Quick answer
A tiny home ADU is a very small house, generally 400 square feet or less, that is permitted as an accessory dwelling unit on a lot with a main home.
Whether it qualifies depends less on size than on what it sits on. A tiny house built on a foundation to the building code can be an ADU. A tiny house on wheels is usually registered as a vehicle and treated as one.
Quick facts
- Model code size
- 400 sq ft or less of floor area, excluding lofts (2024 IRC Appendix BB) 5
- Model code ceiling
- Habitable space at least 6 ft 8 in 5
- California ADU test
- Complete independent living facilities with permanent provisions for living, sleeping, eating, cooking and sanitation 1
- On wheels, often classed as
- Recreational vehicle or park trailer under California vehicle definitions 34
- Decides locally
- The city or county building and planning departments
What a tiny home ADU means, in plain English
A tiny home is a full house shrunk down. It has a place to sleep, cook, eat and wash, in a few hundred square feet.
An ADU (accessory dwelling unit) is a second legal home on a lot that already has, or will have, a main house. A tiny home becomes an ADU only when the city permits it as one.
The phrase covers two very different things. One is a small house on a foundation. The other is a tiny house on a trailer, often called a THOW (tiny house on wheels). The law treats them very differently.
Why the difference matters to you
Buying the wrong kind can cost you the whole purchase. A tiny house on wheels bought for a back yard may never be allowed as a home there.
A permitted tiny ADU is a legal home. You can rent it, insure it as a dwelling and count it when you sell. An unpermitted one can bring a city notice and may have to be removed or emptied.
Size also changes the math. Small units still need sewer, water, power and permits, so those fixed costs weigh more per square foot.
Foundation or wheels: the distinction that decides it
| Tiny house on a foundation | Tiny house on wheels (THOW) | |
|---|---|---|
| What it legally is | A small home built to the residential building code | Usually a recreational vehicle or park trailer, registered as a vehicle 3 4 |
| Designed for | Permanent living | RV: recreational, emergency or other occupancy; park trailer: recreational or seasonal use |
| Fits California's ADU definition? | Yes, if it has complete independent living facilities 1 | Not named in the ADU law; most agencies do not permit it as an ADU unless a local ordinance allows it |
| Permit path | ADU building permit, like any detached ADU | Local zoning rules on parked or occupied vehicles |
What California's ADU law actually says
The decision guide below shows how most California planning departments sort a tiny home. The test is the unit's legal class, not its size.
California's ADU law does not mention tiny homes or vehicles at all.
Government Code 66313(a), as amended by SB 543 effective 1 January 2026, defines an ADU as a unit with complete independent living facilities.
It must have permanent provisions for living, sleeping, eating, cooking and sanitation. It adds that an ADU includes an efficiency unit and a manufactured home.
So the law does not ban a tiny house on wheels by name. It sets a test the unit must meet. A unit on a trailer is usually titled under the vehicle definitions in the Health and Safety Code.
A recreational vehicle (Section 18010) is designed for recreational, emergency or other occupancy and has less than 320 square feet of internal living area.
A park trailer (Section 18009.3) is designed for recreational or seasonal use, with 400 square feet or less of gross floor area. Neither describes a permanent home.
That is why most planning departments decline to process them as ADUs.
A small unit that does meet the test may also fall within the law's efficiency unit category. Once it is an ADU, the normal state review rules apply.
The city must say within 15 business days whether the application is complete. It must decide a complete one within 60 days when a home already stands on the lot (Gov. Code 66317) 2.
- If It sits on a trailer and is titled as a recreational vehicle or park trailer
- Then Usually not an ADU; the vehicle definitions describe recreational or seasonal use, not a permanent home
- If It is built on a permanent foundation and meets the building code
- Then It can be reviewed as an ADU if it has complete independent living facilities
- If It carries a HUD label as a manufactured home
- Then California's ADU definition includes manufactured homes; see the manufactured ADU rules
- If It lacks a kitchen or bathroom
- Then Not an ADU; the law requires permanent provisions for cooking and sanitation
California only. The planning and building departments make the final call. Source: [1] [3] [4]
Source and scope
The definition quoted here is Gov. Code 66313(a), as amended by SB 543 effective 1 January 2026. It is California's rule. Other states define ADUs, and treat tiny homes, in their own ways.
