A prefab ADU typically takes about 4 to 7 months from signed contract to move-in, plus permitting time that should overlap the factory queue. The speed comes from running the factory build and your site prep at the same time. When they don’t overlap, prefab isn’t much faster than site-built.
Here’s the realistic schedule.
Phase 1: Pre-production and permits (about 4 to 8 weeks)
- Choose the model and options: floor plan, finishes, and upgrades
- Site-specific engineering: foundation and utility plans for your lot
- Permit submittal: the city or county has 15 business days to confirm completeness and 60 days to act on a complete application
- Factory slot: popular manufacturers have backlogs
Submit for permits as soon as your site plans are ready, ideally the same week you sign with the factory.
Phase 2: Factory build and site prep (about 8 to 16 weeks, in parallel)
In the factory: the unit is framed, wired, plumbed, insulated, finished, and inspected.
On your lot at the same time: grading, foundation or piers, utility trenching, and any retaining walls. If your site isn’t ready when the unit ships, you may pay storage or rescheduling fees, so coordinate closely.
Phase 3: Delivery and set (1 to 3 days)
A flatbed delivers the unit and a crane sets it on the foundation. You’ll need:
- Clear access for the truck and crane, measured in advance
- A confirmed crane date with a backup day for weather
- A foundation inspected and approved before delivery
High winds can stop crane work, which matters during Santa Ana wind season.
Phase 4: Finish work and inspections (about 4 to 8 weeks)
Utility connections, exterior steps or decks, final grading, interior touch-ups from transport, and the final inspections. Fire sprinkler connections happen here if they’re required.
What does the full schedule look like?
| Phase | Typical duration |
|---|---|
| Pre-production and permits | about 4 to 8 weeks |
| Factory build and site prep (parallel) | about 8 to 16 weeks |
| Delivery and set | 1 to 3 days |
| Finish work and inspections | about 4 to 8 weeks |
| Total | about 4 to 7 months |
Add time for design decisions before you sign and for any correction rounds in plan check.
Where do prefab timelines slip?
- Factory backlogs. Ask for current lead times in writing.
- Site prep not ready. The costly mistake.
- Permit corrections. Clean plans move faster.
- Weather on crane day.
- Utility service upgrades, which run on the utility’s schedule.
Is prefab really faster than site-built?
When the factory build and site work truly overlap, yes. A site-built detached ADU usually takes about 4 to 8 months of construction after permits. When a prefab project is treated as “order and wait,” the advantage shrinks. See prefab vs. site-built ADUs.
Want help planning a prefab project?
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 timeline FAQs
How long does the factory build take?
Often about 8 to 16 weeks once your unit enters production, depending on the manufacturer and backlog.
Can I start site work before the permit is issued?
Generally no. Grading, foundations, and utility work need permits. Plan your schedule around permit issuance.
What happens on delivery day?
The unit arrives on a truck, a crane lifts it onto the foundation, and crews secure it. The set itself often takes a day.
How much does a prefab ADU cost?
Often about $120,000 to $320,000 all-in in California, depending on size and site work. See prefab ADU cost.