Statistics
Prefab and modular ADU statistics
16 of 39 figures are HyreADU calculations. Nobody counts prefabricated ADUs, so this page measures the factory-housing series that is published — and joins it to the ADU permit series and the statutory ceiling.
Written by HyreADU Research Desk Primary-source research and data analysis
Audited by HyreADU Research Desk Provenance and citation audit
How to use this page
There is no published count of prefabricated accessory dwelling units, in California or nationally. HCD’s Annual Progress Report counts ADU units with no prefabrication flag. The Los Angeles permit extract has no prefabrication flag.
Any page quoting “X% of ADUs are prefab” is quoting a vendor estimate, and this page will not join it.
What is published is the HUD-Code manufactured housing series, and it is worth knowing because Government Code § 66313 expressly permits an ADU to be a manufactured home.
In 2025, 3,065 HUD-Code homes were shipped to California, 3.0% of the 102,738 shipped nationally. Against 32,126 ADU permits reported to HCD in the same year, that is 9.5% of ADU permit volume — and not all of those homes became ADUs.
The reason the factory route is smaller than the brochures suggest is size. The average new HUD-Code home shipped in the Census West region in 2025 was 1,369 sq ft — 169 square feet above California’s 1,200-square-foot detached ADU cap in § 66314(a)(5).
Only the single-section line, averaging 944 sq ft, sits comfortably inside it. On price, the factory number is genuinely lower, and genuinely incomplete. The average sales price of a new manufactured home in California in 2025 was $164,800 ($80,500 single-section).
That is the price of the home. It excludes land, site preparation, the foundation, utility connections and installation — the exact items that dominate an ADU budget.
16 of 39 figures on this page are HyreADU calculations (16 hyreadu calculation, 23 agency published, 0 trade / survey estimate). We name no manufacturer and rank no product.
Which of the three legal regimes a unit is in — HUD-Code, HCD factory-built housing, or built to Title 24 — is the question that decides everything else, and it lives on the prefab building-standards study.
What this page is not
- No count of prefab ADUs exists, so this page does not publish one
HCD’s Annual Progress Report Table A2 has unit categories for ADU, single-family detached, single-family attached, 2-to-4, 5-plus and manufactured home — but the ADU category carries no construction-method field. LADBS has no prefabrication flag.
No state or federal file we can reach lets anyone say what share of California ADUs were factory-built. We say that rather than borrow a vendor’s share.
- HUD-Code shipments are not ADU shipments
The Census shipment series counts every HUD-Code manufactured home shipped to the state — for land-lease communities, for rural single-family sites, for replacement of older stock, and for some accessory dwelling units. It is the ceiling on how many ADUs could have arrived by that route, not a measurement of how many did.
- The sales price is the price of the home, not of the project
The Manufactured Housing Survey price is the sales price of the unit. Land, site preparation, the foundation system, utility connections, transport beyond the dealer arrangement, and installation are all outside it. Those are the items an ADU budget is actually made of, which is why a factory price is never a project price.
- We name no manufacturer and rank no product
This desk publishes no vendor list, no product comparison and no recommendation on this page. The regime question is prior to the product question and it is the one people get wrong. Ask which label the unit will carry — a HUD certification label, an HCD insignia, or neither — in writing, before signing anything.
The figures most worth knowing
The legal regimes behind these numbers are set out on the prefab building-standards study.
Three regimes, and only one of them has a national statistic
A factory-built ADU in California is in exactly one of three legal regimes, and the regime is a 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, defined at 24 CFR 3280.2 as a structure “transportable in one or more sections … which when erected on-site is 320 or more square feet, and which is built on a permanent chassis”.
Under 24 CFR 3282.11 no state construction or safety standard covering the same ground may stand unless it is identical to the federal one.
This is the only one of the three the federal government counts, and it is counted in the Census Manufactured Housing Survey — which is why every number in the tables below is about this regime and not the other two.
Two — factory-built housing under HCD’s program. Health and Safety Code § 19971 defines it as a residential building or dwelling unit substantially manufactured offsite where the concealed parts cannot be inspected after installation without damage, assembled on site to the California Building Standards Code, and expressly excluding a mobilehome.
Section 19980 requires it to bear an insignia of approval issued by the department. State design-and-factory regulation, not a city pre-approved plan catalog.
