Research study
HUD Code, factory-built, or site-built?
Three legal regimes hide behind one marketing word. Which one your unit is in decides who inspects it, what foundation it sits on, and whether it is real property.
Written by HyreADU Research Desk Primary-source research and data analysis
Audited by HyreADU Research Desk Statutory and regulatory citation audit
The finding
“Prefab”, “modular” and “manufactured” are not synonyms, and in California they are not even the same body of law. A factory-built ADU sits in exactly one of three regimes, and the regime is a legal fact about the unit rather than a description of how it looks.
One: a HUD-Code manufactured home. Built to the federal Manufactured Home Construction and Safety Standards at 24 CFR Part 3280, and defined in California at Health and Safety Code section 18007 by reference to the federal Act.
The critical consequence is federal preemption under 24 CFR 3282.11: no state standard covering an aspect the federal standards govern may be established or continue in effect unless it is identical to the federal one.
Two: factory-built housing, under California HCD’s program. Health and Safety Code section 19971 defines it as a residential building or dwelling unit wholly or substantially manufactured offsite whose concealed parts cannot be inspected on site without disassembly, assembled to the California Building Standards Code. Section 19980 requires it to “bear insignia of approval issued by the department.” Three: built to Title 24 and inspected by your building department. If it is not a HUD-Code home and carries no HCD factory-built-housing insignia, it is an ordinary building under the California Building Standards Code, whichever place the walls were framed.
All three routes are open to an ADU. Government Code section 66313(a) defines an accessory dwelling unit to include a manufactured home as defined in section 18007, which is why the HUD-Code route exists for ADUs at all.
The consequence people discover late is the foundation and the property status. Health and Safety Code section 18551 allows a manufactured home to be installed as a fixture and real property improvement or “as a chattel”.
Chattel or fixture is the difference between a mortgage and a personal-property loan. Nothing here is legal, tax or financial advice, and we name and rank no manufacturer.
Read this first
- We name no manufacturer, rank no product and price nothing
This page is about which body of law your unit falls under.
It contains no vendor list, no cost comparison and no recommendation, because a page that mixed the two would be a sales page wearing a research page’s clothes.
The regime question is prior to the product question and is usually the one people get wrong.
- The regime is a fact about the unit, not a marketing category
A manufacturer’s brochure describing a product as “modular” tells you nothing legally reliable. What tells you is the label: a HUD certification label for a manufactured home, or an HCD insignia of approval for factory-built housing.
Ask which label the unit will carry, in writing, before you sign. If the answer is neither, it is a building under Title 24 and your building department inspects it.
- A local jurisdiction may still apply its own siting and zoning rules
Federal preemption under 24 CFR 3282.11 is about construction and safety standards for the home itself.
It is not a general license, and it does not displace the state ADU statute, local zoning, setbacks, height, design standards in a historic district, fire hazard severity zone requirements, or the foundation and site-work permits. The preemption is narrower than it is often described.
- Not legal, tax or financial advice
Whether a unit is a fixture or a chattel has tax and lending consequences, and those consequences are specific to your title, your lender and your county.
HyreADU does not practice law, does not give tax advice, and does not originate or broker loans. Take the fixture-versus-chattel question to a lawyer and to your lender before installation, not after.
Why the label decides more than the design does
Two ADUs can be visually identical, arrive on the same truck, and be different legal objects. One carries a red-and-silver HUD certification label and was inspected under a federal program.
The other carries an insignia issued by the California Department of Housing and Community Development and was inspected under a state program.
The third was built by a framer on the lot and was inspected by a city inspector who drove there four times.
The design is not what diverges. What diverges is the approval path, the foundation detail, when and how the unit becomes part of the real property, and — following directly from that last point — which lenders will finance it and how an appraiser will treat it.
HyreADU analysis: most published guidance on prefab ADUs collapses this into a single word because from a buyer’s point of view the units really do look alike.
The collapse is harmless right up to the moment it is not — the moment a lender asks whether the unit is real property, or a building department asks to see an insignia that was never issued. Getting the regime right at the quotation stage costs nothing. Getting it wrong is discovered at the foundation.
