Accessory Dwelling Units

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.