Does a prefab or factory-built ADU need to meet Title 24?

Yes, if it's factory-built housing. The CEC's 2025 ADU FAQ says a factory-built home can be an ADU, and it must comply with Title 24, including the Energy Code (Part 6). A HUD-labeled manufactured home is different: it follows Title 25 and is not subject to the Energy Code.

Factory-built vs. manufactured: why the label matters

• Factory-built housing: a factory-made version of a site-built home, built in modules or panels and transported to its permanent site. California's building standards, including the Energy Code, apply, along with HCD's factory-built housing requirements.
• Manufactured home: a transportable structure built on a permanent chassis, carrying a HUD label. It meets federal HUD standards and California's Title 25, not Title 24 Part 6.
• RVs and park trailers: the CEC's ADU training notes these follow ANSI and NFPA standards rather than the Energy Code. Whether one can be used as an ADU is a zoning question for your city.

Sellers sometimes describe all of these as "prefab." Ask which one you're buying and look for the HCD insignia or the HUD label.

What applies to a factory-built ADU

A new factory-built ADU placed on your lot hasn't been used or occupied before. For energy purposes, it's generally evaluated the same way as a site-built ADU of the same type. A detached unit faces the new-construction rules, including:

• solar PV (unless an exception applies, such as a calculated minimum under 1.8 kWdc);
• battery-ready provisions when the ADU's electrical service is over 125 amps;
• a heat pump baseline for space heating and water heating on the prescriptive path;
• whole-dwelling ventilation and a verified kitchen range hood; and
• ECC-Rater field verification of the measures listed in the compliance documents.

A factory-built unit attached to the house would fall under the addition rules instead.

Who handles the energy paperwork

Ask the manufacturer what energy compliance documentation, if any, comes with the unit. Your building department may still need site-specific items, such as the climate zone for your parcel, PV sizing and placement, utility connections and field verification. Then ask your building department what it needs on top of that. Gaps found at plan check can hold up delivery and installation.

Site-built work still needs plans

Even with a finished box, the site work is yours: the foundation, the electrical feeder and any panel work, water, sewer, gas if any, and PV. That work is permitted locally and must line up with the energy documentation.

What this means for your MEP plans and Title 24 report

We can prepare the site MEP plans that connect the unit, and the Title 24 report or supplemental documentation your building department requires, based on the manufacturer's specifications. Equipment in the unit has to match what's modeled. See the ADU Title 24 guide, permits needed to build an ADU, and our full MEP package.

Sources: energy.ca.gov, energy.ca.gov, energy.ca.gov.