Is a prefab ADU cheaper than a site-built one?

Not automatically. Prefab removes weather delays and change-order risk, which is where site-built budgets usually break, but the factory unit is only part of the cost. Foundation, utility trenching, crane work, and site engineering are quoted separately by most companies, and sales tax on the unit is often excluded from the advertised all-in figure.

What does HCD approval mean for a prefab ADU?

California's Department of Housing and Community Development inspects factory-built housing during manufacturing and places an insignia of approval on the unit before it ships. Because concealed work was already inspected at the factory, your local building department generally does not re-review the structure itself. It still reviews and inspects the foundation, utility connections, and site work.

Do prefab ADUs still need a local permit?

Yes. HCD approval covers the factory-built portion. You still need local approval for the site: foundation design, utility connections, setbacks, and zoning. Under SB 543, effective January 1, 2026, your local agency must tell you in writing whether your ADU application is complete within 15 business days of receiving it.

Prefab ADU Guide (2026): Cost, HCD Approval, and What's Excluded

Q: What's the difference between a modular and a manufactured ADU?

Modular units are built to California's Title 24 building code, the same code as site-built homes, and are treated like site-built structures for permitting. Manufactured units are built to the federal HUD code instead. Both are legal as ADUs in California, but the distinction affects permitting, appraisal, and financing, and companies do not always lead with which one they sell.

What is a prefab ADU?

A prefab ADU is an accessory dwelling unit built in a factory and delivered to your property, rather than framed on site. One California manufacturer publishes all-in prices from $298,800 for a 385 sq ft studio to $498,800 for a 1,195 sq ft three-bedroom. The factory portion is inspected by the state and carries an HCD insignia before it ships, so your city reviews the foundation, utilities, and site work rather than the building itself.

The pitch for prefab is speed and certainty, and both hold up. What doesn’t survive contact with a real quote is the assumption that prefab is the cheap option. It sometimes is. But the advertised number is rarely the number you pay, and the smallest unit on the price list is almost always the most expensive square footage you can buy.

Prefab, modular, manufactured, panelized: they aren’t the same

“Prefab” is a category, not a product. Four things get sold under it, and the differences are not cosmetic.

Modular units are built in sections at a factory to California’s Title 24 building code, the same code that governs site-built houses, then craned onto a permanent foundation. Because the code is the same, local agencies generally treat them like site-built structures for zoning and permitting.

Manufactured units are built to the federal HUD code instead of Title 24. They’re legal as ADUs in California and can be cheaper, but they’re a different regulatory animal, and that difference follows the unit into the appraisal and the loan file.

Panelized units arrive as flat wall and roof panels and are assembled on site. You get factory precision on the framing and site-built everything else, which means more on-site labor and a longer schedule than modular.

Site-built is the conventional route. Everything happens in your backyard.

This matters because companies don’t always volunteer which one they sell. Villa, one of the highest-volume ADU builders in California, states plainly in its own FAQ that its designs are “built to HUD code.” Abodu describes itself as building modular backyard homes. Both are legitimate. They are not equivalent, and you should know which you’re buying before you sign.

What you're actually buying
Type	Built to	Assembled	Local plan check
Modular	Title 24 (state code)	Craned on, 1 to 2 days	Site and foundation only
Manufactured	HUD code (federal)	Set on foundation	Site and foundation only
Panelized	Title 24	Assembled on site, weeks	Full local review
Site-built	Title 24	Built on site, months	Full local review

What HCD approval means, and why it speeds permitting

California’s Department of Housing and Community Development runs the Factory-Built Housing program. HCD defines factory-built housing as a residential building manufactured such that “all concealed parts or processes of manufacturing cannot be inspected before installation,” which is the whole problem it exists to solve. A local inspector can’t check wiring behind a wall that was closed up in Fresno.

So the state inspects it at the factory instead. Every factory-built unit sold in California carries an HCD insignia of approval, placed on the unit before it ships.

The practical consequence: your local building department does not re-review the structure. It reviews the parts that happen on your property, meaning foundation design, utility connections, setbacks, and drainage, and it inspects that work before issuing a certificate of occupancy. That’s a meaningful chunk of plan check removed from the critical path.

It is not a permit exemption. You still apply locally, and you’re still subject to your city’s setback and height rules.

One thing changed in your favor. Under SB 543, effective January 1, 2026, a permitting agency must determine whether your ADU application is complete and tell you in writing within 15 business days. Miss the deadline and the application is deemed complete. If the agency finds it incomplete, it has to give you an itemized list of what’s missing and how to fix it, not a vague rejection.

What a prefab ADU costs in California

Most cost pages give a range and stop. Here are published numbers instead.

Abodu publishes fixed pricing for its full model line, split into a home price (everything done off site) and an installation price (everything done on your property). These are the figures listed on its pricing page as of August 2026.

