Most prefab ADUs in California cost about $120,000 to $320,000 all-in in 2026, and larger premium models can pass $400,000. The factory price covers the unit itself. Foundation, delivery, crane, utility hookups, and permits are extra, and on many lots they add $50,000 to $150,000 or more.
These ranges come from recent California prefab pricing data listed under Sources. They aren’t quotes. Your lot, access, and utilities decide where you land.
How much does a prefab ADU cost by size?
| Size | Typical all-in cost |
|---|---|
| Studio, 300 to 400 sq ft | $90,000 to $180,000 |
| 1 bedroom, 400 to 600 sq ft | $120,000 to $250,000 |
| 2 bedroom, 600 to 800 sq ft | $150,000 to $320,000 |
| Large or premium, 800 sq ft and up | $200,000 to $450,000+ |
Premium brands sit at the top of these ranges. One well-known manufacturer lists an 800 sq ft two-bedroom model at about $329,000 for the home, plus about $96,000 in typical installation, before separately quoted items.
What isn’t included in the factory price?
This is where most budgets go wrong. A factory quote usually covers the finished unit built to code. It usually does not cover:
- Foundation: about $8,000 to $40,000, depending on soil and slope
- Site prep and grading: about $3,000 to $30,000
- Utility connections: about $10,000 to $35,000 for water, sewer or septic, and power
- Delivery and crane: about $5,000 to $25,000
- Installation contractor: about $10,000 to $50,000
- Permits and fees: about $2,000 to $15,000
Ask every prefab company for a written list of exclusions before you sign. Then get a local site contractor to price those exclusions for your address.
Is prefab cheaper than a site-built ADU?
Sometimes. Prefab tends to win on flat lots with easy truck and crane access. It loses its edge on hillside lots, which describes a lot of Saugus and Canyon Country. Foundation and crane work on a slope can erase the factory savings.
For a full comparison of cost, timing, and resale, see prefab vs. site-built ADUs.
Do prefab ADUs need permits in Santa Clarita?
Yes. A prefab ADU gets the same review as any other ADU. The factory supplies engineered plans, but the City of Santa Clarita or LA County still reviews siting, setbacks, the foundation, and utilities.
State law sets the deadlines. The agency has 15 business days to say whether your application is complete, and 60 days to approve or deny a complete application. See how long an ADU permit takes in Santa Clarita.
Will a prefab ADU need fire sprinklers?
Only if the main house is required to have them. State law says an ADU can’t be required to have sprinklers when the primary home doesn’t. If your home is sprinklered, tell the manufacturer before your unit is built so it ships sprinkler-ready. See ADU fire sprinkler rules.
What should you check before buying a prefab ADU?
- Access. Can a flatbed truck and crane reach your backyard? Measure gates and look for overhead lines.
- Foundation. What does the manufacturer require, and what will it cost on your soil?
- Utilities. Who trenches and connects water, sewer, and power, and for how much?
- Warranty. What does the factory cover, and what does the local installer cover?
- Installer. Get the local site contractor’s quote in writing before you commit to the unit.
- Lead time. Ask for the current factory backlog in writing.
Ready to compare prefab and site-built for your lot?
Call (661) 486-8393. If no one answers, leave a voicemail with your name, phone number, address, and the type of ADU you want. You can also use the contact form. We’ll pass your details to a local ADU contractor, who will follow up with you directly. There’s no obligation to hire.
Prefab ADU cost FAQs
Why do prefab ADU prices online look so low?
Most advertised prices cover only the factory unit. Foundation, delivery, crane, utility connections, installation, and permits often add $50,000 to $150,000 or more.
How long does a prefab ADU take?
A realistic schedule is about 4 to 7 months from signed contract to move-in, plus permitting. The factory build and your site prep should run at the same time. See the prefab ADU timeline.
Can I finance a prefab ADU?
Yes. Homeowners use the same options as for any ADU, such as a HELOC, home equity loan, or construction loan. Some lenders handle factory-built units differently, so ask early. See how to finance an ADU.
Does a prefab ADU appraise differently?
A permitted, code-compliant ADU adds value whether it was built in a factory or on site. Appraisers look at permitted square footage, quality, and comparable sales.