Tiny home ADU in California: when a backyard tiny home is legal

What a tiny home ADU is

“Tiny home” isn't a legal term. The state's ADU handbook calls it “an industry term that can mean different structures or vehicles” and says a tiny home is subject to ADU law only if it meets the statutory ADU definition. An older HCD bulletin on tiny homes describes them as anything from 80 to 400 square feet, built to a range of standards or none, with or without a chassis and wheels.

So the useful question isn't “is it tiny?” but “what is it legally?” A backyard tiny home can be an ADU, a recreational vehicle, or an unpermitted structure nobody may legally live in. The answer decides whether California's ADU protections apply to you. For the basics, see what an ADU is.

When a tiny home ADU is legal in California

State law defines an ADU as an attached or detached residential dwelling unit with complete independent living facilities, on a lot with an existing or proposed home. It must include permanent provisions for living, sleeping, eating, cooking and sanitation. The definition expressly includes an efficiency unit and a manufactured home.

Size isn't the obstacle. HCD says a city can't set a minimum size that prohibits an efficiency unit, which may be as small as 150 square feet. The City of San Diego's ADU rules allow new ADUs from 150 to 1,200 square feet.

HCD's tiny home bulletin says any tiny home must be approved as one of a short list of structure types before anyone may live in it. Its summary sorts them into two legal paths:

• On a chassis with axles, 400 square feet or less: a recreational vehicle, park trailer or camping cabin.
• Not on a chassis, placed on a foundation or otherwise permanently fixed to the land, and built to the California Building Standards Code or factory-built housing standards.

Only the second path, plus HUD-code manufactured homes, leads to an ADU. Anything that fits neither is, in the bulletin's words, a structure “in which occupancy is illegal.”

Tiny home on a foundation

A small house framed on site, or assembled from a kit, on a permanent foundation is just a small ADU. It goes through the same plan check, inspections and energy code as a larger one. This is the simplest route to a legal tiny home ADU.

Factory-built and manufactured tiny homes

A unit built in a California-approved factory must carry an HCD insignia of approval; a manufactured home is built to the federal HUD code. Both can be ADUs once your city permits the foundation, placement and hookups. These routes are covered in detail in our guide to the prefab ADU in California.

Tiny home on wheels as an ADU in California

HCD treats most tiny homes on wheels as recreational vehicles or park trailers, built to the NFPA 1192 or ANSI A119.5 standard. RVs are “not intended for occupancy as a permanent dwelling,” and unless a local ordinance allows otherwise, they generally may be occupied only in mobilehome parks or special occupancy parks. A park trailer is limited to 400 square feet of floor area and 14 feet in width.

Because a tiny home on wheels is a vehicle with a chassis, not a building with complete permanent facilities, it doesn't fit the state ADU definition. That means state ADU law doesn't make your city approve one. Some local governments allow them anyway, under their own rules, and San Diego is one of them.

Are tiny homes legal in San Diego?

Yes, in two forms. A tiny home on a foundation can be permitted as an ordinary ADU anywhere in the county that allows ADUs. A tiny home on wheels is legal in the City of San Diego and in unincorporated San Diego County, under different rules.

City of San Diego: the movable tiny house

The city defines a movable tiny house as an accessory structure of 150 to 430 square feet with its own permanent provisions for living, sleeping, eating, cooking and sanitation. San Diego's municipal code allows one per lot in addition to any ADUs and JADUs, approved at the city's lowest review level (Process One). It must:

• Be licensed and registered with the DMV, and certified to NFPA 1192 or ANSI A119.5 with a label showing it.
• Be one story, unable to move under its own power, with no slide-outs or tip-outs, and no separate address.
• Sit behind or beside the house, not in the front yard, at least 5 feet from lot lines and 10 feet from other buildings.
• Be on a lot next to a street at least 20 feet wide, within 150 feet of it, reached by a path at least 5 feet wide.
• Keep its wheels on but hidden, resting with jacks on a paved surface.
• Connect to water, sewer and electricity. Gas connections are prohibited.
• Have sprinklers if the main house does, and meet extra fire rules in a Very High Fire Hazard Severity Zone.
• Stay out of brush management zones. Rentals must be at least 30 days.

