Can a tiny house or park model be an ADU?

The short answer: wheels or a foundation?

If it has wheels, it usually can’t be a legal year-round ADU. If it sits on a permanent foundation and was built to the building code, it can be, if your town’s zoning and your lot allow a second home.

The wheels themselves aren’t really the issue. What matters is the standard the unit was built and certified to, and how you plan to use it. A unit certified as a recreational vehicle (RV) is legally a vehicle for camping and vacations, not a house. A small home built and inspected under the residential building code is a house, however small it is.

Two local approvals stand between any unit and a legal ADU. Zoning, your town’s rules for what may be built where, has to allow a second dwelling on your lot. And the building department has to issue a certificate of occupancy, the paper that says a building passed inspection and people may live in it.

Why a park model or tiny house on wheels usually isn’t a legal ADU

Most units on wheels are built to RV standards. Regular RVs follow a standard called NFPA 1192, and park-model RVs, the larger, cabin-style units, follow one called ANSI A119.5. Both are written for temporary, recreational or seasonal living, and both focus mainly on fire safety and plumbing rather than everything a house code covers (International Code Council and Tiny Home Industry Association, 2023). A park model can be no larger than 400 square feet when set up (Washington State Dept. of Labor & Industries, 2026).

Federal rules say this plainly. An RV escapes the federal manufactured-home standard only if it is designed for recreational use and not as a primary residence. Park-model makers must post a notice in the kitchen, in letters at least half an inch tall, saying the unit is not for primary residence or permanent occupancy (24 CFR §3282.15, Cornell LII).

So the unit’s own paperwork says it isn’t a house. That makes it very hard to get a certificate of occupancy for one as a home, and it runs against the standard the maker certified it to. A park model can make a fine guest cabin or seasonal retreat where your town allows RVs. It is a poor bet as a full-time apartment for a parent or a tenant.

A tiny house on wheels is a little different, because not all of them are built as RVs. The International Code Council and the Tiny Home Industry Association put it this way in their 2023 guide: if a unit on wheels is meant for permanent living, it has to meet either the residential building code or the federal manufactured-home standard, and which one applies depends partly on its size (ICC and THIA, 2023).

In New York: the state’s code office has said the Uniform Code, New York’s statewide building code, doesn’t forbid building a tiny house on a chassis with wheels. The unit still has to meet the code for dwellings, and moving it to a new site falls under the code’s rules for relocated buildings (NY Department of State, 2020). In practice, zoning is usually the harder hurdle.

When a tiny house on a foundation can be an ADU

A tiny house built under the residential code and set on a permanent foundation is a real dwelling. It goes through the same steps as any new home: a building permit, inspections in stages, and a certificate of occupancy at the end. It can be built on site, or in a factory as a modular unit. Our guide to how ADUs are built explains each path.

Whether it can be your ADU then comes down to zoning. Your town has to allow a second home on your lot, in that spot and at that size. Some zoning codes also set a minimum size for any dwelling, which a very small house may not meet. Setbacks, parking and water and sewer or septic all apply the same way they would for a bigger cottage.

In New York: many people believe a home under 144 square feet needs no permit. The exemption towns may adopt covers one-story sheds, playhouses and similar buildings. It never covers a place people live, however small (19 NYCRR 1203.3(a)(1), NY Department of State; NY Department of State, 2020).

Think about who will live there, too. Most tiny houses save space with a sleeping loft reached by a ladder or steep stairs. That works for a young adult or a caregiver. It works badly for an aging parent, or for you later in life. If the ADU is meant for someone who needs one-level living, a slightly larger cottage with the bedroom on the main floor is usually the better plan. Our aging-in-place guide covers what to build in.

The tiny-house rules in the building code, in plain words

The model residential code that most states start from added a tiny-house appendix in 2018, called Appendix Q, and relettered it Appendix AQ in 2021 (ICC and THIA, 2023). An appendix is an optional chapter. It only applies where a state or town adopts it.

In New York: the state adopted it. The 2025 Residential Code of New York State, in force since December 31, 2025 (NY Department of State, 2025), carries it as Appendix BB, “Tiny Houses” (UpCodes, 2025 code). The figures below are from that appendix.

The appendix eases a handful of rules that are hard to meet in a very small home. Everything else in the code, from fire alarms to insulation, still applies.

Just as useful is what the appendix does not do:

• It isn’t a size cap. The 400-square-foot line only defines which homes may use the eased rules (UpCodes). A bigger cottage can meet the building code too. It just follows the regular rules.
• It doesn’t set a minimum size. Any minimum comes from your town’s zoning, not from this appendix.
• It doesn’t cover RVs. A park model certified to an RV standard is still an RV.
• It doesn’t touch zoning. A code-compliant tiny house still needs a lot where a second home is allowed.

HUD code, modular or RV: how to read the label

Sellers use “tiny home” for very different things. The certification label tells you which rulebook a factory-built unit answers to, and that tells you far more than the brochure.

A few details are worth knowing about the HUD path. Under federal law, a manufactured home is at least 8 feet wide or 40 feet long, or at least 320 square feet once set up, and since July 11, 2026 it no longer needs a steel chassis (42 U.S.C. §5402, Cornell LII). If you might ever borrow against it, ask how it will be titled. Freddie Mac, one of the big buyers of home loans, wants a manufactured-home ADU titled as real estate rather than as personal property (Freddie Mac, 2024).

The simple rule: if a seller calls a unit an ADU but its label says RVIA, NFPA 1192 or ANSI A119.5, treat it as an RV until your town tells you otherwise in writing (ICC and THIA, 2023).

How to ask your town

In New York: there is no broad state law that makes ADUs legal everywhere, so each city, town and village writes its own zoning (Mercatus Center, 2025). Your code enforcement officer handles building permits, inspections and the certificate of occupancy (NY Department of State, 2021). Zoning questions usually go to the zoning officer or the town clerk.

Before you call, have the facts ready: the unit’s certification label, its size, whether it will sit on wheels or a foundation, and how it will be used, such as year-round for family, as a guest space or as a rental. Vague questions get vague answers. “Can I have a tiny house?” invites a guess. “Would you issue a certificate of occupancy for this specific unit?” gets a real answer.

Questions worth asking:

• Does zoning allow a second dwelling on my lot, and may it be detached?
• Is there a minimum size for a dwelling in my zoning district?
• Would you treat this unit as a dwelling, a manufactured home or an RV?
• Could it get a certificate of occupancy as a year-round home? What would you need to see, such as the label, plans or a foundation design?
• Are RVs or park models allowed on a house lot at all, and is there a limit on how long someone may stay in one?
• Does it need a special use permit, which is approval from a board after a public hearing, or a site plan review?
• What will you need for water, sewer or septic, and power?

Ask for the answers by email, or write down who you spoke to and when. Rules change, and a written answer is worth far more than a remembered phone call. Our guide to permits and the approval process explains what comes next, and the property checker is a good place to start.

What this means for you

• Wheels usually mean “not a legal home.” A park-model RV is certified as not for permanent living. A tiny house on wheels only stands a chance if it was built to a dwelling standard and your zoning allows it.
• A foundation and a permit are the usual path. A small home built under the residential code, inspected and given a certificate of occupancy, can be an ADU if your town and your lot allow it.
• Read the label before you read the price. A New York insignia, a HUD label and an RV seal lead to three very different answers.
• The tiny-house appendix helps, but only with the building code. It eases ceilings, lofts and ladders. It does nothing about zoning.
• Get your town’s answer in writing before you buy. A unit you can’t legally live in is an expensive shed. For questions about contracts, titles or loans, talk to a lawyer or lender. Nothing here is legal advice.

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.