MRSC: Accessory Dwelling Units

An accessory dwelling unit (ADU) is a small, self-contained residential unit located on the same lot as an existing single-family home.

An ADU has all the basic facilities needed for day-to-day living independent of the main home, such as a kitchen, sleeping area, and a bathroom. As the term "accessory" implies, ADUs are generally defined to be smaller in size and prominence than the main residence on the lot. Some definitions include specific size limits, and a location that is not readily visible from the street.

In theory, an ADU may be created as a separate unit within an existing home (such as in an attic or basement), an addition to the home (such as a separate apartment unit with separate entrance), or in a separate structure on the lot (such as a converted garage). See the examples shown below.

Some communities, however, only allow ADUs that are within or attached to the main residence, and exclude ADUs housed in a separate structure. Whether attached or detached from the main residence, most codes require that the main residence and the ADU must be owned by the same person and may not be sold separately.

ADUs are sometimes called "mother-in-law apartments" or "granny flats," because they are often used to house extended family. Other codes use terms such as "accessory apartment," "accessory living unit," or "secondary unit," to have a similar meaning.