Prefab and Factory-Built ADU Title 24 Requirements
A prefab ADU with a HUD label is a manufactured home under Title 25, outside the Energy Code. A factory-built unit is Title 24 like any other, but the factory plan cannot settle what depends on your lot: solar size for your climate zone, electrical service and any gas connections. Those go to the building department.
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- Upload your plans
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- Permit-ready report by email
Read the label, then the lot
The Energy Commission says manufactured homes carry a HUD label, meet Title 25 and are not subject to the Energy Code. HCD adds that a manufactured home can be an ADU, yet ADUs are not allowed on a lot inside a mobilehome park. The compliance manual notes that on private property the local building department may hold jurisdiction. The prefab ADU page covers the factory approval process.
Solar follows the climate zone
A new detached factory-built ADU needs newly installed PV unless an exception applies, and the size comes from Equation 150.1-C using your zone. The same 500 square foot unit calculates to about 1.35 kW in zone 5, under the 1.8 kWdc exception, and about 2.25 kW in zone 15. A design approved for one zone does not carry its PV size to another. Modules may be added to the main house system on the same lot if they are in the ADU permit.
Electrical and gas are site work
- Service over 125 amps needs battery-ready provisions; a 200 amp panel qualifies only with a marked 225 amp busbar.
- Gas water heating, a gas cooktop or a gas dryer connection brings electric-ready circuits.
What to hand the building department
- The manufacturer approved plan set with its energy compliance documents for your climate zone.
- A site-side compliance document for solar and the electrical service.
- Any ECC-Rater verification the plan set leaves for site (the role formerly called a HERS rater).
If the unit shares a wall, floor or ceiling with the house it is an addition and needs no PV. See ADU solar, detached ADU Title 24 and manufactured homes and Title 24. Start at ADU Title 24 reports.
Common Title 24 Questions
- Do ADUs need Title 24 in California?
- What plans are needed for an ADU Title 24 report?
- How much does an ADU Title 24 report cost?
- How long does an ADU Title 24 report take?
- Does a garage conversion ADU need Title 24?
- Does a JADU need a Title 24 report?
Frequently Asked Questions
Not by default. PV size depends on your climate zone, so confirm in writing whether the factory documents include it or whether it is left to site.
No. HCD says a lot in a mobilehome park allows only one manufactured home, mobilehome or recreational vehicle, so ADUs are not permitted there.
Explore ADU and JADU
- Two-Story ADU Title 24 Requirements
- Title 24 for ADU Builders
- Title 24 for an ADU Above a Garage
- Attached ADU Title 24 Report
- Detached ADU Title 24 Report
View all ADU and JADU guides → · Permits and Compliance · Title 24 guide