The three regimes, side by side
Every cell is drawn from a retrieved standard or statute, cited in the row. Where a distinction depends on facts we did not verify, the cell says so rather than guessing.
| Question | HUD-Code manufactured home | Factory-built housing (HCD program) | Built to Title 24 and inspected locally |
|---|---|---|---|
| Governing construction standard | The federal Manufactured Home Construction and Safety Standards, 24 CFR Part 3280 — “[a] reasonable standard for the construction, design, and performance of a manufactured home which meets the needs of the public including the need for quality, durability, and safety.” | The California Building Standards Code. Section 19971 requires factory-built housing to be “assembled onsite in accordance with building standards published in the California Building Standards Code and other regulations.” | The California Building Standards Code (Title 24). The 2025 edition was published 1 July 2025, effective 1 January 2026. |
| What defines it | “A structure, transportable in one or more sections, which in the traveling mode is 8 body feet or more in width or 40 body feet or more in length or which when erected on-site is 320 or more square feet, and which is built on a permanent chassis.” 24 CFR 3280.2. California ties the term to construction “on or after June 15, 1976”, HSC § 18007. | A residential building or dwelling unit wholly or substantially manufactured offsite where “all concealed parts or processes of manufacture cannot be inspected before installation at the building site without disassembly, damage, or destruction.” HSC § 19971. | Nothing special. It is a building, and the Code does not separately name factory pre-assembly. |
| What it expressly is not | — | “Factory-built housing does not include a mobilehome, as defined in Section 18008, a recreational vehicle, as defined in Section 18010.5, or a commercial modular, as defined in Section 18012.5.” HSC § 19971. | — |
| What the unit must carry | The federal certification label under the federal program. | “[S]hall bear insignia of approval issued by the department.” HSC § 19980. | No factory label. Approval is the building department’s permit, inspections and certificate of occupancy. |
| Can the state or city impose its own construction standard? | No, where the federal standards cover the subject. “No State manufactured home standard … shall be established or continue in effect … unless it is identical to the Federal standards”; and no State may require the home “to be subject to State inspection” or “that a State label be placed on the manufactured home.” 24 CFR 3282.11. | Not as to the closed construction the insignia covers. States may, however, “establish or continue in force consumer protections, such as warranty or warranty performance requirements” — 24 CFR 3282.11, in the parallel federal context. | Yes — the local building department is the enforcement authority throughout. |
| Is it allowed to be an ADU? | Yes, expressly. Government Code § 66313(a) defines an ADU to include a manufactured home as defined in Health and Safety Code § 18007. | Yes. It is a residential dwelling unit built to the state code. | Yes. |
| Foundation and property status | Under HSC § 18551 it “may be installed on a foundation system as either a fixture or improvement to the real property … or … as a chattel.” On the real-property route, once installed in compliance it “shall be deemed a fixture and a real property improvement”, and the enforcement agency records a document to that effect within five business days of the certificate of occupancy. | It is a building assembled onsite to the state code and becomes part of the realty in the ordinary way. | Part of the realty in the ordinary way. |
| Who issues the building permit for installation? | “Prior to installation … the manufactured home, mobilehome, or commercial modular owner or a licensed contractor shall obtain a building permit from the appropriate enforcement agency.” HSC § 18551. | The local enforcement agency, for the site work and assembly. | The local building department, for everything. |
Compiled by the HyreADU Research Desk from 24 CFR §§ 3280.2 and 3282.11 (Legal Information Institute), Health and Safety Code §§ 18007, 18551, 19971 and 19980, and Government Code § 66313, each retrieved individually on 2026-09-05. Code edition dates from the California Building Standards Commission.
The row that matters most for money is the foundation row. A manufactured home installed as a chattel remains personal property; installed on a foundation system under Health and Safety Code section 18551 with the recorded document, it becomes a fixture and a real property improvement.
Which of those it is determines whether a conventional mortgage product is available, how an appraiser treats it, and how it is assessed.
Decide that before the unit is ordered, with your lawyer and your lender in the room — not on delivery day.
What federal preemption does and does not do
Federal preemption is the most over-claimed idea in the prefab conversation, in both directions. The text of 24 CFR 3282.11 is narrower than the enthusiasts say and broader than the sceptics say.
