ADU comparisons
Prefab vs stick-built ADU: time, code and site access
By the HyreADU research desk. Last checked .
The short answer
Prefab vs stick-built ADU is a choice about where the building is made. A prefab unit is built in a factory and set on a foundation your site crew prepares.
A stick-built ADU is framed on your lot. Prefab can shorten time on site, but not permit review, and it needs truck or crane access. Stick-built fits odd lots and custom plans.
| Factor | Prefab (factory-built) | Stick-built (site-built) |
|---|---|---|
| Where it is built | Factory, then delivered | On your lot |
| Approvals in California | Factory work under HCD’s factory-built program or the HUD Code; local permit for site, foundation and hookups | Local building permit for the whole job |
| Time on site | Often shorter | Longer, weather-dependent |
| Site access | Truck or crane must reach the pad | Materials carried in |
| Design freedom | Limited to the catalog and its options | Any plan that meets code |
| Still on your bill | Foundation, utilities, delivery, set, finish work | Everything |
When prefab wins
Prefab wins on an open, flat lot with a clear path for a truck or crane, when a catalog layout fits your needs.
Factory work is not slowed by rain on your site, and inspections of the factory-built parts happen at the plant.
In California, factory-built housing is regulated by HCD under Health & Safety Code § 19971 and following.
When stick-built wins
Stick-built wins on a sloped lot, a lot reached only by a narrow side yard, or when you want a layout no catalog offers. It is also the route for additions, conversions and attached ADUs, which a factory box rarely suits. Read custom ADU builders for who does that work.
The part prefab does not remove
Prefab does not remove permit review, the foundation, the trench to the sewer or the electrical service. Those site lines are where budgets most often move, so ask every supplier which of them are in their price. Compare the lines in the ADU bid comparison, and see the prefab ADU definition.
A mistake to avoid
The mistake to avoid is ordering a prefab unit before the permit is issued. Factory slots, delivery dates and crane bookings are hard to move, and a permit delay can leave a finished unit waiting with storage charges. Line up the permit first, then lock the factory schedule.
What to do next: check your lot with the ADU feasibility checker, read the questions to ask an ADU builder, then tell us about your project. Matching is not live yet, so we reply with information, not a quote.
Get ADU optionsQuestions
Is prefab cheaper than stick-built?
Not always. The unit price leaves out site work, delivery, set and hookups. Compare complete bids.
Does prefab skip the permit?
No. Your city still permits the foundation, utilities and installation.
What is the difference between prefab and modular?
Modular is one kind of prefab: whole boxes built in a factory. See panelized vs modular.
Can prefab units go on a slope?
Sometimes, on a raised foundation, if a crane can reach. Site work rises with the slope.
Sources
- California HCD, Factory-Built Housing program. Checked September 5, 2026.
- Health & Safety Code § 19971 (factory-built housing). Checked September 5, 2026.
- 24 CFR § 3280.2 (HUD manufactured home definitions). Checked September 5, 2026.
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