A factory can be building your accessory dwelling unit (ADU) while the application to power it is still waiting in line. In PG&E territory, new electric service for an ADU averaged 303 days from application to energized, based on PG&E's own published data. A finished unit set on a finished foundation still can't be lived in without power.
That's the problem with asking whether prefab or custom is better. The build method changes how the walls get made. It doesn't change your lot, your utility, or your city's inspections. Those decide most of your schedule and a good share of your cost.
We design and build custom ADUs, so you should read this knowing that. But there are lots where a factory-built unit is the right choice, and we'll tell you which ones. There's also one question that matters more than speed or price: when a factory builds part of your ADU and a contractor builds the rest, who answers for the handoff between them?
What counts as "prefab" when you're building an ADU?
"Prefab" covers four different ways of building an ADU. They behave very differently on a real lot.
Modular
Whole room-sized sections are built and finished in a factory. They're trucked to your property and set on a foundation with a crane. Most of the building arrives done.
Panelized
Walls, floors, or roof sections are built in a factory and shipped flat. A crew assembles them on site. Much more of the work still happens in your yard.
Manufactured home
A complete home is built on a steel chassis under federal manufactured-housing standards. It goes on a permanent foundation at your property. This is a detached-only option.
Kit
A package of pre-cut or pre-made parts is delivered for a crew to assemble. In practice, most of the construction is still done on site.
You'll also see "preapproved plans" offered as a shortcut. A preapproved plan isn't a way of building. It's a design your city has already reviewed, and it can shorten plan review for a detached ADU. It can be built with a site-built crew or with panels. Our Q&A guide to California ADU rules covers the permit basics.
Does a factory-built ADU skip city permits and inspections?
No. A factory-built ADU moves part of the review to the state. Everything that happens on your property stays with your city or county.
For factory-built housing, the California Department of Housing and Community Development (HCD) certifies private agencies to do the factory side. A design approval agency reviews the unit's plans. A quality assurance agency inspects the unit while it's being built. Each approved unit carries an HCD insignia that tells your local building department the factory work was inspected.
Your local building department still reviews zoning and the site plan. It still inspects the foundation, the utility connections, and the installation of the unit. It still signs off on the final inspection before anyone can live there. A modular ADU goes through the same foundation inspection as a site-built one.
One detail surprises people. Under HCD's rules, changing a factory-built unit after it's built but before it's installed needs approval from HCD. Changing it after installation needs approval from your local building department. Moving a window or adding an outlet isn't a decision the crew can make on the spot.
Is a prefab ADU actually faster to finish?
Sometimes. A prefab ADU is faster only when the factory work and the site work happen at the same time and nothing outside both of them is slower.
A factory-built project runs on two tracks. One track is the factory: design approval, production, and delivery. The other is your property: permits, grading, foundation, trenching, and utility service. The project finishes when the slower track finishes.
The speed advantage is real when these are true:
- The design fits your lot without changes.
- The factory has an open production slot.
- A truck and crane can reach the spot where the unit will sit.
- Your utility service doesn't need a major upgrade.
It shrinks or disappears when any one of those fails. A redesign sends the plans back for review. A blocked crane position stops delivery. A long utility wait holds up move-in no matter how fast the walls went up.
A site-built ADU does more of its work one step after another, so its time in your yard is usually longer. But a straightforward site-built design on an easy lot can still finish ahead of a factory unit that's waiting on a production slot or a redesign.
How long does it take to get power to a new ADU?
In PG&E territory, completed ADU electric service projects averaged 303 days from application to power, with an average invoiced cost of $4,831.86. Those are PG&E's own published averages. They cover projects with applications submitted from January 2023 that were finished by August 2025. They leave out projects that needed a larger line extension, which take longer.
PG&E also breaks that time down by who was responsible for it. On average, PG&E accounted for 111 days, the customer side accounted for 114 days, and permitting agencies accounted for 28 days. PG&E notes that these periods overlap, so they don't add up to the total. The useful part is the middle number. The customer side, which means you and your builder, held the project about as long as the utility did. Slow responses, incomplete submittals, and site work that isn't ready all count against that side.
The Sacramento Municipal Utility District (SMUD) publishes a different measure. It reports only the last step. After your local agency approves the meter panel inspection, a SMUD crew typically installs the meter and turns on service within seven business days. SMUD doesn't publish a comparable start-to-finish average.
SMUD's process has one sequencing rule that matters for prefab. You need a plan check number from your local building department before you can apply to SMUD. So the utility track can't start until your plans are in your city's hands.
For a factory-built ADU, that means the power question should be settled before you choose a build method, not after. A fast factory doesn't help if the utility side has a long wait. Our guide to whether your property can support an ADU covers how to check your electrical panel and service before you get that far.
Is a prefab ADU cheaper than building custom?
That isn't proven for ADUs in Sacramento or the Bay Area. We haven't found any current independent data comparing the finished, all-in cost of factory-built and site-built ADUs here.
National research on modular construction comes mostly from apartment buildings and institutional projects, not backyard units. It finds that modular can lower costs in some conditions and raise them in others. Transport distance, crane work, factory volume, and how hard the site is all push the number one way or the other.
The bigger issue is what an advertised unit price includes. A factory price usually covers the building. To compare it fairly against a site-built quote, add everything else the project needs:
- Design and engineering for your specific lot
- Grading, drainage, and any demolition
- The foundation
- Trenching for water, sewer, and electrical lines
- Utility company work and fees
- Delivery and crane time
- Permits and city fees
- Energy compliance work, including solar where it's required
- Finish work after the unit is set
- A contingency for what turns up in the ground
Once both quotes cover the same list, you can compare them. Before then, the lower number may simply be the one with more left out.