What it forecloses
Conflicting state construction and safety standards: “[n]o State manufactured home standard regarding manufactured home construction and safety which covers aspects of the manufactured home governed by the Federal standards shall be established or continue in effect … unless it is identical to the Federal standards.”
State inspection of the certified home, and a state label: a State may not require a home certified as conforming to the federal standards “to be subject to State inspection” or mandate “that a State label be placed on the manufactured home.”
Parallel enforcement systems: “No State may establish or keep in effect through a building code enforcement system or otherwise, procedures or requirements which constitute systems for enforcement of the Federal standards … which go beyond this system.”
What it leaves alone
Consumer protection: “A State may establish or continue in force consumer protections, such as warranty or warranty performance requirements, which respond to individual consumer complaints.”
Participation in the federal scheme: states may act as State Administrative Agencies or Program Implementation Agencies within the federal framework.
HyreADU analysis: and — critically for an ADU — the whole of land use. The preemption clause is about the home as a manufactured product.
It is not about setbacks, height, lot coverage, design review, fire hazard severity zone requirements, floodplain elevation, septic capacity or the foundation.
Those are governed by the California ADU statute and by local ordinance, and every one of them is a live question for an ADU project.
Do not let anyone tell you a HUD label makes a lot buildable.
The ADU statute is the other half of the answer
Government Code section 66313(a) puts a manufactured home inside the definition of an accessory dwelling unit, which means the by-right approval machinery in section 66323 and the maximum-standards ceiling in section 66314 apply to it in the ordinary way.
Section 66321 caps what a local ordinance may demand on size and height.
Section 66317 requires the permitting agency to approve or deny within 60 days and, on denial, to “return in writing a full set of comments to the applicant with a list of items that are defective or deficient and a description of how the application can be remedied.”
So the analysis for a HUD-Code ADU is two-layered and the layers do not interfere: federal law governs how the box is built; California law governs whether and where the box may be placed.
The defining provisions, in one place
If you want to check a supplier’s claim, these are the six provisions to check it against. Each is quoted from the source we retrieved, with its amendment note where the source carried one.
| Provision | What it establishes | Amendment note as retrieved |
|---|---|---|
| 24 CFR § 3280.2 | Defines a manufactured home as “[a] structure, transportable in one or more sections, which in the traveling mode is 8 body feet or more in width or 40 body feet or more in length or which when erected on-site is 320 or more square feet, and which is built on a permanent chassis and designed to be used as a dwelling with or without a permanent foundation when connected to the required utilities …” and defines a federal construction and safety standard as “[a] reasonable standard for the construction, design, and performance of a manufactured home which meets the needs of the public including the need for quality, durability, and safety.” | Retrieved copy is truncated after “permanent chassis”; the source notes the definition continues with plumbing, heating and electrical specifications and exclusions, which we have not reproduced. |
| 24 CFR § 3282.11 | Federal preemption: “No State manufactured home standard regarding manufactured home construction and safety which covers aspects of the manufactured home governed by the Federal standards shall be established or continue in effect … unless it is identical to the Federal standards.” No State inspection of a certified home, no State label, and no parallel enforcement system “which go beyond this system”. States may keep “consumer protections, such as warranty or warranty performance requirements”. | None shown on the retrieved copy. |
| Health and Safety Code § 18007 | California’s manufactured home definition: a structure “constructed on or after June 15, 1976, is transportable in one or more sections, is eight body feet or more in width, or 40 body feet or more in length”, referencing the National Manufactured Housing Construction and Safety Act of 1974, 42 U.S.C. § 5401 et seq. A structure meeting everything but the size specifications may qualify where the manufacturer voluntarily certifies. | None recorded at retrieval. |
| Health and Safety Code § 19971 | Factory-built housing: offsite manufacture where “all concealed parts or processes of manufacture cannot be inspected before installation at the building site without disassembly, damage, or destruction”, assembled onsite “in accordance with building standards published in the California Building Standards Code and other regulations”. Excludes “a mobilehome, as defined in Section 18008, a recreational vehicle, as defined in Section 18010.5, or a commercial modular, as defined in Section 18012.5.” | None recorded at retrieval. |
| Health and Safety Code § 19980 | Factory-built housing “shall bear insignia of approval issued by the department.” This is the label that separates regime two from regime three. | None recorded at retrieval. |
| Health and Safety Code § 18551 | Installation on a foundation system “as either a fixture or improvement to the real property … or … as a chattel”. A building permit must be obtained “[p]rior to installation … from the appropriate enforcement agency”. On the real-property route, within five business days of the certificate of occupancy the enforcement agency “shall record a document naming the owner of the real property, describing the real property with certainty, and stating that a manufactured home … has been affixed to that real property by installation on a foundation system”, after which it “shall be deemed a fixture and a real property improvement”. | Amended by Stats. 2018, Ch. 254, Sec. 1 (AB 1943), effective 5 September 2018. |
| Government Code § 66313(a) | Defines an accessory dwelling unit and expressly includes a manufactured home as defined in Health and Safety Code § 18007 — the provision that opens the federal route to ADUs. | Amended by Stats. 2025, Ch. 520 (SB 543), operative 1 January 2026. |
Federal provisions retrieved from the Legal Information Institute and California provisions from the Legislative Counsel’s codes service, each on 2026-09-05. Where a retrieved text was truncated by its source, the truncation is recorded rather than filled in.
