A prefab ADU in California costs about $145,000 to $420,000 installed for 400 to 800 square feet, per our California cost guide, and the factory unit is only about 49% of that (ADU Wizard estimate). The rest is your lot: foundation, utilities, permits, site prep, delivery and the crane.
Last verified: October 22, 2026
Every prefab company I have priced against leads with the same number: the price of the box. It is a real number. It is also usually about half of what you will spend, and the other half is the part that decides whether prefab saves you anything at all.
So this page does what the brochures do not. It splits a prefab install into the factory unit and every line that stays on your lot, prices each one, and puts the total next to a site-built detached ADU of the same size. If you are shopping companies rather than costs, I am verifying California prefab firms against the state license board in a separate guide to California prefab ADU companies. For the site work around a prefab unit, you still need a local general contractor; our guides to the best ADU builders in California and the best ADU builders in Los Angeles are a good place to start.
Prefab vs Modular vs Panelized vs Manufactured
"Prefab" covers four different ways of building, and they are permitted, financed and installed differently. The code each one is built to matters most, because it decides who inspects what.
| Method | Where it is built | Code it is built to | Typical lead time | Size range |
|---|---|---|---|---|
| Modular (volumetric) | Whole boxes built in a factory, finished inside, trucked and craned onto a foundation | State building code. In California, units certified under the Factory-Built Housing program that HCD runs | 3 to 6 months of factory time | Roughly 300 to 1,200 sq ft, in one or more boxes |
| Panelized | Wall, floor and roof panels built in a factory, assembled on your foundation by a crew | Local building code, inspected on site like a stick build | 4 to 10 weeks for panels, then on-site assembly | Any size the plans allow |
| Manufactured (HUD code) | Built on a permanent steel chassis in a factory, delivered in one or two sections | Federal HUD code (24 CFR Part 3280), not the local building code | 2 to 4 months of factory time | At least 320 sq ft by federal definition |
| Kit | A package of shell materials shipped to you, assembled by you or a crew | Local building code, inspected on site | Weeks to ship, then a full on-site build | Usually 100 to 1,000 sq ft |
Codes: California Health and Safety Code 19960 and following (factory-built housing); 24 CFR Part 3280 (HUD manufactured home standards); the 320 sq ft floor is the federal definition in 42 U.S.C. 5402. Lead times and size ranges are my planning ranges, not guarantees.
The kit row is a different animal: you are buying materials, not a finished building. I break that down separately in ADU kits: what's in the box and real installed cost.
Where the Money Goes on a Prefab ADU
Here is a 600 square foot prefab ADU in California, split into the lines on a real install. The installed total comes from the prefab ranges in our California ADU cost guide (interpolated between its 400 and 650 sq ft rows). The site lines are our cost engine's California numbers for the same work on a site-built detached unit, because a foundation is a foundation no matter what sits on it. The factory line is what is left.
| Line | California, 600 sq ft (estimate) | Who does it |
|---|---|---|
| Factory unit, delivery, crane set and on-site finish work | $96k to $146k | Manufacturer, its trucker and crane operator, and the set crew |
| Site prep and grading | $20k to $33k | Your local contractor (or the city, for fees) |
| Foundation | $22k to $37k | Your local contractor (or the city, for fees) |
| Utility trenching and connections | $17k to $29k | Your local contractor (or the city, for fees) |
| Permits and fees | $12k to $20k | Your local contractor (or the city, for fees) |
| Site plan, engineering and design | $15k to $25k | Your local contractor (or the city, for fees) |
| Contingency | $7k to $12k | Your local contractor (or the city, for fees) |
| Installed total | $189k to $302k | About $409 per sq ft at the midpoint |
Source: ADU Wizard cost engine (California detached breakdown) and the ADU Wizard California cost guide. Estimates, not bids. The factory line is the installed total minus the site lines.
I cannot split that first line further with numbers I can source, so I am not going to make them up. What I can tell you is that it hides four different bills: the unit, the truck, the crane and the finish work after the set (steps, decks, trim at the seams, final tie-ins). Make every quote split it four ways. The crane is the one I see missing most often, and it is never cheap when the yard is behind the house.
