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Prefab Versus Site-Built ADUs
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A factory-built ADU and a site-built ADU both have to meet California ADU law and your local code, and both still need site work such as foundation, utility connections, and permits. The HCD ADU Handbook (March 2026) notes that a so-called tiny home is subject to ADU law if it meets the definition of an ADU.
How The Process Differs
| Step | Factory-Built | Site-Built |
|---|---|---|
| Where the main work happens | In a factory, then delivered | On your lot |
| Foundation and utilities | Still done on site, often while the unit is made | On site, in sequence with the build |
| Delivery | Needs truck and crane access to the yard | Needs access for materials and crews |
| Design flexibility | Often a catalog of floor plans with options | Fully custom |
| Weather exposure | Less during construction | More during construction |
We are not quoting cost or schedule differences. Published comparisons come mostly from companies that sell one approach, and the totals depend on site work, which is the same for both.
Costs That Do Not Go Away
- Site survey, permits, and city fees
- Foundation and grading
- Sewer, water, electric, and gas connections
- Delivery, crane, and set fees for factory units
- Finish work after the unit is placed
Ask any factory builder whether their price includes site work and installation, or only the unit.
Permits And Rules
An ADU built from a factory-made unit still has to meet California ADU law and your local requirements. The HCD ADU Handbook (March 2026) notes that a so-called tiny home is subject to ADU law if it meets the definition of an ADU, and points to HCD Information Bulletin 2016-01 on tiny homes for more on tiny homes. Many cities also publish pre-approved plans. See Permits And Timeline.
Questions To Ask A Factory Builder
- Is the price for the unit only, or delivered and installed on my lot?
- Who handles permits, utility hookups, and the foundation?
- What happens if my site needs unplanned work such as retaining walls?
- What does the warranty cover, and who services it?
- How are deposits held while my unit is in production?
Some factory and prefab ADU companies have closed with projects unfinished. If that happened to you, see Your ADU Company Closed. What Now?.
Questions To Ask A Site-Built Contractor
- Is the price fixed, or cost plus?
- Who pays when hidden problems turn up in a conversion or in the soil?
- What is the payment schedule, and how does it track completed work?
Check any contractor on the Contractors State License Board license lookup. More in Questions To Ask An ADU Builder.
Related guides: Detached ADU, ADU Cost In California.
Common Questions
Is a prefab ADU cheaper than a site-built one?
We do not quote a difference. Published comparisons come mostly from companies that sell one approach, and totals depend on site work, which is the same for both.
What costs stay the same for a prefab ADU?
Site survey, permits and city fees, foundation and grading, and sewer, water, electric, and gas connections. Factory units add delivery, crane, and set fees.
Does a prefab ADU skip the permit process?
No. A unit built from a factory-made unit still has to meet California ADU law and your local requirements.
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