How to use this table. Take a supplier’s written answer to “which label will this unit carry?” and check it against the second, fourth and fifth rows.
A HUD-Code home is governed by the first two rows and California cannot impose conflicting construction standards on it.
Factory-built housing is governed by the fourth and fifth and must carry an HCD insignia.
If neither applies, the unit is an ordinary building under the California Building Standards Code and the local building department enforces all of it.
How to establish which regime you are actually buying
Five questions, all of which should be answered before a deposit, and all of which a legitimate supplier will answer in writing without hesitation.
- 1 Ask which label the unit will carry
A federal certification label under 24 CFR Part 3280, an HCD insignia of approval under Health and Safety Code section 19980, or neither.
Those are the only three answers, and “it is modular” is not one of them.
If the answer is “neither”, the unit is a building under Title 24 and your local building department is the enforcement authority for the whole of it.
- 2 Ask whether it will be installed as a fixture or as a chattel
For a manufactured home this is the section 18551 question, and it is the financing question in disguise.
Installed on a foundation system under subdivision (a), the home “shall be deemed a fixture and a real property improvement” and the enforcement agency records a document within five business days of the certificate of occupancy. Installed as a chattel under subdivision (b), it does not.
- 3 Confirm the site work scope in writing, because it is where prefab budgets move
All three regimes converge from the foundation onward. Grading, foundation, utility connections, setting, weather-sealing at the marriage line, decks and stairs, and the certificate of occupancy are local, permitted, licensed work.
Ask the supplier explicitly which of these is in the quoted price and which is not. Our utility connection study covers what an agency may and may not charge for the connections.
- 4 Take the regime question to your lender before you take it to your architect
Whether the finished unit is real property changes the product set available to finance it. Our financing study sets out how narrow that product set already is for ADUs generally; a chattel-status unit is narrower still. This is not financial advice, and your lender is the only source that can answer it for your file.
- 5 Check the site constraints that federal preemption does not touch
Fire hazard severity zone requirements, floodplain elevation, septic capacity, setbacks, height. None of these is displaced by a HUD label. See our studies on fire code, flood zones and septic systems.
Six terms people use interchangeably and should not
- Manufactured home
- Federally: “A structure, transportable in one or more sections, which in the traveling mode is 8 body feet or more in width or 40 body feet or more in length or which when erected on-site is 320 or more square feet, and which is built on a permanent chassis and designed to be used as a dwelling with or without a permanent foundation when connected to the required utilities.” 24 CFR 3280.2. In California, Health and Safety Code section 18007 adds the date threshold of 15 June 1976 and the reference to the National Manufactured Housing Construction and Safety Act of 1974, 42 U.S.C. § 5401 et seq.
- Factory-built housing
- A California statutory category, not a description. Health and Safety Code section 19971: a residential building or dwelling unit wholly or substantially manufactured offsite where “all concealed parts or processes of manufacture cannot be inspected before installation at the building site without disassembly, damage, or destruction”, assembled onsite to the California Building Standards Code. It “does not include a mobilehome … a recreational vehicle … or a commercial modular.”