Look at the site lines together. Site prep, foundation, utilities, permits, design and contingency add up to about 51% of the installed price, and none of them gets cheaper because the walls were built indoors. That is the honest ceiling on what prefab can save you.
Prefab vs Site-Built at 400, 600 and 800 Square Feet
Same sizes, both ways. Site-built is our cost engine's detached estimate; prefab installed is our California cost guide's published range (600 sq ft interpolated).
| Size | Prefab installed, California | Site-built detached, California | Prefab vs site-built (midpoints) | Site-built, national average |
|---|---|---|---|---|
| 400 sq ft | $145k to $230k | $164k to $273k | About 14% less | $90k to $150k |
| 600 sq ft | $189k to $302k | $246k to $410k | About 25% less | $135k to $225k |
| 800 sq ft | $250k to $420k | $328k to $546k | About 23% less | $180k to $300k |
Source: ADU Wizard cost engine and California cost guide. Estimates.
The gap is real, roughly 14% to 25% on these numbers, and it is smallest on the smallest unit, because the fixed site costs are a bigger share of a small build. For the full site-built picture at each size, see what a 600 sq ft ADU costs and what an 800 sq ft ADU costs across every type and city we cover.
Well, sort of. Those percentages assume a lot that cooperates. On the lots where prefab struggles, below, I have seen the saving go to zero.
How Long Prefab Takes: Factory Time vs Permit Time
The pitch is speed, and the factory part is fast. The trouble is that the factory is rarely the slow part of an ADU. Here is how a prefab job usually runs on the projects I see, as planning ranges.
| Phase | Typical time | Runs in parallel? |
|---|---|---|
| Site visit, model selection, site plan | 2 to 6 weeks | No, everything waits on it |
| Permit review (California) | Up to 60 days after a complete application, plus correction rounds | Factory build can start once plans are set, if the company allows |
| Factory build | 4 to 16 weeks depending on method and backlog | Yes, with site prep and foundation |
| Site prep, foundation, utility trench | 4 to 8 weeks | Yes, with the factory build |
| Delivery and crane set | 1 to 2 days | No |
| Hookups, finish work, final inspection | 3 to 8 weeks | No |
The 60-day limit is Gov. Code 66317. Everything else is my planning range.
In California, ask any prefab company early how its units handle the Energy Code's solar rules; our California ADU solar requirements page covers them.
The real time saving is the overlap: the box gets built while your foundation goes in. The real time sink is everything before the factory starts. Cities with pre-approved plans shorten it a lot, and our pre-approved ADU plans guide covers how those programs work, and our ADU timeline by type and city puts prefab next to site-built units and every city permit timeline we track. If what you want is a small factory-built cottage, our backyard cottage guide prices it as an ADU and as a guest house.
When Prefab Saves Money, and When It Doesn't
Prefab wins when the lot is easy. These are the conditions where I have watched it stop winning.
- Tight backyards. A module needs a path or a lift. If the only route is a narrow side yard, you are paying for a bigger crane to fly it over the house.
- Slopes. Factory boxes want a flat, level foundation. Stepped footings and retaining walls are site work, priced like site work.
- Crane access. Overhead power lines, trees and street parking restrictions can mean a bigger crane, a street closure permit, or both.
- Delivery distance. Freight and escort costs scale with miles and with oversize-load rules. A distant factory can erase the saving before the unit arrives.
- Small units. The site costs barely shrink with the unit, so a 400 sq ft prefab saves the smallest percentage in the table above.
- A custom layout. The savings live in standard models. Move enough walls and you are paying factory prices for a custom build.
The flip side: on a flat lot with clear access, a short utility run and a standard model, prefab is the most predictable ADU I know how to build.
