What is an ADU?

What counts as an ADU?

An accessory dwelling unit, or ADU, is a complete second home on a lot that already has a main house. “Complete” is the important word. It has its own place to sleep, its own bathroom, its own kitchen and its own way in. Building codes call that a dwelling unit: a home with permanent space for living, sleeping, eating, cooking and washing up (New York Uniform Code, §1219.2(a)(13)).

“Accessory” means it goes with the main house. It is the smaller of the two homes, and it shares the lot, the driveway and usually the utilities. In many places it can’t be sold on its own. The Town of Clarkson, for example, bars selling an ADU separately from the house (Town of Clarkson Code §140-23.4).

The same idea goes by plenty of other names: granny flat, in-law apartment, accessory apartment, backyard cottage, carriage house, casita. Each town’s code picks its own words, so when you look up your rules, search for both “accessory dwelling unit” and “accessory apartment.”

A space without independent cooking facilities may not meet the definition of a dwelling unit. That does not decide whether sleeping, working or another use is allowed. Ask the municipality to review the actual use, whether the structure is attached or detached, and any occupancy approval required. If words like “zoning” or “setback” are new to you, the glossary has short definitions.

In New York: The code distinguishes a dwelling unit from a guestroom used for living or sleeping; leaving out a kitchen does not remove the need to check the intended use (Uniform Code, §1219.2(a)(13) and (15)). A change of use also needs the required occupancy or compliance certificate (19 NYCRR §1203.3(d)).

What kinds of ADUs are there?

ADUs come in five common shapes. ADU types compares them side by side, with sizes, costs and the approvals each one tends to need.

Interior conversion. Part of the existing house becomes a separate apartment. Often that’s a basement, sometimes an attic or one wing of a floor. It reuses space you already have and adds nothing to the footprint, which is the ground a building covers. The usual hurdles are headroom, an escape window in every bedroom and fire-resistant walls and floors between the two homes.

Attached addition. A new wing goes onto the house, with its own entrance, kitchen and bath, sharing at least one wall with the main home. Because it is new construction, it has to fit your setbacks, the minimum distances a building must keep from your property lines. Built at ground level, it can be a good one-floor home for an older parent.

Detached backyard cottage. A small house of its own in the yard. It gives both households the most privacy and is the easiest kind to build on one level with no steps. It also tends to cost the most, because it needs its own foundation, roof and utility lines (UC Berkeley Center for Community Innovation, 2021).

Garage conversion or apartment over a garage. An existing garage becomes living space, or a new apartment goes on top of one. A conversion starts with walls and a roof already standing. An upstairs apartment adds no footprint but comes with stairs, which rules it out for many older people.

Junior ADU (JADU). This is a California legal term for a small unit of 500 square feet or less carved out of a single-family house, which may share a bathroom with the main home (California Government Code §66313). New York has no such category, so you’ll mostly see the term in California articles and ads.

Why do people build ADUs?

Most people build an ADU for a person, not a spreadsheet. In a 2021 AARP survey of adults 50 and older, 62% said they would consider building one for a loved one, and 60% said they would consider living in one themselves (AARP, 2021). People open to living in one gave three top reasons: being close to someone while keeping their own space (69%), getting help with daily tasks (68%) and saving money (48%) (AARP, 2021).

The need behind those answers is large. About 63 million Americans, roughly one adult in four, were family caregivers in 2025 (AARP and National Alliance for Caregiving, 2025). In 2021, 18% of people in the U.S. lived in a multigenerational home, and money was the reason given most often (Pew Research Center, 2022).

The reasons we hear most fall into a few groups. What will you use an ADU for? walks through each one.

• A parent who needs help nearby but wants their own front door. See building an ADU for aging in place.
• An adult child who can’t afford rent yet. ADU types matches each kind of unit to goals like this one.
• A live-in caregiver who needs a real bedroom and some privacy. The aging in place guide covers layouts with room for one.
• Rent from a long-term tenant, where your town allows renting the unit. Renting an ADU covers the rules and whether the rent will cover the cost.
• Downsizing. Some owners move into the ADU and rent out or hand down the main house. Check first: some towns require the owner to live in the main house, and renting it out can cost you part of your tax break when you sell. See moving into the ADU and renting out the house.
• An office or guest space now, with possible housing later if the town approves that use. ADU types explains what to check before planning a conversion.

Uses change over time. In a survey of California ADU owners, 51% rented their unit out, 16% housed a relative for free and about 16% used it as an office, studio or guest space (UC Berkeley Center for Community Innovation, 2021). That’s California; we haven’t found a similar survey for New York. Since a unit built for a parent today may house a caregiver or a tenant later, design it to work for more than one kind of household, and don’t count on rental income until you’ve checked your town’s rules on renting.

How is an ADU different from an addition, a guest house, a tiny house or a duplex?

An ADU is a complete, separate home that comes with a main house. Each look-alike misses on one of those points.