- Modular
- Not a term the California Health and Safety Code uses for housing. In trade usage it usually means a unit built offsite to the California Building Standards Code — which, if it carries an HCD insignia, is factory-built housing, and if it does not, is an ordinary building inspected locally. Because the word carries no legal content, do not accept it as an answer to “which regime is this?”
- Mobilehome
- A separate defined term at Health and Safety Code section 18008, and expressly excluded from factory-built housing by section 19971. The practical significance for an ADU project is that the exclusions in section 19971 are the statute drawing a boundary, and a supplier who uses “mobilehome”, “manufactured home” and “modular” interchangeably has not read it.
- Fixture versus chattel
- The distinction Health and Safety Code section 18551 turns on. A manufactured home “may be installed on a foundation system as either a fixture or improvement to the real property … or … as a chattel.” On the real-property route, once installed in compliance it “shall be deemed a fixture and a real property improvement to the real property to which it is affixed”, and the enforcement agency records a document naming the owner and describing the property. Chattel status keeps it personal property.
- Efficiency unit
- Relevant because a small prefab ADU may be one. Government Code section 66313(c) gives “efficiency unit” the meaning in Health and Safety Code section 17958.1, and section 66321(b)(1) bars a local minimum square footage requirement “that prohibits an efficiency unit.” The prefab route and the very small unit therefore reinforce one another legally.
The part that is not a paperwork question
Whichever regime a unit is in, the site work is real construction and is licensed and permitted.
Setting a unit on a foundation, connecting it to electrical service, gas and sewer, and sealing the structure are not homeowner tasks. Foundation and structural work is a general building contractor’s work with an engineer’s design where required; electrical connection is a licensed C-10 electrical contractor’s work; gas, water and sewer connection is a licensed C-36 plumbing contractor’s work.
Health and Safety Code section 18551 is explicit that a building permit must be obtained “[p]rior to installation … from the appropriate enforcement agency”, and that the certificate of occupancy is the enforcement agency’s. A prefab unit that arrives complete does not arrive approved.
HyreADU recommendation: before signing, get the supplier to state in writing which of foundation design, foundation construction, crane or set, utility connections, weather sealing, and the certificate of occupancy are inside the quoted price.
The gap between a factory price and a finished-on-your-lot price is entirely made of those items, and it is the single most common source of surprise in prefab ADU budgets.
What we could not verify
HCD’s own factory-built housing program documentation. The Department of Housing and Community Development’s site returned HTTP 403 throughout this retrieval, as it did throughout the previous one.
We therefore worked from the Health and Safety Code sections themselves — 19971 for the definition and exclusions, 19980 for the insignia — and we quote no HCD program guidance, no fee schedule and no list of approved plan-approval agencies. Those are the documents to ask HCD for directly.
The full text of 24 CFR 3280.2 and the certification-label mechanics. The Electronic Code of Federal Regulations blocked automated retrieval, so we worked from the Legal Information Institute’s copy.
The definition of “manufactured home” we quote is truncated in that copy after “permanent chassis”, and the retrieval noted that the definition “continues with additional specifications regarding plumbing, heating, electrical systems, and exclusions.” We have not reproduced the remainder and you should not assume our excerpt is complete.
Anything about relative cost, build quality, timeline or resale between the three regimes. We have not researched it here and we publish no comparison. Our construction duration study deals with timelines on the evidence we do have.
How lenders and appraisers in fact treat each regime. This page states the property-law consequence of section 18551 and stops. It does not report lender practice, because we did not retrieve underwriting guidelines and would not generalise from any that we had.
Questions
Can an ADU be a manufactured home in California?
What is the difference between a manufactured home and factory-built housing?
Is a modular ADU the same as a prefab ADU?
Does a HUD-Code ADU still need a local building permit?
Does a manufactured home ADU count as real property?
Can my city refuse a HUD-Code manufactured home as an ADU?
Which regime is fastest or cheapest?
What label should I look for on delivery?