What I Check on the Lot Before a Prefab Company Visits
Most of the prefab jobs that go sideways were decided before anyone signed anything, on the walk from the street to the back of the lot. This is the walk I do, with a tape measure and a phone camera.
| Check | What I am looking for | Why it matters on a prefab job |
|---|---|---|
| The path in | Width of the clearest route from the street to the pad, including gates and side yards | Decides whether a module rolls in on a trailer, gets craned over the house, or does not fit |
| Overhead | Power and phone lines, tree canopy, roof eaves along the crane swing | Lines near the swing can mean a bigger crane, a different setup spot, or a utility shutoff |
| Crane setup spot | A flat, firm area on the street or driveway near the pad | A longer reach from the street needs a bigger crane, and some streets need a closure permit |
| Grade at the pad | Slope across the footprint and where water goes when it rains | Factory boxes want a level foundation; slope is site work |
| Utility distance | Feet from the panel, water and sewer lateral to the unit | Same trench a site-built unit needs, and the most common budget miss either way |
| Panel capacity | Spare room in the main electrical panel | An all-electric factory unit can still trigger a service upgrade |
If the path, the overhead and the crane spot all pass, prefab is usually the cheaper, faster route on your lot. If two of them fail, I would get a site-built bid before I paid any factory deposit. And if the company will not send someone to look at the lot before quoting, that tells you how much of the bill they expect you to discover later.
Prefab ADU Rules in California
California treats a prefab ADU as an ADU, full stop. The state's definition in Government Code 66313 expressly includes a manufactured home as defined in Health and Safety Code 18007, and a factory-built unit is held to the same size, setback and height rules as any other detached ADU. That means the 800 square foot unit a city must allow, four-foot side and rear setbacks, and a 16 foot height floor (18 near major transit). The full detail is on our California ADU permit guide.
What changes is who inspects the box. A unit certified under the state's factory-built housing program is inspected at the factory; your city inspects the foundation, the connections and the set. That split is why the site work still needs a licensed local contractor even when the unit does not.
New California Rules for Factory-Built ADUs in 2027
Three 2026 bills change the rules around factory-built units from January 1, 2027. None of them says "ADU" in its digest, so it helps to know which kind of prefab each one reaches. A modular or factory-built unit inspected under the state's Factory-Built Housing Law (Health and Safety Code 19960 and following) is "factory-built housing." A HUD-code manufactured home is a different category under the Manufactured Housing Act, and state law says a manufactured home is not factory-built housing (Health and Safety Code 18007(c)).
AB 1815: no stricter local building code
AB 1815 (Wicks, Chapter 782, Statutes of 2026) bars a city, county or district from imposing local building standards on a "factory-built housing structure" that exceed or differ from the California Building Standards Code. A city can still amend the code for local climate, geology or topography, including wildfire and flood risk, but only with an express finding filed with the Building Standards Commission (Health and Safety Code 19993.1). When it is unclear whether a project qualifies, a rebuttable presumption favors treating it as factory-built. Your city's zoning rules, setbacks and height still apply through ADU law.
AB 2058: who inspects the install, and what it costs
AB 2058 (Harabedian, Chapter 783, Statutes of 2026) lets the owner choose a state-regulated quality assurance agency, instead of the city, to inspect the installation of factory-built housing, once HCD adopts rules for it, due by January 1, 2029 (Health and Safety Code 19992.1). After that, a city's installation inspection fee is capped at 50 percent of the site-built equivalent unless it adopts a cost finding, and it charges no installation inspection fee at all if you use the state agency, though sitework like grading and foundations is still inspected locally. Separately, a city may charge a permitting fee on a factory-built housing structure above 50 percent of the site-built equivalent only with a written cost finding (Section 19992.2). No inspector may damage or take apart the unit to inspect it. Our ADU permit cost guide shows what those base fees look like by city.
SB 996: manufactured homes without a chassis
SB 996 (Padilla, Chapter 995, Statutes of 2026) redefines a "manufactured home" in Health and Safety Code 18007 to match federal law: a structure built on or after June 15, 1976, with or without a foundation system. HCD must adopt rules by July 1, 2027 so a manufactured home built without a permanent chassis is treated the same as one with a chassis. Because California's ADU definition borrows Section 18007's meaning of manufactured home, this widens which HUD-code units can count as an ADU once those rules are in place.
One bill did not make it. AB 2012, which would have changed how manufactured homes are transported, was vetoed on September 18, 2026, so delivery and transport rules are unchanged. All of these are tracked on our California ADU law tracker.
