Accessory Dwelling Unit

An accessory dwelling unit (ADU) is a secondary, independent living space located on the same parcel as a primary residence. To count as a dwelling unit it generally must include its own provisions for living, sleeping, cooking, and sanitation — in practice, a kitchen and a bathroom rather than just an extra room. ADUs go by many regional names: granny flats, in-law units, backyard cottages, secondary suites, and casitas. What unifies them is that the unit is accessory to a main home — smaller in scale, on the same lot, and usually under the same ownership.

ADUs come in several forms. A detached or attached ADU is the most common distinction: a detached unit is a standalone structure, while an attached unit shares at least one wall with the house. A junior ADU (JADU) is a smaller variant created inside the existing footprint of the home. Garage and basement conversions reuse existing space and are often the most affordable path.

ADUs are popular because they add housing capacity without new subdivisions, can generate rental income, and let families house aging parents or adult children nearby. Building one is construction of a dwelling unit, so it requires a building permit and, on completion, typically a certificate of occupancy before it can be legally occupied.

Rules for what is allowed, and how it is approved, vary significantly by state and city — California in particular has passed legislation to make ADUs easier to build. Treat this as a general overview and confirm specifics with your local jurisdiction.