The code also sets design rules: double-pane windows, a sloped roof over at least half its area, and no single-piece composite or interlocking metal siding. An RV-style unit with metal siding or a flat roof won't meet them as built, so check before you buy.

Unincorporated San Diego County

In a September 2025 director's determination, the County of San Diego said a tiny home on wheels can count as a dwelling, used as a primary home or an accessory dwelling, once it gets a building permit. The county accepts only the ANSI A119.5 park-model standard and adds its own rules: no more than 430 square feet, one story and 16 feet tall, at least 6 feet from other structures, fire-resistant exterior walls, a Class A roof, sprinklers (ADU exemptions apply), a paved pad and continued DMV registration. If you remove it, you file a demolition permit.

Sources: SDMC § 141.0318 and the county determination. Other cities in the county, such as Chula Vista or Oceanside, set their own rules; ask the local planning counter.

Permits for a backyard tiny home

Every version people live in needs a permit. San Diego's building code exempts one-story sheds and playhouses up to 120 square feet of roof area, but not if the structure is used for a separately regulated use such as an ADU. Placing a factory-built home or manufactured home requires a building permit.

An ADU application gets ministerial review: the city must approve or deny a complete application within 60 days. For the City of San Diego steps, see how to get an ADU permit in San Diego. Size, height and setback limits are in our summary of California ADU laws.

Utilities

A legal tiny home needs real hookups, not an extension cord and a hose. The city requires a movable tiny house to connect to water, sewer and electricity, and the county requires the same, with building permits. A tiny home ADU on a foundation connects like any ADU. Snap ADU's sitework guide lists some of the costs that can come with that in San Diego:

• A separate SDG&E meter: $10,500–$15,000.
• A sewer pump if the unit sits below the sewer line: about $5,000.
• Fire sprinklers, where required: $5,000–$10,000.

How much does a tiny home cost?

The answer depends on which kind you mean, and most published tiny home prices leave out the site work a legal unit in California needs.

Clever Tiny Homes, a tiny home builder, says a professionally built tiny home on wheels typically costs $70,000 to $150,000, and $90,000 to $180,000 or more in California. Bob Vila's cost guide, citing Angi and HomeAdvisor, gives national figures for tiny houses on foundations; they don't reflect California code or San Diego site costs.

For a permitted tiny home ADU in San Diego, the realistic benchmark is a small ADU. Snap ADU's published prices put a 500 sq ft detached unit at about $300,000 all-in, including design, permit management, roughly $35,000 of level-lot sitework and standard finishes. Per square foot, small units cost more, because foundations, utilities and permits don't shrink with the house. City fees for a 499 sq ft ADU came to $10,734 in Snap ADU's February 2026 table (a builder's figure, not an official fee schedule), and state law bars impact fees on ADUs of 750 sq ft or less. See what an ADU costs in San Diego or try the ADU cost calculator.

Tiny home on wheels vs tiny home on foundation vs ADU

Sources: HCD tiny home bulletin, Gov. Code § 66313, and SDMC §§ 141.0302 and 141.0318.

Who can build or install a tiny home ADU

The Contractors State License Board says a B, General Building contractor can build or install ADUs, and a C-47 contractor can install a manufactured-home ADU. A tiny home dealer that only sells the unit may not be licensed for the foundation, trenching and hookups, so find out who is, and check a contractor's license before you sign. State law caps the deposit on a home improvement contract at $1,000 or 10%, whichever is less. Read how to hire an ADU contractor, then compare verified ADU builders in San Diego. Looking at a kit from a big-box store? See whether an Amazon or Home Depot tiny home can be an ADU.

Common questions