ADU builders
Modular ADU: which law the unit is built to
By the HyreADU research desk. Last checked .
The short answer
A modular ADU is built in sections in a factory and set on a foundation on your lot.
In California the key fact is which law it is built to: state factory-built housing (Health & Safety Code § 19971) or the federal HUD code for manufactured homes (24 CFR Part 3280). The two have different rules for installation and inspection.
Two different legal objects
A factory-built modular unit is built to the state building code under California’s factory-built housing program. A manufactured home is built to the federal HUD code and carries a HUD label.
The CSLB says a C-47 contractor may install a manufactured home; a B contractor builds or installs other ADUs. See which standard is your prefab built to.
What is still built on site
The foundation, utility trenches and connections, the crane set, and any decks or steps. Site access for a truck and crane can decide whether modular works at all. Ask for a site visit before you pay a deposit.
What to ask a modular company
Which standard is the unit built to? Who designs the foundation? Who holds the contractor license for site work? Who pulls the local permit? What is in the factory price, and what is not? Put the split in writing.
What to do next: read the questions to ask an ADU builder, check what decides ADU cost, then tell us about your project. Matching is not live yet, so we reply with information, not a quote.
Get ADU optionsQuestions
Is modular faster?
Factory work can run while the site is prepared. The permit and site work still take their time.
Does a modular ADU need a local permit?
Yes, for the foundation, utilities and placement.
Is a modular ADU a mobile home?
Not if it is factory-built housing to the state code. A HUD-code home is a different legal object.
Can I use a preapproved plan with modular?
Some cities list factory-built plans in their programs. Check yours.
Sources
- Health & Safety Code § 19971 (factory-built housing). Checked September 5, 2026.
- 24 CFR § 3280.2 (HUD manufactured home definitions). Checked September 5, 2026.
- CSLB Fast Facts: Accessory Dwelling Units (2025). Checked October 27, 2026.
HyreADU is a publisher. It does not design, permit or build ADUs, has no partner builders, and gives no legal, tax or engineering advice. Rules vary by state and city; California rules are labeled as such.