Will a factory-built ADU physically fit onto your lot?
A modular ADU fits when a truck can get the sections to your street and a crane can lift them over everything between the street and the foundation. Both can be checked before you sign anything.
Look at these on your property:
- Street access. Narrow streets, tight turns, and parked-car-lined blocks limit what a delivery truck can do.
- Crane position. The crane needs a stable spot within reach of the foundation.
- Overhead lines. Power and phone lines between the street and the backyard can block a lift.
- Side yards and trees. A narrow side yard or a large tree can rule out the path a section has to travel. Our article on planning an ADU around a tree covers that decision.
- Slope. A steep lot adds foundation and access problems a standard unit wasn't designed around.
Panelized and kit systems ship as smaller pieces, so they handle tight sites better than full modules. Site-built construction handles the hardest sites best, because nothing has to arrive in one piece.
Fire hazard zones add another check. Lots in mapped fire hazard areas can require specific vents, siding, roofing, and windows. A factory unit that already has those assemblies can make the paperwork simpler. One that doesn't may need redesign, which costs back the time the factory was supposed to save.
Is a custom-built ADU higher quality than prefab?
The evidence doesn't show that either one is better. Comparisons of modular and site-built projects have found few real differences in materials and performance. Quality depends on the specific team and how carefully the work is done and checked.
What differs is where the risk sits. A factory unit is built indoors and inspected in the plant, which can make it more consistent. It also faces risks a site-built ADU doesn't have:
- Damage during transport or lifting
- Connections between sections that have to be sealed correctly on site
- Utility hookups made by a different crew than the one that built the unit
- Inspection split between a state-certified agency and your city
A site-built ADU has the opposite profile. Every stage happens in your yard, under your city's inspector, with more exposure to weather and to differences between crews.
Who's responsible when something goes wrong between the factory and your yard?
With a factory-built ADU, there are often two parties. The manufacturer builds the unit. A site contractor handles the foundation, utilities, setting, and finish work. Each may have its own contract and its own warranty.
That split is where problems can fall through. Before you sign, get written answers to these questions:
- Who pays to fix damage that happens during delivery or the crane lift?
- If a leak shows up at a connection, is that the factory's warranty or the site contractor's?
- Who coordinates the delivery date with the foundation, the inspections, and the utility work?
- If the unit doesn't fit the foundation as built, who corrects which part?
Then ask the questions that apply to any builder, including us. Recent years have shown why. More than one Sacramento-area ADU builder has stopped work with customers' projects unfinished and their payments gone. Those weren't factory-built projects. Failure risk doesn't follow the build method.
- What's the down payment? California limits a contractor's down payment on a home improvement job to $1,000 or 10% of the contract price, whichever is less.
- What is each payment tied to? Payments should follow finished, checkable work, not dates on a calendar.
- Who owns partly finished work? If a manufacturer has your money and the unit isn't delivered, know where that leaves you.
- Is the license active and bonded? You can check any California contractor's license and bond on the Contractors State License Board (CSLB) website.
Our setup is one contract. We draw the plans, process the permits, and build the ADU, so there isn't a handoff between a designer, a factory, and a site crew. Our labor warranty is one year, and manufacturers' warranties cover the products we install. Our license is CSLB #1058444.
Which approach fits your ADU project?
Match your ADU to the situation below that sounds most like your lot. Your lot usually decides more than the build method does.
A flat lot with easy access and a plan that fits as-is
Get a modular quote. This is where a factory-built ADU works best. If a standard model fits your lot without changes, a truck and crane can reach it, and your power service is straightforward, modular deserves a serious look next to any custom bid.
A narrow infill lot or overhead lines in the way
Look at panelized or site-built construction. Smaller pieces can get where full modules can't. On many Bay Area infill lots, the crane path decides this before anything else does.
A hillside lot or a fire hazard zone
Site-built construction usually fits best. The foundation, drainage, access, and fire-resistant construction shape these projects more than the walls do.
An attached ADU that has to match your house
Site-built or panelized construction usually fits best. Rooflines, floor heights, waterproofing at the connection, and fire separation from your home are all specific to your house.
A garage conversion or junior ADU
Prefab isn't really the decision. A conversion or a junior accessory dwelling unit (JADU) works inside a structure you already have, so it's a renovation. Our comparison of converting a garage versus building a backyard ADU covers that choice.
You need to move someone in soon
Settle the power question first. In PG&E territory especially, the utility timeline can outweigh any difference between build methods. Choose how to build after you know how long power will take.
What should you know before you ask anyone for an ADU price?
Five answers make any ADU quote, factory-built or custom, more honest:
- Access. Whether a truck and crane can reach your backyard, and what's overhead.
- Your electrical panel rating. It's printed on the panel.
- Your utility. PG&E, SMUD, or another provider, because each runs a different process.
- Whether a standard plan fits. A model that needs changes loses much of its factory advantage.
- How you'll use the unit. A rental, a parent, or a home office each points to a different layout.
If your lot matches the first situation above, price a modular unit alongside a custom one. If it matches any of the others, bring these answers to a design-build conversation. We start with feasibility and take the project from ADU design through permits and construction. The process works best when you know roughly what you want the unit to do and you're ready to let the lot tell you what it can hold.
