What Is an ADU? Accessory Dwelling Units Explained

An ADU, or accessory dwelling unit, is a second, smaller home on the same lot as a house: a backyard cottage, a converted garage, a basement apartment or an addition, with its own kitchen, bathroom and place to sleep. AARP defines it as a small residence that shares a single-family lot with a larger, primary dwelling, and notes that it can be located within, attached to or detached from the main house. It is a complete home, but it stays on the same lot as the main house and usually under the same ownership.

ADUs go by many names: granny flat, in-law suite, backyard cottage, carriage house, garage apartment, casita, ohana unit, and in Canada laneway house, garden suite or secondary suite. Below are the types, what makes a unit an ADU in law, how big they are, the building code side, what they cost, and a measured 800 sq ft plan. For the rules state by state, see ADU laws by state.

What makes it an ADU

The legal definition is set by each state or city. California's, in Government Code § 66313, is typical: an attached or detached residential dwelling unit that provides complete independent living facilities for one or more persons, on a lot with a proposed or existing primary residence, with permanent provisions for living, sleeping, eating, cooking and sanitation. In practice that means:

• A kitchen with cooking appliances, not just a wet bar or a microwave.
• A bathroom, or in some places a shared one (see the junior ADU below).
• A separate entrance in most local definitions, so the unit can be used without passing through the main house.
• The same lot as a main house, which is usually a single-family home.

A guest room with a bathroom but no kitchen is not an ADU, and in many places neither is a suite that can only be reached through the house. That distinction matters, because ADU laws give rights (to build, to rent, to limited parking) that an ordinary addition does not have, and a unit that does not meet the definition does not get them.

Types of ADU

Each type has its own guide: garage conversion floor plans, basement apartment floor plans and the in-law suite addition. A duplex is different: two primary units in one building, usually under middle housing rules rather than ADU rules, as in Oregon's middle housing law.

How big is an ADU?

Most ADUs fall between a studio in a 400 sq ft garage and a two-bedroom unit of about 1,000 sq ft. The limits come from local zoning, within whatever floor a state law sets. Some state examples, from laws we have checked:

• California: local caps may not be below 850 sq ft for a studio or one-bedroom or 1,000 sq ft for two or more bedrooms, and an 800 sq ft unit is guaranteed; see California ADU size limits.
• Washington: cities may not cap ADUs below 1,000 sq ft, and most urban lots may have two; see Washington's HB 1337.
• Arizona: larger cities must allow an ADU of 75% of the house or 1,000 sq ft, whichever is less; see Arizona's casita law.

Your city's ordinance sets the actual cap, along with setbacks, height, parking and whether the owner must live on the lot. Read it before you design: the same backyard can allow 800 sq ft in one city and 1,200 in the next.

An 800 sq ft two-bedroom ADU

FloorGen's 800 sq ft 2-bed ADU template is a detached unit 40 × 20 ft to the outside of its walls (800.1 sq ft), with 700.5 sq ft of net floor inside. Two bedrooms are at the west end, a bath and laundry alcove in the middle, and an open kitchen, dining and living room at the east end, with the front door into the living room.

More layouts at this size are in 800 sq ft ADU floor plans, and the size itself is put in perspective in how big is 1,000 sq ft.

The building code side

Most US states build houses to the International Residential Code, and an ADU is built as a dwelling: habitable rooms of at least 70 sq ft and 7 ft in every direction (2021 IRC R304), ceilings of at least 7 ft (R305), an escape opening in every sleeping room (R310), and smoke and carbon monoxide alarms. An attached ADU usually needs a fire-rated wall or floor between it and the house, as a duplex does (2021 IRC R302.3). The 2024 IRC adds an optional appendix written for ADUs, Appendix BC, which applies only where a state or city adopts it.

FloorGen's IRC profile checks room areas and widths, ceiling heights, the escape window in each bedroom, stairs and door swings as you draw. It does not check zoning, setbacks, parking, fire separations or alarms.

What an ADU costs

Costs vary more with the type and the region than with the size. The best survey data is from California: the Terner Center at UC Berkeley reported in 2021 a median statewide construction cost of $150,000, or $250 per sq ft, from the state's first ADU owner survey. Conversions of an existing garage or basement usually cost less than a new detached unit, because the shell is already built; a new detached unit carries the most site work and utility cost. Get local bids before you commit, since prices have moved since that survey.