An HCD Insignia of Approval on a factory-built unit or component is evidence the factory work was approved under Health & Safety Code §§ 19960–19997 and Title 25. The local building department still permits the foundation, site, utilities and installation.
Some city galleries (Berkeley; some LADBS listings marked 'HCD Approved') admit HCD-approved designs into a local preapproval program under § 65852.27(a)(5)(B). That admission is a local act. It does not make the HCD FBH program itself a city pre-approval.
There is no published unit count for this program that we have been able to retrieve, so there is no row for it below — an absence, stated, rather than an estimate.
Three — built to Title 24 and inspected locally. Whether the walls were framed in a factory or on the lot, if the unit is neither a HUD-Code home nor carrying an HCD insignia, it is an ordinary building under the California Building Standards Code and your building department inspects it.
This is what most of the trade means by “modular”, and the Code does not separately name it.
It is therefore inside the ordinary ADU permit counts and cannot be separated out of them.
All three routes are open. Government Code § 66313 defines an accessory dwelling unit to include a manufactured home as defined in Health and Safety Code § 18007. That is a deliberate legislative choice, and it is the reason the HUD-Code column belongs on an ADU statistics page at all.
How many factory homes actually arrive in California
The Census has published shipments to states since 1994. California’s series tells a long story that has nothing to do with ADUs and everything to do with why the factory route is smaller here than the marketing implies: the state took 10,783 units in 2005 and 3,065 in 2025.
| Year | Shipped to California | Shipped nationally | California share | California ADU permits reported to HCD | Shipments ÷ ADU permits | Class |
|---|---|---|---|---|---|---|
| 2018 | 3,988 | 96,555 | 4.1% | 12,050 | 33.1% | Agency published + HyreADU calculation |
| 2019 | 3,890 | 94,615 | 4.1% | 12,992 | 29.9% | Agency published + HyreADU calculation |
| 2020 | 3,232 | 94,390 | 3.4% | 13,116 | 24.6% | Agency published + HyreADU calculation |
| 2021 | 3,344 | 105,772 | 3.2% | 21,163 | 15.8% | Agency published + HyreADU calculation |
| 2022 | 4,022 | 112,882 | 3.6% | 26,224 | 15.3% | Agency published + HyreADU calculation |
| 2023 | 2,951 | 89,169 | 3.3% | 29,309 | 10.1% | Agency published + HyreADU calculation |
| 2024 | 2,813 | 103,314 | 2.7% | 31,515 | 8.9% | Agency published + HyreADU calculation |
| 2025 | 3,065 | 102,738 | 3.0% | 32,126 | 9.5% | Agency published + HyreADU calculation |
Shipment and national columns are Census figures. The share and ratio columns are HyreADU calculations. ADU permit counts are HCD Annual Progress Report Table A2 as processed for the permits-versus-completions study.
The final column is an upper bound, not a prefab share: it assumes every HUD-Code home shipped to California became an ADU, which is emphatically not the case. 2026 is not tabulated because the Census column is year to date through May (1,148 units).
The interesting number is how small the ceiling is. Even treating every manufactured home shipped to the state as an ADU, the route cannot account for more than 9.5% of 2025 ADU permits. Anyone claiming a large prefab share of California ADUs is claiming it for regimes two and three, which nobody counts.
The size problem: the product line is built above the cap
Product size against the statutory cap is the finding that makes the shipment numbers make sense. California caps a detached ADU at 1,200 square feet. The HUD-Code product line is not built for that ceiling — it is built for lots where the ceiling does not exist.
| Product line | Average floor area | Median floor area | Against the 1,200 sq ft detached cap | Class |
|---|---|---|---|---|
| HUD-Code, all sizes, Census West, 2025 | 1,369 sq ft | 1,325 sq ft | 169 sq ft over | Agency published + HyreADU calculation |
| HUD-Code, single-section, Census West, 2025 | 944 sq ft | 916 sq ft | 256 sq ft under | Agency published + HyreADU calculation |
| HUD-Code, double-section, Census West, 2025 | 1,532 sq ft | 1,511 sq ft | 332 sq ft over | Agency published + HyreADU calculation |
| HUD-Code, all sizes, United States, 2025 | 1,471 sq ft | 1,343 sq ft | 271 sq ft over | Agency published + HyreADU calculation |
| New-construction ADU permits, City of Los Angeles | 975 sq ft | 920 sq ft | 12.5% of permits over | HyreADU calculation |
Floor-area figures are Census, by region — the West region is not California, and there is no state-level floor-area table.
