What does ADU stand for? In housing and real estate it means accessory dwelling unit: a second, smaller living space on the same lot as a single family home. It has its own place to sleep, cook and bathe, and it may sit in the backyard, over a garage, in a basement or attached to the side of the main house.
The term is mostly a planning and zoning word. City codes, building permits and property listings use it, while homeowners tend to say granny flat, in-law suite, casita or backyard cottage. All of those can be ADUs if they meet the local rules.
Quick facts
Stands for
Accessory dwelling unitType
Initialism, said A D UPlural
ADUsUsed in
Zoning codes, permits, real estate listings, housing newsOther names
Granny flat, in-law unit, carriage house, casita, backyard cottageRelated term
JADU, junior accessory dwelling unit
What does ADU stand for in housing?
Each word in the phrase narrows the meaning:
- Accessory means it comes second to the main house on the lot. It is not the primary home.
- Dwelling means a place where people live, not a shed or a workshop.
- Unit means a complete, separate home with its own kitchen and bathroom.
So an ADU is a full small home that shares land with a bigger one. A finished basement with a bedroom but no kitchen is usually not an ADU under most codes; it is just extra space in the main house.
Common types of ADUs
| Type | Where it sits | Example |
|---|---|---|
| Detached | A separate building in the yard | A one bedroom cottage behind the house |
| Attached | Joined to the main house by a shared wall | A side addition with its own entrance |
| Garage conversion | Inside an existing garage, or above it | A studio over a two car garage |
| Interior | Carved out of space in the home | A basement apartment with a kitchenette |
Some states also allow a junior unit, or JADU, created within the walls of the main house and usually limited to a small size. Rules for it are often looser.
What does ADU stand for in real estate listings?
In a listing, ADU tells buyers the lot already has, or legally allows, a second home. That matters for several reasons: it can house a relative, bring in rental income or give space for guests.
Listing line: "Three bed ranch with permitted ADU, separate entrance and laundry." This means the backyard or garage unit was approved by the city, so it can be lived in legally, and it has its own door.
Watch for the word permitted. A space built without approval may be called an unpermitted unit, and a buyer may need to bring it up to code or stop using it as a home.
Why cities talk about ADUs
Many local governments have changed their zoning to make these units easier to build. Supporters see them as a way to add homes without large new buildings, and to let older owners age in place with family nearby. Some neighbors raise concerns about parking, privacy and crowding. Rules vary widely on lot size, height, setbacks, owner occupancy and whether the unit may be rented short term.
Because each city writes its own code, the same backyard plan can be allowed in one town and refused in the next.
Using ADU in a sentence
- "We are turning the old garage into an ADU for my mother."
- "The city now allows two ADUs on larger lots."
- "Is the ADU included in the square footage?"
- "Our permit for the ADU took four months."
Since the first letter is said "ay," write "an ADU." The plural is ADUs with no apostrophe. If your reader may not know the term, spell out accessory dwelling unit the first time and put the letters after it.
Other meanings and similar letters
Outside housing, the letters are rare, and a few groups use them as their own names. When you see them in a city notice, a contractor quote or a home listing, the housing sense is almost certain.
Nearby terms include HOA, the homeowners association that may also have rules about second units, and HUD, the federal housing department. For more letters people spot in daily life, see the famous acronyms list, or read what an acronym is.
Questions people ask
What does ADU stand for in real estate?
It means accessory dwelling unit, a second home on the same lot as a main house. Listings use it to show buyers that the property has, or can have, extra living space.
Is a tiny house an ADU?
It can be, if it sits on a lot with a main home and meets local building rules. A tiny house on wheels is often treated differently from one on a fixed foundation.
What is the difference between an ADU and a JADU?
A JADU, or junior accessory dwelling unit, is built inside the walls of the main home and is usually smaller. It may share some facilities with the main house, depending on local code.
Can you rent out an ADU?
In many places, yes, but rules differ. Some cities ban short term rentals or require the owner to live on the lot, so check your local code first.
How do you pronounce ADU?
Say the three letters, "ay dee you." It is not pronounced as one word.