ADU comparisons
Modular vs manufactured ADU: which building code applies
By the HyreADU research desk. Last checked .
The short answer
Modular vs manufactured ADU is about which code the unit is built to. A manufactured home is built to the federal HUD Code on a permanent steel chassis.
A modular unit is factory-built to the same state building code as a site-built home; in California HCD’s factory-built housing program covers it. Both still need a local permit for the foundation and hookups.
| Factor | Modular ADU | Manufactured ADU |
|---|---|---|
| Code it is built to | State building code (California: factory-built housing under HCD) | Federal HUD Code, 24 CFR Part 3280 |
| Chassis | None; set on a foundation | Permanent steel chassis |
| Label | State insignia | HUD certification label |
| Who installs in California | B General Building contractor | A C-47 manufactured housing contractor may install |
| Treated like a site-built home | Generally yes | Depends on local rules and the foundation |
When modular wins
Modular wins when you want a unit that inspectors, appraisers and lenders treat like a site-built house. It is built to the same code and set on a permanent foundation. Modules can be stacked or joined, so layouts can be larger than a single box. See modular ADU builders.
When manufactured wins
A manufactured home can win on a simple lot where a HUD-Code unit meets local ADU rules and a C-47 installer is available.
California allows manufactured homes as ADUs where they meet local standards. Ask your lender early how it treats a HUD-Code unit, because rules differ by loan program. Read the manufactured ADU definition.
How to tell which one you are buying
Ask the supplier for the label. A HUD-Code home carries a HUD certification label on each transportable section. A California factory-built unit carries an HCD insignia. If a supplier cannot say which, ask why before you sign anything.
A mistake to avoid
The mistake to avoid is mixing up the words in a sales conversation. “Modular”, “manufactured”, “prefab” and “factory-built” are used loosely in advertising, but the code a unit is built to changes who installs it, how it is inspected and how a lender may treat it. Ask for the label and the code in writing.
What to do next: check your lot with the ADU feasibility checker, read the questions to ask an ADU builder, then tell us about your project. Matching is not live yet, so we reply with information, not a quote.
Get ADU optionsQuestions
Is a manufactured home the same as a mobile home?
Homes built after the HUD Code took effect in 1976 are called manufactured homes. The older term is mobile home.
Can a modular ADU have two stories?
Yes, modules can be stacked where the local height limit allows.
Do both need a foundation?
Both are set on a foundation the local building department approves.
Which license installs each in California?
The CSLB names B General Building for an ADU and C-47 for installing a manufactured home.
Sources
- 24 CFR § 3280.2 (HUD manufactured home definitions). Checked September 5, 2026.
- California HCD, Factory-Built Housing program. Checked September 5, 2026.
- Health & Safety Code § 19971 (factory-built housing). 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, lending or engineering advice.
Rules vary by state and city; California rules are labeled as such. Spotted an error? Tell us through our corrections page. Licensed professionals can also contribute as named sources.