Accessory Dwelling Unit

An accessory dwelling unit (ADU) is a secondary housing unit on a residential lot — a self-contained living space with its own kitchen, bathroom, sleeping area, and entrance, built in addition to the primary dwelling on the same lot. ADUs come in three primary physical configurations: detached (a separate freestanding structure, sometimes called a granny flat or backyard cottage), attached (an addition that shares at least one wall with the primary dwelling), and conversion (an existing space such as a garage, basement, or attic transformed into a habitable unit). ADUs are permitted on residential lots in all fifty states as of 2026, though the specific rules governing size, setbacks, permits, and fees vary significantly by jurisdiction.