Most ADUs take about 5 to 14 months from first planning to move-in, depending on the type. Plan on 1 to 3 months for feasibility and design, 1 to 3 months for permits, and 1 to 8 months of construction. Garage conversions and Junior ADUs are usually fastest; custom detached builds take longest.
These are typical ranges, not guarantees. Here’s the schedule phase by phase.
Phase 1: Feasibility and decisions (2 to 4 weeks)
Check your lot, get a contractor’s site visit, choose the ADU type, set a budget, and talk to a lender if you’re financing. Skipping this phase usually costs time later. See is my lot eligible for an ADU?
Phase 2: Design and plans (3 to 8 weeks)
- Stock or pre-approved plans: a few weeks to adapt to your lot
- Custom design: 4 to 8 weeks or more
- Prefab: site-specific foundation and utility plans while you wait for a factory slot
This phase includes structural engineering, the Title 24 energy report, and utility planning. It’s also when to confirm fire sprinkler requirements.
Phase 3: Permits (1 to 3 months)
State law sets two deadlines for the City of Santa Clarita and LA County:
- 15 business days to tell you whether your application is complete
- 60 days to approve or deny a complete application
Correction rounds add time, because your designer has to revise and resubmit. HOA review runs on its own timeline, so start it at the same time. See how long an ADU permit takes.
Phase 4: Construction (by ADU type)
| ADU type | Typical construction time |
|---|---|
| Junior ADU | about 1 to 3 months |
| Garage conversion | about 2 to 6 months |
| Prefab | about 1 to 2 months on site after the factory build |
| Attached ADU | about 3 to 5 months |
| Detached site-built | about 4 to 8 months |
Inspections happen at set stages, such as foundation, framing, rough utilities, insulation, and final. Each failed inspection adds days.
Phase 5: Final inspection and occupancy (1 to 2 weeks)
The final inspection, any punch-list fixes, and sign-off. Then the unit is ready to live in or rent.
How long does each ADU type take overall?
| ADU type | Typical total, start to finish |
|---|---|
| Junior ADU | about 5 to 8 months |
| Garage conversion | about 5 to 11 months |
| Prefab | about 6 to 12 months, including the factory build |
| Detached site-built | about 8 to 14 months |
What causes the biggest delays?
- Design changes after submittal. Freeze the design before you submit.
- Slow responses to corrections. Answer plan check comments within days.
- Utility surprises. Confirm panel size, sewer or septic, and water service early.
- Contractor availability. Line up your builder before the permit is issued.
- HOA review started late. Run it alongside the city or county review.
- Utility company scheduling. Service upgrades run on the utility’s schedule.
How can you speed it up?
Use stock or pre-approved plans where they fit, hire a designer and contractor who have permitted ADUs locally, answer every question within 48 hours, and have financing in place before construction starts.
Ready to start your timeline?
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.
ADU timeline FAQs
What’s the fastest type of ADU to build?
Usually a Junior ADU or a garage conversion, because they reuse existing structure.
How long does the city have to approve my ADU permit?
60 days after your application is complete. The city must also tell you within 15 business days whether your application is complete.
Is prefab faster than site-built?
It can be, when the factory build and site prep run at the same time. See the prefab ADU timeline.
Can I live in my house during construction?
Usually yes for a detached ADU. A Junior ADU or attached ADU means more disruption inside the home.