Written and audited by
HyreADU Research Desk
Primary-source research, data analysis and fact checking
We are a research desk, not a builder. We read the permit extract, the statute, the HCD return or the fee schedule ourselves, and publish each figure with its source and retrieval date.
Where a number cannot be traced to a primary source, we leave it out and say what we could not verify. Our store-based claims cover California only.
- CA
- the only state this desk will make store-based claims about
- 5
- jurisdictions with extracted ADU permit evidence
- 735
- CSLB-verified companies in the California store
- 0
- national claims from a one-state store
How this desk works
- Primary sources only. Permit counts come from the city or county that issued the permit. Production counts come from HCD’s Annual Progress Report. Rents come from HUD or the Census. We do not cite an article that cites a source; we download the source and compute the figure ourselves.
- This is a California site. The company store is 734 California firms and one New Mexico firm. Permit evidence exists for five named jurisdictions: Los Angeles, San Francisco, Sacramento, San José and unincorporated Marin. A number from that store is titled to those places, never to the United States.
- A permit is not a completion, and a license is not an ADU grade. California licenses no ADU classification. Being named on an ADU permit is evidence of engagement in that jurisdiction, not of quality, completion, or work anywhere else. Owner-builder permits are excluded from contractor counts.
- Calculation is labeled as calculation. Figures we derive are never presented as something HCD, HUD, the Census or a city published. Terner Center research is cited as Terner’s, never restated as ours.
- We do not design, permit or build ADUs, and we take no payment for placement, ranking or a favorable mention. Pages that look like rankings are not: they publish public-record counts and let the reader decide.
- Nothing here is legal, tax or financial advice. Zoning, underwriting and appraisal practice vary by jurisdiction, lender and appraiser. The useful next step on a specific lot is the planning counter and a licensed professional.
Data as of 24 CFR §§ 3280.2 and 3282.11 retrieved from the Legal Information Institute 2026-09-05; Health and Safety Code §§ 18007, 18551, 19971 and 19980 and Government Code § 66313 retrieved from leginfo.legislature.ca.gov 2026-09-05; California Building Standards Code edition dates from the California Building Standards Commission 2026-09-05. Authorship on this site is organizational: the analysis belongs to the desk rather than to a named individual, and we do not publish credentials we do not hold.
Our editorial policy sets out how we source, date and correct what we publish.
Sources & retrieval dates
- 24 CFR § 3280.2 — definitions in the Manufactured Home Construction and Safety Standards , “Manufactured home” as retrieved: “A structure, transportable in one or more sections, which in the traveling mode is 8 body feet or more in width or 40 body feet or more in length or which when erected on-site is 320 or more square feet, and which is built on a permanent chassis and designed to be used as a dwelling with or without a permanent foundation when connected to the required utilities …” The retrieved copy notes the definition continues with plumbing, heating and electrical specifications and exclusions, which we have not reproduced. “Federal manufactured home construction and safety standard” is defined as “[a] reasonable standard for the construction, design, and performance of a manufactured home which meets the needs of the public including the need for quality, durability, and safety.” Retrieved from the Legal Information Institute; the Electronic Code of Federal Regulations blocked automated retrieval on this date. Retrieved 2026-09-05.
- 24 CFR § 3282.11 — federal preemption , “No State manufactured home standard regarding manufactured home construction and safety which covers aspects of the manufactured home governed by the Federal standards shall be established or continue in effect … unless it is identical to the Federal standards.” States may not require a certified home “to be subject to State inspection” or “that a State label be placed on the manufactured home”, and “[n]o State may establish or keep in effect through a building code enforcement system or otherwise, procedures or requirements which constitute systems for enforcement of the Federal standards … which go beyond this system.” States may, however, “establish or continue in force consumer protections, such as warranty or warranty performance requirements, which respond to individual consumer complaints.” Retrieved 2026-09-05.
- California Health and Safety Code § 19971 — definition of factory-built housing , A residential building or dwelling unit wholly or substantially manufactured offsite, including units where “all concealed parts or processes of manufacture cannot be inspected before installation at the building site without disassembly, damage, or destruction”, and which is “assembled onsite in accordance with building standards published in the California Building Standards Code and other regulations.” “Factory-built housing does not include a mobilehome, as defined in Section 18008, a recreational vehicle, as defined in Section 18010.5, or a commercial modular, as defined in Section 18012.5.” Retrieved 2026-09-05.
