Accessory Dwelling Units Accessory dwelling units, or ADUs, are a new buzzword in the housing world even though the concept is nothing new. Sometimes called “mother-in-law apartments” or “granny flats,” ADUs are self-contained residential units located on the same lot as an existing single-family home. According to the Municipal Research and Services Center, ADUs have all the basic facilities needed for day-to-day living, such as a kitchen, sleeping area and a bathroom, but are generally smaller than the main residence. An ADU may be created as a separate unit within an existing home (such as an attic), as an addition to the home (a separate unit with separate entrance), or as a separate structure on the same lot (such as a converted garage or new build). Other terms include “accessory apartment,” “accessory living unit,” “secondary unit,” or “junior dwelling unit.”