The comparisons against 1,200 square feet are HyreADU calculations against Government Code § 66314(a)(5).
The Los Angeles row is our own measurement of 8,536 new-construction ADU permits and is set out fully on the size digest.
What this does and does not prove. It does not prove that manufactured homes cannot be ADUs — they plainly can, and § 66313 says so.
It shows that the mass-market factory product is sized for a different job, so an ADU buyer is shopping the narrow single-section end of a line whose economics come from the wide end.
That is a different proposition from “buy the factory home, it is cheaper”.
What a factory home costs, and what that price leaves out
The Census publishes average sales price by state and by size of home. California is the most expensive state in the Pacific division for a new manufactured home, and the single-section number is the one an ADU buyer should be looking at.
| Geography | All sizes | Single-section | Double-section | Double ÷ single | Class |
|---|---|---|---|---|---|
| California | $164,800 | $80,500 | $185,000 | 2.30× | Agency published + HyreADU calculation |
| Census West region | $145,100 | $89,000 | $167,600 | 1.88× | Agency published + HyreADU calculation |
| United States | $127,200 | $85,600 | $157,100 | 1.84× | Agency published + HyreADU calculation |
| California above the national average | +30% | -6% | +18% | — | HyreADU calculation |
Census Manufactured Housing Survey, average sales price by state, 2025, retrieved 2026-09-07. Sales price is the price of the home. It excludes land, site preparation, foundation and installation. Price columns are Census; ratio and premium rows are HyreADU calculations.
The exclusions are the point. A California ADU project pays for land it already owns, then site preparation, a foundation system, utility connections, permit and impact fees, and installation. None of that is in $80,500. Our California cost study and the cost calculator exist so those lines are entered explicitly rather than assumed away.
Price per square foot, and why the comparison misleads
A per-square-foot comparison between a factory home and a site-built ADU is the single most common way this topic is got wrong. Here is the comparison, computed honestly, with the reason it does not mean what it appears to.
| Basis | Figure | What is inside it | Class |
|---|---|---|---|
| HUD-Code, all sizes, Census West, 2025 | $106/sq ft | Sales price $145,100 ÷ average floor area 1,369 sq ft. The home only. | HyreADU calculation |
| HUD-Code, single-section, Census West, 2025 | $94/sq ft | Sales price $89,000 ÷ 944 sq ft. The size class an ADU buyer is shopping. | HyreADU calculation |
| HUD-Code, double-section, Census West, 2025 | $109/sq ft | Sales price $167,600 ÷ 1,532 sq ft. | HyreADU calculation |
| LA new-construction ADU permits, median | $120/sq ft | Declared permit valuation ÷ recorded floor area, 8,528 permits. Interquartile $102–$152. | HyreADU calculation |
Every row here is a HyreADU calculation, and every row is a ratio between two series that were not built to be divided by each other.
The factory rows divide a Census price by a Census regional floor area.
The Los Angeles row divides a declared permit valuation by a declared floor area — and a declared valuation is a fee input, not a construction cost.
Do not conclude that prefab is $14 per square foot cheaper. The factory figure excludes land, site work, foundation, connections and installation.
The permit figure excludes work carried on separate trade permits and is not a contract price either. Two incomplete numbers with different exclusions do not make a comparison; they make a headline.
We publish both so the exclusions are visible, and we decline the headline.
The three labels, and what each one commits you to
- HUD certification label
- A red-and-silver label affixed at the factory certifying construction to 24 CFR Part 3280. Federal preemption under 24 CFR 3282.11 means no state may impose a different construction standard, and no state label may be required for the same ground. It also means the unit is a manufactured home for California purposes under Health and Safety Code § 18007 — with all the foundation and property-status consequences that follow.
- HCD insignia of approval
- Required by Health and Safety Code § 19980 for factory-built housing. The unit was built to the California Building Standards Code and the concealed work was inspected under the state program rather than by your city. This is a state design-and-factory approval, not a city’s pre-approved plan catalog — the two are frequently confused.
