What you can build

Prefab ADUs: modular, manufactured and factory-built units

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

Three ways to build an ADU

• Manufactured (HUD code). Federal rules define a manufactured home as a structure "transportable in one or more sections" that is at least 8 body feet wide or 40 body feet long when moving, or at least 320 square feet when set up. It must be "built on a permanent chassis and designed to be used as a dwelling with or without a permanent foundation."
• Modular or factory-built (state-approved). California calls factory-built housing "a factory-constructed version of a site-built residential building." It is built off site and then moved to its permanent location. Oregon calls these "prefabricated structures": buildings made "in whole or substantial part" off site using closed construction and assembled on site. Oregon's definition expressly excludes manufactured dwellings.
• Site-built. The ADU is built on your lot. In California, local building departments enforce the state building standards for site-built homes.

[1][3][6][7]

Manufactured homes and the HUD code

HUD's Office of Manufactured Housing Programs enforces the federal construction and safety standards and installation standards. It enforces them "both directly and through state agencies," inspects factories, and authorizes "a certification label for each section of a compliant home." HUD's installation program was set up in 2008 "to ensure that all states have minimum installation standards and operating installation programs in place for manufactured homes."

The federal definition may change. HUD published a proposed rule, "Revising the Definition of 'Manufactured Home' to Lower Housing Costs," on June 12, 2026, with comments due August 11, 2026. It would let a section that forms part of an upper floor be built and moved without a permanent chassis. As of October 2026 it was still a proposal, not a final rule. Check HUD's manufactured housing page for the outcome.

HUD says homeowners can resolve complaints about a manufactured home through State Administrative Agencies and HUD. [2][8]

Modular and factory-built units: state approval

In California, HCD's Factory-Built Housing program regulates factory-built homes and building parts made for sale in the state. It works like this:

• An HCD-approved Design Approval Agency reviews and approves the plans.
• An HCD-approved Quality Assurance Agency inspects the unit while it is built in the factory.
• Each unit or component must carry a California Insignia of Approval before it ships.
• Units are designed to the California Building Standards Code.

Some decisions stay with your city or county. HCD lists zoning, local snow and wind loads, fire zones, setbacks and yard rules, site development and property lines, and architectural and aesthetic review as reserved for the local building agency.

In Oregon, state law requires plan approval and inspection of prefabricated structures by certified or approved parties. It also requires an insignia of compliance or certification stamp for certain transactions. [3][4][6][9]

You still need local permits

A factory-approved unit doesn't skip the local permit. In California, the local building department "is responsible for inspecting the assembly and installation" of factory-built housing on site, using the approved plans. It may charge for that inspection "by local ordinance." Changes made during or after installation need approval from the design agency before installation, or from the local building department during or after it.

Zoning, setbacks and site development stay under local control as well, and so does the ADU permit itself. See the ADU permit process.

In Washington, a city may require a manufactured home to be new, set on a permanent foundation with the space underneath enclosed, built to local design standards, and thermally equivalent to the state energy code. [4][10]

Can a manufactured or prefab home be an ADU?

• California: yes for manufactured homes. State law says an ADU includes "a manufactured home, as defined in Section 18007 of the Health and Safety Code." Newly built detached ADUs must generally meet California's solar requirement, but HCD notes that manufactured homes are excluded.
• Oregon: inside urban growth boundaries, local governments must allow manufactured homes and prefabricated structures on all land zoned for single-unit homes. They may not apply standards that wouldn't apply to a site-built home of the same housing type on the same land, with limited exceptions. Historic districts are exempt.
• Washington: homes built to the federal manufactured housing standards "must be regulated for the purposes of siting in the same manner as site built homes, factory built homes, or homes built to any other state construction or local design standard."

The Oregon and Washington laws cover where manufactured and prefab homes can be placed in general and are not written specifically for ADUs. Ask your city whether one can be permitted as an ADU on your lot.

Whatever the construction method, local zoning rules such as setbacks still apply. See what counts as an ADU. [4][5][10][11]

Pre-approved ADU plans

Some cities offer pre-approved ADU plans, which are designs the city has already reviewed. California law required local agencies to set up a program to pre-approve ADU plans by January 1, 2025, and to post them online. Seattle says that with a pre-approved backyard ADU plan, "in most cases, you can get your permit in just 2-6 weeks." Phoenix offers free pre-approved detached ADU designs that residents must submit "as is without modifications." [5][12][13]

Costs

UC Berkeley's 2021 statewide survey of California ADU owners reported costs for all ADU types together. The median construction cost was $150,000, or $250 per square foot. See our ADU cost guide, financing guide and hiring help guide. [14]

Questions people ask