want several generations living close by, they enable Baby Boomers to care for elderly parents while respecting their independence, provide private quarters for adult children still at home or, rented out, second units can generate income to pay the mortgage or provide for retirement. In-Laws, Outlaws, and Granny Flats is the first book to explore the many designs, uses and benefits of this time-honored and emotionally satisfying living arrangement. In-law units take many forms and they're all shown here: attic, basement and garage conversions, bump-out additions, carve-out suites, and backyard cottages. This book covers every aspect of turning one house into two homes. With more than 200 color photographs, 50 floor plans and architectural details, and a lively, personable voice, In-laws, Outlaws, and Granny Flats is perfect for homeowners who want richer lives and a more secure future.