- Neither label
- Then it is a building under Title 24, whatever the brochure calls it, and your local building department inspects it like any other house. This is the majority of what the trade markets as “modular”, and it is invisible in every prefabrication statistic because it is counted as an ordinary ADU permit.
- Fixture or chattel
- Health and Safety Code § 18551 lets a manufactured home be installed on a foundation system as a fixture and real-property improvement, or as chattel. Under the real-property route, once installed in compliance, the enforcement agency records a document to that effect within five business days of the certificate of occupancy. Fixture or chattel is the difference between a mortgage and a personal-property loan — and it is decided at installation, not afterwards.
- Pre-approved plan
- A fourth thing entirely, and the one most often mistaken for prefabrication. Government Code § 65852.27 (AB 1332) required every local agency to develop an ADU preapproval program by 1 January 2025. A pre-approved plan is a set of drawings a city has already reviewed. It says nothing about whether the unit is built in a factory. We surveyed 35 programs for the pre-approved plans study.
Ask which label the unit will carry, in writing, before you sign. If the answer is “neither”, that is a legitimate answer — it just means your building department, not a factory inspector, is the authority on the concealed work.
Figures we will not repeat
- A “percentage of ADUs that are prefab”
No agency publishes the denominator or the numerator. HCD counts ADU permits with no construction-method field; the Census counts HUD-Code shipments with no end-use field. Every share we have traced originates with a vendor or an industry association describing its own market. The absence is the finding, and we print it as one.
- A factory sales price quoted as an ADU project cost
$164,800 is the Census average sales price of the home in California. Land, site preparation, foundation, utility connections, permit and impact fees, transport and installation are all outside it. A page that puts that number beside a site-built total is comparing a component with a project.
- “Modular” used as if it were a legal category
It is not one. California law knows HUD-Code manufactured homes, factory-built housing bearing an HCD insignia, and buildings under Title 24.
“Modular” is a marketing word that spans the second and third and is sometimes used loosely for the first.
The regime, not the word, decides the inspection path, the foundation, and whether the unit is real property.
- Federal preemption described as a general exemption
24 CFR 3282.11 preempts conflicting state construction and safety standards for a HUD-Code home. It does not displace the California ADU statute, local zoning, setbacks, height, fire-hazard-severity-zone requirements, or the permits for the foundation and site work. The preemption is narrower than it is usually described, and the narrowness is where projects get stuck.
Citing these figures
Journalists and planning desks are welcome to cite these tables. Link the prefab building-standards study for the legal argument and this page for the numbers.
Where a figure is labeled Agency published, cite the Census Manufactured Housing Survey table or the statutory section directly — the URLs are in the source list and the files are small enough to check in a minute.
Where it is labeled HyreADU calculation, attribute it to HyreADU and say what was divided by what: “HyreADU analysis of Census Manufactured Housing Survey shipments and HCD Annual Progress Report ADU permits.”
Carry the caveat on the ratio. Shipments-to-ADU-permits is an upper bound computed on the assumption that every manufactured home shipped to California became an accessory dwelling unit. It is a ceiling, not a prefab share, and citing it as the latter would be exactly the error this page was written to prevent.
Questions
What percentage of ADUs are prefab or modular?
Can an ADU legally be a manufactured home in California?
How much does a prefab ADU cost?
Is a prefab ADU cheaper per square foot than site-built?
Why are manufactured homes usually too big to be an ADU?
What is the difference between manufactured, factory-built and modular?
Does a HUD-Code ADU become real property?
Is a pre-approved plan the same as a prefab ADU?
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 Census Manufactured Housing Survey annual tables (shipments to states; average sales price by state; floor area by region) retrieved 2026-09-07; HCD Annual Progress Report Table A2 retrieved 2026-09-05; 24 CFR §§ 3280.2 and 3282.11, Health and Safety Code §§ 18007, 18551, 19971, 19980 and Government Code §§ 66313 and 66314 retrieved 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
- U.S. Census Bureau, Manufactured Housing Survey — Annual Shipments to States, 1994–2026 , California and national annual shipments, 1994–2026. The 2026 column is year to date through May. California 2025: 3,065; national 2025: 102,738; California peak 10,783 in 2005. Retrieved 2026-09-07.