Abodu published pricing, August 2026
Model	Size	Home	Installation	All-in	Per sq ft
Studio	385 sq ft	$234,800	$64,000	$298,800	$776
One	500 sq ft	$260,800	$64,000	$324,800	$650
Two	625 sq ft	$293,800	$65,000	$358,800	$574
Two Plus	800 sq ft	$328,800	$96,000	$424,800	$531
Three	1,070 sq ft	$364,800	$100,000	$464,800	$434
Three Plus	1,195 sq ft	$393,800	$105,000	$498,800	$417

Source: Abodu published fixed pricing, retrieved August 2026. Per-square-foot figures calculated from the all-in totals. Prices change, so verify directly.

Look at the right-hand column, because it’s the least intuitive number on this page.

The same published prices, plotted. Going bigger nearly halves what you pay per square foot. Plotted from Abodu published fixed pricing, retrieved August 2026.

The studio costs $776 per square foot. The largest model costs $417. Going bigger nearly halves your cost per square foot. That’s not a discount for volume. It’s because a kitchen, a bathroom, a panel, and a water heater cost roughly the same whether they serve 385 square feet or 1,195. In a small unit, those fixed costs are spread over almost nothing.

If you’re choosing between a studio and a one-bedroom to save money, you are paying a large premium per square foot for a unit that will rent for less and appraise for less. That trade sometimes makes sense. It should be a decision, not an accident.

The 750-square-foot line

There’s a second reason size deserves thought. Under Government Code § 66311.5, renumbered from § 66324 by SB 543, an ADU with 750 square feet or less of interior livable space is exempt from local impact fees. Above 750, fees may be charged, and they’re prorated against the square footage of the primary dwelling.

Separately, an ADU or JADU under 500 square feet is treated as not increasing assessable space for school developer fee purposes, which generally takes school fees off the table too.

Those thresholds sit right in the middle of the model lineup. A 625-square-foot unit is under both impact-fee and proration exposure. An 800-square-foot unit isn’t. The jump between them costs more than the price list shows.

What “turnkey” excludes

This is the section worth reading twice.

Abodu is unusually specific about its exclusions, which makes it a useful worked example. Its published pricing includes design, civil engineering, permits and plan check, fire sprinklers, and delivery in the home price. Foundation, site work, utility trenching up to 50 feet, and a crane set with an allowance up to 100 feet sit in the installation price.

Quoted separately, per their own list:

Boundary survey, staking, and certification, $6,000
Water, sewer, and power trenching beyond 50 feet
Craning beyond 100 feet
Demolition or tree removal
Unique site engineering
Sales tax

That last one matters. Sales tax on a factory-built unit is assessed on the unit, and on a $234,800 home that is not a rounding error. It is excluded from the “expected all-in” figure.

The trenching and crane allowances are the other two to watch. Fifty feet of trenching covers a lot of ordinary backyards and none of a deep lot. A hundred feet of crane reach is fine over a driveway and useless if the only approach is over a two-story house and a mature oak.

Questions to put to any prefab company in writing, regardless of who you’re talking to:

Is this unit built to Title 24 or HUD code?
Is sales tax included in the quoted total?
How many feet of utility trenching are included, and what’s the per-foot rate beyond it?
What crane reach is included, and what happens if my site needs more?
Who pays if the soils report comes back requiring an engineered foundation?
Does the price include the boundary survey?
What are the school and impact fees for my specific jurisdiction, and are they in this number?

Any company that answers all seven without hedging is worth taking seriously. The answers vary enormously, and the variance is where the surprises live.

Site requirements: whether prefab is even possible on your lot

A modular unit arrives in one or two pieces on a truck and gets lifted into place. That imposes constraints a site-built project doesn’t have.

Access. The truck has to reach your property and have room to maneuver. Narrow hillside streets, tight cul-de-sacs, and low-hanging utility lines are common disqualifiers.

Crane clearance. The crane needs a setup pad and a clear arc to swing the module over whatever stands between the street and the pad, whether that’s the house, trees, or power lines. Reach beyond the included allowance costs real money, and some sites can’t be reached at any price.

Grade. A steep or sloping lot means an engineered foundation, and the soils report drives that decision. This is the most common source of change orders on an otherwise fixed-price prefab job.

Utilities. Distance from the existing sewer lateral, water main, and electrical panel determines your trenching cost. Panel capacity matters too. If your main service can’t carry another dwelling, a service upgrade lands on your bill.

A reputable company will assess all of this before quoting you a firm number. If someone gives you a fixed price before anyone has looked at your lot, the price isn’t fixed.

Is prefab right for you?

Prefab makes the most sense when your site is straightforward, you value schedule certainty over customization, and you’re building at a size where the per-square-foot math isn’t punishing, which in practice means 600 square feet and up.

It makes less sense on a difficult lot, where the site-specific exclusions can erase the advantage, or when you want a layout nobody manufactures. Two-story ADUs and garage-conversion ADUs are generally outside what prefab companies offer. Villa, for instance, states it doesn’t build two-story units or garage-top ADUs at all.

The summary: prefab buys you a schedule you can plan around and a number that mostly holds. It does not reliably buy you a lower total cost. Price it against a site-built bid on the same square footage before you decide.

Prefab ADUs in California: What They Cost and What's Actually Included