6 Questions to Ask a Prefab Company
1. Which license do you hold, and who holds the license for the work on my lot?
In California, a HUD-code manufactured home is installed by a contractor with CSLB's C-47 General Manufactured Housing license. A modular unit is certified at the factory, but the foundation, utilities and set are construction on your property and need a licensed contractor, usually a B General Building contractor. Look the number up on CSLB yourself.
2. Who pulls the permits, and who answers the plan checker?
Some companies run the whole permit; some hand you a set of drawings and wish you luck. Get it in writing, including who pays for correction rounds.
3. What exactly does the price include?
Ask for the quote split into the factory unit, delivery, the crane, the foundation, utility trenching and connections, permits and fees, site prep and on-site finish work. Any line marked "by owner" is a line you are paying someone else for.
4. What is the warranty, and who honors it?
There are usually two: the factory's warranty on the unit and the installer's on the site work. Find out who you call when a leak shows up at the seam between them.
5. How much is the deposit, and when is it refundable?
Factory slots are often reserved with a deposit before your permit is issued. Ask what happens to it if the city says no, or if your lot cannot take a crane.
6. What is your delivery radius, and what does distance cost?
Delivery is priced on distance, road access and escort requirements. A factory three states away can erase the savings before the box arrives.
Paying for a Prefab ADU
Prefab changes when the money is due. A factory may want a large deposit to hold a production slot and more before the unit ships, which is earlier than a site-built job would draw the same dollars. Ask any lender, before you sign with a factory, whether it will fund a deposit on a unit that is not yet on your lot. Our ADU loans comparison runs real budgets through a HELOC, a home equity loan, a cash-out refinance and a construction loan, and a HELOC's draw-as-you-go structure tends to fit a factory payment schedule best.
Frequently Asked Questions
How much does a prefab ADU cost?
In California, about $145,000 to $420,000 installed for 400 to 800 square feet, per the prefab ranges in our California cost guide. Roughly half of that is the factory unit; the rest is foundation, utilities, permits, site prep, delivery and the crane, which every quote should list separately.
Is a prefab ADU cheaper than building on site?
Usually, but by less than the marketing suggests. At 400 to 800 square feet, our published California prefab ranges come in about 14% to 25% under our cost engine's California site-built estimate, because the foundation, utilities and permits cost the same either way. On a tight or sloped lot the gap can close entirely.
What is the difference between prefab and modular?
Prefab is the umbrella term for anything built in a factory. Modular is one kind: whole three-dimensional boxes, finished inside, set by a crane. Panelized homes ship as flat panels, and manufactured homes are built to the federal HUD code on a steel chassis.
Can a manufactured home be an ADU in California?
Yes. California's ADU definition in Government Code 66313 expressly includes a manufactured home as defined in Health and Safety Code 18007. It still has to meet the same ADU rules on size, setbacks and height, and it sits on a foundation your city approves.
How long does a prefab ADU take?
Plan on six to twelve months from contract to move-in. The factory part is often the short part. Design, permits and site work before the set, and utility hookups after it, usually take longer than building the box.
Do prefab ADUs need permits?
Yes. The unit may be certified at the factory, but the city still permits and inspects the foundation, the utility connections and the set. In California the city has 60 days to act on a complete ADU application under Government Code 66317.
Do prefab ADUs cost less to permit in California in 2027?
Some will. From January 1, 2027, AB 2058 lets a city charge a permitting fee on a factory-built housing structure above 50 percent of the site-built equivalent only if it adopts a written cost finding. Its matching cap on installation inspection fees waits for HCD regulations, due by January 1, 2029. A HUD-code manufactured home is not factory-built housing, so these caps do not reach it.
Can my city add its own building rules to a factory-built ADU?
Mostly no, from January 1, 2027. AB 1815 bars a city from imposing local building standards on a factory-built housing structure that exceed or differ from the state building code, unless it finds a change necessary for local climate, geology or topography, such as wildfire or flood risk, and files that finding with the state. Zoning rules like setbacks and height still apply under ADU law.
If I were buying prefab today, I would price my lot before I priced a single model: utilities, access and the crane path. If those three are easy, prefab is a great way to build. If any one of them is hard, get a site-built detached ADU bid too and let the numbers decide.