Rules in other states
Rules outside California differ a lot. Some states and cities have written rules for movable tiny homes. Others treat them only as RVs. See ADU laws by state and ask your local planning office in writing.
The building code's tiny house appendix
Two numbers define the model appendix. Both apply only where a state or city has adopted it.
The International Residential Code (IRC) is a model building code many states start from. It has a tiny house appendix that relaxes a few rules for very small homes. In the 2024 IRC it is Appendix BB. Earlier editions lettered it Appendix Q, then AQ.
It applies to tiny houses used as single homes. It defines a tiny house as a home of 400 square feet or less in floor area, not counting lofts.
The appendix mainly covers ceiling height (at least 6 feet 8 inches in living space and hallways), loft size and access, and emergency escape openings.
It is a model code. It applies only where a state or city adopts it.
California builds from its own Residential Code, so ask the building department whether any tiny house rules are in force where you live.
- 400 sq ftLargest floor area for a tiny houseLofts are not counted
- 6 ft 8 inLowest ceiling in living space and hallwaysModel code minimum
Source: [5]
Sources: [5]
Types of tiny home and how each is treated
| Type | What it is | Usual path |
|---|---|---|
| Site-built tiny house | Framed on your lot on a foundation | ADU building permit |
| Prefab tiny house | Built in a factory, set on a foundation | ADU permit; see prefab ADU |
| Small manufactured home | HUD Code home on a chassis | Named as an ADU in California; see manufactured ADU |
| Tiny house on wheels | Built on a trailer, titled as a vehicle | Usually not an ADU; local rules only |
| Shed or studio with no kitchen | A room, not a home | Accessory structure permit; not an ADU |
Worked example: two tiny homes, two outcomes
Two hypothetical neighbors in California each want a small backyard home.
| Neighbor A | Neighbor B | |
|---|---|---|
| Unit | A 360 sq ft cottage on a foundation | A 240 sq ft tiny house on a trailer |
| Living facilities | Kitchen, bath, sleeping area, permanent hookups | Kitchen and bath, hookups by hose and cord |
| Legal class | A home built to the residential code | Likely an RV under Section 18010 (less than 320 sq ft) 3 |
| Fits Gov. Code 66313(a)? | Yes, permanent provisions 1 | Unclear at best; the city decides |
| Likely outcome | ADU permit | Permit only if a local rule allows movable tiny homes |
Neighbor B's unit is smaller, but size did not decide it. What it sits on, and how it connects, did.
How to turn a tiny home into a permitted ADU
- Choose a foundation-built unit. If you already own a tiny house on wheels, ask planning in writing whether your city allows it as an ADU or a movable home.
- Confirm the lot allows a detached ADU and check the space with the feasibility checker.
- Get plans drawn for the unit, including the foundation and utility connections.
- Submit with a site plan for the ADU permit.
- Build, connect water, sewer and power, and pass final inspection.
Where you will see it on paper
- Sales listing: "RVIA", "park model" or a VIN usually means a vehicle, not a home.
- Permit set: a foundation plan and utility layout show a permanent home.
- City letter: a notice about an occupied RV or trailer is a zoning issue, not an ADU review.
- Lease: renting an unpermitted unit as a home can create legal trouble. See long-term rental ADU.
What drives the cost
HyreADU does not publish a tiny home price, because we have no dated, sourced dataset for it.
The drivers are the same as any ADU. Foundation, sewer and water runs, electrical capacity, and permit fees do not shrink much with floor area. That is why cost per square foot rises as units get smaller. Read more on ADU cost per square foot and run numbers in the cost calculator.
Why build tiny, and the limits
What it does well
- Fits lots where a larger unit would break setback or coverage limits.
- Less floor area means fewer materials and lower energy use.
- A permitted tiny ADU counts as a legal home for rental and resale.
Where it falls short
- Site work, utility connections and fees do not shrink with floor area, so cost per square foot rises.
- Lofts reached by ladder or steep stairs limit who can live there.
- A unit on wheels bought before checking local rules may never be allowed as housing.