- California Health and Safety Code § 19980 — insignia of approval , Factory-built housing “shall bear insignia of approval issued by the department.” This is the label that distinguishes factory-built housing from a unit built to Title 24 and inspected wholly by the local building department. Retrieved 2026-09-05.
- California Health and Safety Code § 18007 — definition of manufactured home , A structure “constructed on or after June 15, 1976, is transportable in one or more sections, is eight body feet or more in width, or 40 body feet or more in length”, referencing compliance with the National Manufactured Housing Construction and Safety Act of 1974, 42 U.S.C. § 5401 et seq. The section also permits a structure meeting all requirements except the size specifications to qualify where the manufacturer voluntarily files certification and complies with the federal standards. Retrieved 2026-09-05.
- California Health and Safety Code § 18551 — installation on a foundation system , “A manufactured home, mobilehome, or commercial modular may be installed on a foundation system as either a fixture or improvement to the real property … or a manufactured home or mobilehome may be installed on a foundation system as a chattel.” A building permit must be obtained “[p]rior to installation … from the appropriate enforcement agency.” Within five business days of the certificate of occupancy the enforcement agency “shall record a document naming the owner of the real property, describing the real property with certainty, and stating that a manufactured home … has been affixed to that real property by installation on a foundation system.” Once so installed the home “shall be deemed a fixture and a real property improvement.” Amended by Stats. 2018, Ch. 254, Sec. 1 (AB 1943), effective 5 September 2018. Retrieved 2026-09-05.
- California Government Code § 66313 — ADU definitions , Subdivision (a) defines an accessory dwelling unit as “[a]n attached or a detached residential dwelling unit that provides complete independent living facilities for one or more persons and is located on a lot with a proposed or existing primary residence”, with permanent provisions for living, sleeping, eating, cooking and sanitation, and expressly includes a manufactured home as defined in Health and Safety Code section 18007. Subdivision (c) gives “efficiency unit” the meaning in Health and Safety Code § 17958.1. Retrieved 2026-09-05.
- California Government Code § 66321 — size and height standards a local ordinance may not impose , Relevant to small prefab units: no minimum square footage requirement “that prohibits an efficiency unit”; no maximum below 850 sq ft of interior livable space, or 1,000 sq ft with more than one bedroom; and specified minimum height allowances of 16, 18 and 25 feet depending on configuration and transit proximity. Retrieved 2026-09-05.
- California Government Code § 66317 — the 60-day ministerial deadline and the comments requirement , “The permitting agency shall either approve or deny the application to create or serve an accessory dwelling unit within 60 days from the date the permitting agency receives a completed application if there is an existing single-family or multifamily dwelling on the lot.” On denial the agency must “return in writing a full set of comments to the applicant with a list of items that are defective or deficient and a description of how the application can be remedied.” Retrieved 2026-09-05.
- California Building Standards Commission — code editions and effective dates , “The 2025 California Building Standards Code (Cal. Code Regs., Title 24) will be published July 1, 2025, with an effective date of January 1, 2026.” The 2022 edition “was published July 1, 2022, with an effective date of January 1, 2023.” Establishes the vintage of the state code a factory-built or site-built unit is constructed to. Retrieved 2026-09-05.
One question, asked before the deposit
Which label will this unit carry — a federal certification label, an HCD insignia of approval, or neither? Every other question on this page follows from the answer, and a supplier who will not answer it in writing has told you something.
HyreADU does not design, permit, build or sell accessory dwelling units, does not practice law, and does not give tax or financial advice. This page is informational and is not legal, tax or financial advice.
It describes federal regulations and California statutes as retrieved on 5 September 2026, quotes no manufacturer and ranks no product, and makes no comparison of cost, quality, timeline or resale between the three regimes.
Whether a particular local requirement is preempted as to a particular manufactured home is a legal question this page does not answer.
Installation, foundation, structural, electrical, gas and plumbing work is licensed and permitted work in California, and a prefabricated unit that arrives complete does not arrive approved.
Take the fixture-versus-chattel question to a lawyer and a lender before the unit is ordered.