Prefab ADUs in California: costs, approvals and what's included

What a prefab ADU is

“Prefab” isn't a legal category. It's a sales term for any prefab accessory dwelling unit built largely off site. California law sorts these into two very different groups. A factory-built (modular) unit is wholly or substantially made off site, to the California Building Standards Code, and assembled on your lot. A manufactured home is built on a permanent chassis to the federal HUD code.

Both can be ADUs. State law defines an ADU as a unit with complete independent living facilities on a lot with a home, and says an ADU “also includes” a manufactured home. Once permitted, a prefab ADU follows the same size, setback and parking rules as any other; see our plain-English summary of California ADU laws or start with what an ADU is.

Modular, manufactured and kit ADUs

Sources: Health and Safety Code and federal rules cited below.

Modular ADUs (HCD factory-built housing)

Every factory-built home sold to a first user in California must carry an insignia of approval issued by the Department of Housing and Community Development. HCD says the insignia tells local building officials the unit was inspected and approved against its approved plans. Your city still inspects the installation.

Manufactured home ADUs (HUD code)

In California a manufactured home is a structure built on or after June 15, 1976, transportable in one or more sections, built on a permanent chassis, and either at least 8 body feet wide or 40 feet long in transit or at least 320 sq ft when set up. Each transportable section carries a permanent HUD certification label. HUD says homes built since 1976 must meet its federally preemptive code.

One change to watch: the federal 21st Century ROAD to Housing Act, which became law in July 2026, lets manufactured homes be built without a permanent chassis. HUD still has to issue construction standards for chassis-free homes, so they aren't on the market yet.

Panelized ADUs and ADU kits

Kits ship walls, floors and roof as panels that a crew assembles on a foundation. California's factory-built definition turns on whether concealed parts can be inspected without taking them apart. Closed panels with wiring and plumbing already inside can fall under HCD's program; open framing panels are typically reviewed and inspected by your city like site-built work. Ask the seller which path its system takes before you buy.

Tiny homes on wheels

A tiny home on a trailer is generally not an ADU. HCD says a “tiny home” is subject to ADU law only if it meets the statutory ADU definition. Santa Cruz County, citing HCD guidance, says tiny homes on wheels “may not be permitted as” ADUs; it treats them as park trailers, a type of recreational vehicle.

Is a tiny home ADU allowed in San Diego?

The City of San Diego allows one movable tiny house per lot as a separate use, on top of any ADUs and JADUs. The rules are strict:

• Registered with the DMV, one story, and certified to the NFPA 1192 RV or ANSI A119.5 park-model standard.
• Behind or beside the house, at least 5 feet from lot lines and 10 feet from other buildings, within 150 feet of a street at least 20 feet wide.
• Wheels stay on but hidden; connected to water, sewer and power, with no gas.
• Not allowed in brush management zones; rentals must be at least 30 days.

Because it isn't an ADU, a movable tiny house doesn't get the protections of state ADU law. If you want a small unit that counts as an ADU, put it on a permanent foundation.

Who approves a prefab ADU in California

Two sets of approvals apply, and buyers often assume the first covers the second. It doesn't.

Sources: factory-built rules, manufactured home foundation rules, ADU law. Local agencies must inspect the installation of factory-built housing and can charge an inspection fee. HCD says changes made after installation need the local building department's approval. A manufactured home set on a foundation as part of the real property needs a building permit first.

The permit itself works like any ADU: ministerial review, a 15-business-day completeness check and a decision within 60 days. In San Diego the process runs through Development Services; see how to get an ADU permit in San Diego. One perk: the city notes the state solar-panel requirement applies to newly built detached ADUs that are not manufactured.

Who can install it

The Contractors State License Board says a B, General Building contractor can build or install ADUs, and a C-47, General Manufactured Housing contractor can install an ADU that is a manufactured home built off site. A factory or dealer that only sells you the box may not be licensed to do the foundation, trenching and hookups. Find out who holds the license for the site work, then check a contractor's license before you sign. The same contract rules apply: a down payment of no more than $1,000 or 10%, whichever is less, and no payments ahead of completed work.

Prefab ADU cost, all in

Advertised prices often cover only the unit. Abodu's published pricing (October 2026) is unusually clear about the split. Its 800 sq ft two-bedroom lists a home price from $328,800, installation from $96,000, and an expected all-in from $424,800. Installation there covers the foundation, site work, up to 50 feet of utility trenching and a crane set up to 100 feet. A $6,000 boundary survey, longer trenching or crane distances, demolition, tree removal, unusual engineering and sales tax are quoted separately.

Across turnkey prefab offers, Snap ADU found 800 sq ft two-bedroom units in California ran about $280K to $430K all-in (October 2026), including unit, permits, site work, utilities and installation. Snap ADU builds site-built ADUs, so treat that comparison as one builder's survey.

Sources: site costs, fees, survey. These are builder figures, not official city fees. State law bars impact fees on ADUs under 750 sq ft, and an ADU needs sprinklers only if the main house is required to have them. For the full picture, see what an ADU costs in San Diego or run the numbers in our ADU cost calculator.

Prefab vs stick-built ADU

Snap ADU says smaller prefab units can save money over comparable site-built ones, but larger and two-story units are “often a wash” once everything is counted. Time on site drops from months to weeks. Payment timing is the difference to check most carefully. With a home improvement contract, California law keeps payments in line with finished work. A unit purchase agreement may be structured differently, so find out how much is due before your unit is delivered. Planning a two-level unit? See our guide to the two-story ADU.

Timeline

The factory saves time on site, not in permitting. Snap ADU's October 2026 guide puts the factory build at an average of 4–6 months after permits, and the installation at a few days to a few weeks, and says the total time is “typically about the same” as site-built. The permit clock is the same for every ADU: 60 days to approve or deny a complete application. Using a pre-approved ADU plan in San Diego is another way to shorten review.

When prefab and ADU companies fail

• Anchored Tiny Homes (Fair Oaks), which sold tiny homes and ADUs, closed its offices in 2024. A former executive said 450 customers were left stranded; one customer reported losing a $28,000 payment.
• Multitaskr (Chula Vista) collapsed in 2024, leaving San Diego owners with loans for unfinished ADUs; the average claim per ADU topped $200,000. The state revoked its license in 2025.

For the full checklist, read how to hire an ADU contractor.

Questions to ask a prefab seller

• “Is this unit HCD factory-built or a HUD-code manufactured home?” Ask to see the insignia or label on a delivered unit.
• “What exactly is in the installation price?” Get trenching length, crane distance, foundation type and survey in writing.
• “Who pulls the city permit, and what license does that company hold?” Look for a B or C-47.
• “Can a crane reach my yard?” Power lines, trees and narrow side yards decide this.
• “How much do I pay before the factory finishes, and what happens to it if you close?”
• “Are city fees, school fees and a separate meter included?”

If you'd rather work with a local general contractor who can install a prefab unit or build on site, compare verified ADU builders in San Diego.

Common questions