An addition gives the main house more room. It becomes an ADU only when it’s built as a separate home with its own kitchen, bath and door. At that point the building code usually treats the whole house as a two-family dwelling, and the two homes must be separated by walls and floors rated to hold back fire for at least 1 hour, or ½ hour if the building has sprinklers (International Code Council, 2021 IRC §R302.3).

A guest room, backyard office and storage building can have different requirements. Approval for one use does not automatically allow another. Before planning a future conversion to housing, confirm the zoning route and required building and occupancy approvals.

A tiny house on wheels turns on its certification, not its wheels. Many are built and labeled as recreational vehicles, and park-model RVs follow a standard written for seasonal or temporary living (International Code Council and Tiny Home Industry Association, 2023). Federal rules require a park model to carry a notice that it isn’t meant as a permanent home (24 CFR §3282.15), so it usually can’t be a legal year-round ADU. A tiny house built to the building code on a permanent foundation is different: it’s a real house, and it can serve as an ADU if your town and your lot allow it. Can a tiny house or park model be an ADU? goes deeper.

In New York: The 2025 residential code includes a tiny-house appendix for homes of 400 square feet or less, not counting lofts (2025 Residential Code of New York State, Appendix BB). It eases a few rules that are hard to meet in very small homes, such as ceiling heights and the stairs or ladders to a sleeping loft. Zoning still decides whether one may sit on your lot.

A duplex is one building with two homes, usually planned together and often similar in size. A house with an attached ADU can look a lot like one, and the building code may treat it the same way. Zoning is where they part. An ADU is the smaller home that rides along with a single-family house, and towns often attach conditions such as a size cap, a bedroom limit, owner occupancy (the owner must live in one of the two homes) or a ban on selling it separately (Town of Clarkson Code §140-23.4). A duplex is a two-family home, and your zoning code decides on its own terms where those are allowed.

Who decides whether an ADU is allowed?

Two sets of rules apply, and people often mix them up (International Code Council and Tiny Home Industry Association, 2023):

• Zoning decides what may be built and where. It covers whether an ADU is allowed on your lot at all, how big it can be, how far it must sit from the property lines and who may live there. Your city, town or village writes it.
• The building code decides how it must be built to be safe: structure, fire protection, ways out, insulation, plumbing and wiring. Your local building department enforces it through permits and inspections.

A unit can pass one test and fail the other. A well-built cottage that meets every building code rule is still not allowed if your zoning has no room for a second home on the lot.

States sit above towns, and they take different approaches. Some have passed laws that make towns allow ADUs, within limits. California requires its cities and counties to allow them (California SB 477, 2024), and Washington bars many of its cities and counties from capping an ADU below 1,000 square feet (RCW 36.70A.681). Even there, towns still set details like height and setbacks within the state’s limits. In other states, the state stays out of it and each town writes its own rules. Our ADU laws by state map shows where your state stands and when we last checked.

Where towns decide, the approval path varies too. Some towns list ADUs as a permitted use, meaning they are allowed if you meet the written rules. Others require a special use permit, which is approval from a local board, usually after a public hearing, often with conditions attached.

In New York: No state law makes towns allow ADUs. Each city, town and village writes its own zoning (Mercatus Center, 2025), so a village’s rules can differ from the town around it. Near Rochester the range is wide. The Village of Fairport allows ADUs only inside or attached to the house, up to 650 square feet (Village of Fairport Code §551-26), while the Town of Canandaigua lists a house plus one attached or detached ADU as a permitted use in several districts (Town of Canandaigua Code Ch. 220). New York City began allowing ADUs of up to 800 square feet in December 2024 (NYC Department of City Planning, 2024). Our New York ADU rules page covers the state grant and tax programs.

To learn what applies to your own lot, call your town’s building or code enforcement office. Ask whether an ADU is allowed in your zoning district, what approval it needs and what limits come with it. How to check your town’s rules walks through it step by step.

What this means for you

• An ADU is a complete second home. Its own kitchen, bathroom, sleeping space and entrance. A space that is not an ADU still needs approval for its actual use; leaving out a kitchen does not automatically allow someone to sleep there.
• Start with the person. Who will live there now, and who might later? That answer points you to the right type and size.
• Wheels usually rule it out. A tiny house counts only if it’s built as a permanent home and your zoning allows a second dwelling.
• Zoning comes first. Your town decides whether an ADU may go on your lot. Find out before you pay for plans or sign with a builder, and get the answer in writing.
• Private rules matter too. A deed restriction or homeowners association rule can block an ADU even where zoning allows one, so read your deed.
• This isn’t legal advice. Your town’s code office has the final say on your lot. A lawyer can help with deeds, HOA rules or a variance request, and an engineer or designer with anything structural.

Want a head start on your own property? Try the property checker.

General information, not legal, tax or financial advice. Your municipality decides what can be built, and a professional should review decisions about money and contracts.