Does a Prefab ADU Need Title 24?
A factory-built ADU, the usual modular unit, must comply with Title 24, Energy Code included, exactly as a site-built one does. A manufactured home built on a chassis to HUD standards is governed by Title 25 instead, and the Energy Commission states it is not subject to the Energy Code.
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Two kinds of prefab, two sets of rules
"Prefab" covers buildings the law treats very differently. The Energy Commission’s 2025 ADU guidance draws the line:
- Factory-built housing is a factory-constructed version of a site-built residential building, transported to its permanent site. It must comply with Title 24, including Part 6, the Energy Code.
- A manufactured home is built on a permanent chassis, is transportable in one or more sections, and is at least 8 body feet wide or 40 body feet long in travelling mode, or 320 square feet once erected. It carries HUD certification, must meet Title 25, and is not subject to the Energy Code.
Both can be ADUs. HCD’s ADU Handbook notes that state ADU law includes a manufactured home in its definition of an ADU (Government Code §66313). The compliance manual adds that Title 25, not Title 24, also governs additions to a manufactured home.
What makes a unit factory-built
HCD’s Factory-Built Housing Handbook describes units manufactured so that concealed parts cannot be inspected on site without disassembly or damage: whole modules, or closed wall, floor and roof panels with the wiring and plumbing already inside. Open-framed prefabricated construction, which an inspector can still see into on site, is not factory-built housing, so it is reviewed like any site-built ADU.
Each factory-built unit leaves the factory with an HCD Insignia of Approval. The building department must check for it before installation, and a unit without one may not be installed.
Who reviews the energy compliance
For a factory-built ADU the work is split. Every model needs plan approval under HCD’s Title 25 regulations, carried out by a third-party Design Approval Agency that HCD certifies, which checks the design against the California Building Standards Code; the handbook lists Part 6 among the codes that apply. Your city or county then:
- reviews the parts of the project not covered by the approved factory-built plans, typically the foundation and other site work
- issues the permit and inspects the installation and assembly, including site electrical, mechanical and plumbing work
- may not re-review plans already approved by HCD or its agency, apart from local matters such as zoning, setbacks and fire zones
So ask the manufacturer early whether its approved plan set includes the energy compliance documents for your climate zone, and which site-built items it leaves to you. Anything outside the approved design is documented and permitted locally, as described on what is the CF1R for an ADU.
A detached factory-built ADU is new construction
Placed as a new detached structure, a factory-built ADU is a newly constructed building, and the requirements are those on detached ADU Title 24: solar PV under §150.1(c)14 unless an exception applies, ventilation under §150.0(o) whatever its size, BESS-ready wiring where the electrical service is over 125 amps, and electric-ready provisions if gas appliances are installed. HCD’s handbook confirms that manufactured homes are excluded from the solar requirement for new ADUs.
A factory-built ADU joined to the house, sharing a wall, floor or ceiling, is an addition instead, and follows the attached ADU rules.
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
No. A modular unit is normally factory-built housing, approved through HCD’s factory-built program and subject to Title 24. A manufactured home is built on a permanent chassis to HUD standards and governed by Title 25.
Not for the Energy Code. The Energy Commission states that manufactured homes must meet Title 25 and are not subject to the Energy Code, and the compliance manual says the same of additions to them.
A new detached one does, unless an exception in §150.1(c)14 applies. The Energy Commission allows new modules added to an existing system on the same lot, if they are part of the ADU permit and sized to the code.
The local building department inspects the installation and assembly on site, and any later alterations. Once installed, HCD’s handbook says the unit is treated like a site-built structure for its life.