What you can build

Putting a tiny house in your backyard

Checked against state laws and official agency sources · Last reviewed October 9, 2026

Wheels or foundation: the first question

"Tiny home" is not a legal category by itself. California HCD calls it "an industry term that can mean different structures or vehicles." Its 2016 tiny homes bulletin, which HCD's March 2026 ADU Handbook still points readers to, says a tiny home must be approved as one of these before anyone can legally live in it:

• a home built to the state building code,
• factory-built housing,
• a HUD-code manufactured home,
• a recreational vehicle (RV) or park trailer, or
• a camping cabin.

The bulletin sums up two legal routes. One is a unit built on a chassis with axles, 400 square feet or less (not counting a loft), treated as an RV, park trailer or camping cabin. The other is a unit "not constructed on a chassis with axles" that is placed on a foundation or otherwise permanently attached to the land, and built to the building code or to factory-built housing standards.

A unit that fits none of these categories is, in HCD's words, "a noncomplying residential structure in which occupancy is illegal." [1][2]

Tiny houses on foundations and the building code

A tiny house on a foundation is generally treated as a regular home under the building code (or, in California, may be approved as factory-built housing). In California, HCD says a tiny home that isn't an RV, park trailer, manufactured home, factory-built home or camping cabin "is a dwelling unit and must comply with the" state building code.

The 2021 International Residential Code has an optional appendix for small homes, Appendix AQ, "Tiny Houses." In the 2024 edition it was relettered Appendix BB. The 2021 text defines a tiny house as "a dwelling that is 400 square feet (37 m2) or less in floor area excluding lofts." It relaxes some rules. For example, habitable space needs a ceiling height of at least 6 feet 8 inches, and bathrooms and kitchens at least 6 feet 4 inches. The model appendix also covers lofts, ladders, compact stairs and escape windows in lofts.

The appendix applies only where your state or city has adopted it: "The provisions contained in this appendix are not mandatory unless specifically referenced in the adopting ordinance." Seattle's current (2021) residential code adopts it with state and local changes, including taking the loft rules out of the appendix and pointing to the main code instead. Ask your building department whether it uses the tiny house appendix. [2][5][6]

Tiny houses on wheels: RVs and park trailers

California's 2016 bulletin treats a tiny house built on a chassis with axles, 400 square feet or less, as an RV, park trailer or camping cabin. Under that bulletin:

• An RV has less than 320 square feet of interior living area and 400 square feet or less of gross area. It is "not intended for occupancy as a permanent dwelling."
• A park trailer is up to 400 square feet set up (lofts that meet state rules aren't counted) and no more than 14 feet wide. It is designed "for recreational or seasonal use only."
• Unless a local ordinance allows otherwise, RVs and park trailers generally may be lived in only in mobilehome parks or special occupancy parks.

Seattle says "tiny houses on wheels are treated like camper trailers," and "you cannot live in a tiny house on wheels (or similar equipment such as RVs and boats) on lots in Seattle city limits."

Washington state protects tiny houses on wheels in one setting. Cities may not bar an RV or "tiny house with wheels" used as a primary residence in a manufactured or mobile home community, though fire, safety and utility rules still apply. [2][3][7]

Can a tiny house be an ADU?

In California, possibly. HCD says state ADU law "does not specifically reference 'tiny homes,'" but if a unit meets the legal definition of an ADU, "it is subject to ADU Law." That definition requires complete independent living facilities, including permanent provisions for living, sleeping, eating, cooking and sanitation. It also includes manufactured homes.

In Washington, the state's law on tiny house communities defines a "tiny house" and a "tiny house with wheels" as a dwelling "to be used as permanent housing with permanent provisions for living, sleeping, eating, cooking, and sanitation built in accordance with the state building code." Cities may also adopt rules for tiny house communities. In Seattle, a tiny house on a foundation is a backyard ADU (DADU).

Some cities allow movable tiny houses on house lots. San Diego's municipal code defines a "movable tiny house" as a unit of 150 to 430 square feet with permanent provisions for living, sleeping, eating, cooking and sanitation. Under the code, a movable tiny house:

• is built to RV standards (NFPA 1192 or ANSI A119.5), certified and labeled,
• is licensed and registered with the California DMV,
• is no more than one story and can't move under its own power,
• sits behind or beside the main home, not in the front yard, with the wheels, axles and hitch hidden from view,
• is connected to water, sewer and electric service (gas hookups are not allowed), and
• is not rented for less than 30 days in a row.

San Diego allows one movable tiny house per lot in addition to the ADUs and junior ADUs its code allows. The city's older Information Bulletin 403 (October 2020) allows only one ADU or movable tiny house plus one junior unit; the code was amended after that, so go by the current code and confirm with the City of San Diego Development Services Department. [1][3][4][8][9][10]

Factory-built and manufactured tiny homes

A tiny home can also be built in a factory. In California, factory-built housing is built off site "for placement on a foundation" and generally must meet the same standards as site-built homes. HCD's 2016 bulletin said HCD "has not approved any FBH units as tiny homes" as of that date.

A manufactured home counts as an ADU in California. Under state law a manufactured home is at least 8 feet wide or 40 feet long when moving, or at least 320 square feet when set up, and is built on a permanent chassis. A smaller unit can also count if the manufacturer certifies it to the federal manufactured housing standards. See our prefab ADU guide. [1][2]

How to get a backyard tiny house approved

• Call your city first. Ask whether it treats a tiny house on a foundation as an ADU, whether it has adopted the IRC tiny house appendix (Appendix AQ, or Appendix BB in the 2024 edition), and whether it allows movable tiny houses at all.
• Plan it as an ADU. A foundation tiny house permitted as an ADU follows the same steps as any other ADU. See the ADU permit process and what counts as an ADU.
• Check utilities. In California, a detached ADU that isn't made from existing space may be required to have its own utility connection.

[1][3][5][6]

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