Misconceptions that cost money
- Myth California law lets you put a tiny house on wheels in any back yard.
- Reality The ADU law does not mention them, and most agencies treat them as vehicles. Some cities have local rules for movable tiny homes; those are local choices, not state law.
- Myth Small means no permit.
- Reality Size does not exempt a home. Any unit people live in needs a building permit and must meet the ADU definition 1.
- Myth A certified tiny house on wheels is code-built.
- Reality RV or park-trailer certification is a vehicle standard, not the residential building code.
What to ask before you buy or build
- Does my city allow a tiny house on wheels as a home at all? Get the answer in writing.
- Is the unit titled as a vehicle, or built to the residential code?
- Does it have permanent provisions for cooking and sanitation?
- Can the seller supply plans and foundation details for a permit?
- Does the loft meet the local code for height, size and access?
Your tiny home ADU checklist
- Check the lot's space with the size envelope calculator.
- Decide: foundation-built, prefab or manufactured.
- Confirm the unit meets the ADU definition.
- Get plans and a site plan drawn.
- Apply for the permit and track the city's deadlines.
- Build or set the unit and pass final inspection.
Key takeaways
- A tiny home can be an ADU when it is a permanent, code-built home.
- A tiny house on wheels is usually a vehicle, not an ADU, in California.
- The IRC tiny house appendix covers homes of 400 sq ft or less, where adopted.
- Ask your city in writing before you buy anything.
Related guides, tools and research
- ADU typesTiny, prefab, manufactured and site-built compared
- ADU size statisticsHow small permitted units really are
- ADU size distributionWhere tiny units fall in the data
- ADU type selectorWhich type suits your lot
Questions about tiny home adu
Can a tiny house on wheels be an ADU in California?
Usually not. The ADU law requires permanent provisions for living, and a unit on wheels is generally titled as a recreational vehicle or park trailer. A few cities allow movable tiny homes under local rules, so ask your planning department.
How big is a tiny home ADU?
The model building code's tiny house appendix covers homes of 400 square feet or less, not counting lofts. California's ADU law sets no tiny-home size; local standards and the efficiency-unit rules apply.
Does a tiny house ADU need a foundation?
To be permitted as an ADU it generally needs to be a code-built home with permanent utility connections. In practice that means a foundation or an approved permanent support system.
Do I need a permit for a tiny house in my back yard?
Yes, if anyone will live in it. A home needs a building permit regardless of size. The city also checks zoning, setbacks and utilities before anyone moves in.
Can I live in a tiny house in my back yard full time?
Only if it is permitted as a home. A permitted tiny ADU can be lived in full time. An RV or trailer parked in a yard is governed by local zoning, and many cities limit living in one.
Can I rent out a tiny home ADU?
A permitted tiny ADU can be rented like any ADU, subject to local rental rules. Short-term rental rules are set locally, so check them first. Renting an unpermitted unit as a home carries legal risk.
Is a park model home an ADU?
Usually not in California. A park trailer is defined for recreational or seasonal use with 400 square feet or less of floor area (Health and Safety Code 18009.3). That does not describe a permanent home.
What is the minimum ceiling height in a tiny house?
Where the IRC tiny house appendix is adopted, living space and hallways need at least 6 feet 8 inches. Lofts have their own rules. Ask your building department which code applies to you.
Sources
- California Legislature, Government Code 66313 (ADU definitions, as amended by SB 543, effective 1 Jan 2026), retrieved .
- California Legislature, Government Code 66317 (completeness notice and 60-day decision, as amended by SB 543, effective 1 Jan 2026), retrieved .
- California Legislature, Health and Safety Code 18010 (recreational vehicle), retrieved .
- California Legislature, Health and Safety Code 18009.3 (park trailer), retrieved .
- International Code Council, 2024 International Residential Code, Appendix BB Tiny Houses, retrieved .
Expert review
Written by the HyreADU Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Statutes read on leginfo.legislature.ca.gov and the 2024 IRC appendix read on codes.iccsafe.org on 7 October 2026. Whether California currently adopts the tiny house appendix was not confirmed and is not stated. The worked example is hypothetical.
Suggest a correction. We fix errors and say what changed.