- U.S. Census Bureau, Manufactured Housing Survey — Average Sales Price of New Manufactured Homes Sold or Intended for Sale by Size of Home by State, 2014–2025 , Sales price is the price of the home. It excludes land, site preparation, foundation and installation. California 2025: all sizes $164,800, single-section $80,500, double-section $185,000. Retrieved 2026-09-07.
- U.S. Census Bureau, Manufactured Housing Survey — Average and Median Square Feet of Floor Area by Region, 2014–2025 , Census disclosure-avoidance approval ID CBDRB-FY26-0220. West region 2025: all sizes 1,369 sq ft average, single-section 944 sq ft, double-section 1,532 sq ft. Published by region; there is no state-level floor-area table. Retrieved 2026-09-07.
- California Department of Housing and Community Development — Housing Element Annual Progress Report, Table A2 , The ADU permit counts this page divides shipments against. 32,126 ADU permit units in 2025 and 31,515 in 2024, on the event-year basis set out in the permits-versus-completions study. HCD file last updated 2026-09-04. Retrieved 2026-09-05.
- 24 CFR Part 3280 — Manufactured Home Construction and Safety Standards, and 24 CFR 3282.11 — preemption , The federal definition of a manufactured home at § 3280.2 (320 square feet erected on site, permanent chassis), and the preemption of non-identical state construction and safety standards at § 3282.11. Retrieved 2026-09-05.
- California Health and Safety Code §§ 19971 and 19980 — factory-built housing , The definition of factory-built housing, its exclusion of mobilehomes, and the requirement that it bear an insignia of approval issued by the department. Retrieved 2026-09-05.
- California Health and Safety Code § 18551 — installation on a foundation system , Fixture-and-real-property-improvement installation versus chattel, and the five-business-day recording requirement following the certificate of occupancy. Retrieved 2026-09-05.
- California Government Code § 66313 — ADU definitions , Defines an accessory dwelling unit to include a manufactured home as defined in Health and Safety Code § 18007, which is what opens the HUD-Code route. Retrieved 2026-09-05.
- California Government Code § 66314 — maximum standards for an ADU , The 1,200-square-foot detached cap at (a)(5) that every size comparison on this page is drawn against. Retrieved 2026-09-05.
- HCD Factory-Built Housing program , State design-and-factory regulation, not a city pre-approved plan catalog. An HCD Insignia of Approval on a factory-built unit or component is evidence the factory work was approved under Health & Safety Code §§ 19960–19997 and Title 25. The local building department still permits the foundation, site, utilities and installation. Some city galleries (Berkeley; some LADBS listings marked 'HCD Approved') admit HCD-approved designs into a local preapproval program under § 65852.27(a)(5)(B). That admission is a local act. It does not make the HCD FBH program itself a city pre-approval. Program page and handbook, used to confirm that the HCD insignia is a state factory approval and not a city pre-approved plan catalog. Retrieved 2026-09-05.
- City of Los Angeles Department of Building and Safety — Building Permits Issued from 2020 to Present (Socrata resource pi9x-tg5x) , The site-built comparison rows: 8,536 new-construction ADU permits, median floor area 920 sq ft, median implied declared valuation $120 per square foot. Retrieved 2026-09-05.
Ask which label it will carry
HUD certification label, HCD insignia, or neither. That answer, in writing, decides the inspection path, the foundation, the property status and the financing — before any price comparison means anything.
HyreADU does not design, permit or build accessory dwelling units, names no manufacturer and ranks no product. Statistics are informational. They are not legal, tax, financial or construction advice.
Census Manufactured Housing Survey shipments count all HUD-Code homes shipped to a state for every use, not accessory dwelling units; no agency publishes a count of prefabricated ADUs, and no share is offered here.
Manufactured Housing Survey sales prices are the price of the home and exclude land, site preparation, foundation, utility connections, transport beyond the dealer arrangement and installation. Floor-area figures are published by Census region, not by state.
Ratios between series are HyreADU calculations and are labeled as such. Statutory and regulatory text was retrieved on the date shown; California ADU law was recodified twice between 2024 and 2026 and section numbers change.