ADU Construction Timeline
How long does it take to build an ADU? Most projects take 10–18 months from planning to move-in. Here's every phase, city-level permitting timelines, and how to avoid costly delays.
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ADUs: –
Permits: – · Updated April 2026
ADU Timeline at a Glance
Every ADU project moves through six phases. Permitting and construction are the longest — and the most variable. Here's how a typical 12–14 month project breaks down.
Planning
2–4 weeks
Design
6–16 weeks
Permitting
8–36 weeks
Site Prep
2–4 weeks
Construction
16–40 weeks
Inspection
2–4 weeks
Phase 1: Planning & Feasibility
Before you hire a designer or break ground, you need to know what's possible on your lot. This phase is about answering fundamental questions: Can you build an ADU? How big? What type? What will it cost?
What Happens in This Phase
- Zoning check: Verify your lot allows ADUs. Check setbacks, height limits, lot coverage, and parking requirements with your city planning department.
- Site assessment: Walk your property. Identify utility connections, slope, access, trees, and any easements that could affect placement.
- Budget range: Establish a rough budget. Use our ADU Cost Estimator to get a personalized breakdown.
- ADU type decision: Decide between detached new construction, garage conversion, attached addition, JADU, or prefab. Each has different timelines and costs.
- Financing plan: Determine how you'll pay — savings, construction loan, HELOC, or DSCR loan.
Speed tip: Many cities offer free pre-application consultations. Schedule one early — a 30-minute meeting can save months of wrong-direction design work.
Phase 2: Design & Architecture
The design phase turns your vision into permit-ready construction documents. This is where costs, layout, and aesthetics get locked in.
| Design Step | Duration | What Happens |
|---|---|---|
| Schematic Design | 2–4 weeks | Floor plans, site layout, initial elevations, and concept review |
| Design Development | 2–4 weeks | Material selections, structural approach, MEP layout, energy compliance |
| Structural Engineering | 2–4 weeks | Structural calculations, foundation design, seismic/wind compliance |
| Construction Documents | 2–4 weeks | Final permit-ready drawings with all details, specs, and Title 24 calcs |
Pre-approved plans can cut this to 1–2 weeks. California's AB 1332 requires cities to offer pre-approved ADU plan programs. These plans skip most of the design phase and get faster permitting — typically 30–60 days total. Learn about ADU permit fees →
Design fees typically run $4,000–$14,000 for standard plans or $15,000–$30,000 for fully custom designs. This is a soft cost — part of the 15–20% of your budget that goes to non-construction expenses.
Phase 3: Permitting
Permitting is the most unpredictable phase and the most common source of delays. Your timeline depends heavily on which city you're in, whether you're using pre-approved or custom plans, and how complete your application is on first submission.
Permitting Steps
Application Submission
Submit plans, engineering, Title 24 energy calcs, and fees. Many cities accept online submissions.
Completeness Check (1–3 weeks)
City confirms your application is complete. Under SB 543, California cities must do this within 15 business days.
Plan Review (4–12 weeks)
Zoning, building, fire, and public works departments review your plans. This is where most delays happen.
Revisions (0–3 cycles, 2–4 weeks each)
Address plan check comments and resubmit. Each cycle adds 2–4 weeks. Respond within 48 hours to maintain priority.
Permit Issuance (1–2 weeks)
Pay remaining fees, pick up the permit, and you're cleared to build.
California's 60-Day "Shot Clock"
California law requires cities to approve or deny complete ADU applications within 60 days. SB 543 (effective October 2025) tightens this further — cities must confirm completeness within 15 business days and cannot introduce new requirements after resubmission. If your city takes more than 30 business days, AB 253 allows you to hire a private plan checker.
Permit Timelines by City
Actual timelines vary based on project complexity, application completeness, and city staffing levels.
| City | Plan Review | Total Permit Time | Notes |
|---|---|---|---|
| Los Angeles (pre-approved) | 2–3 weeks | 30–60 days | AB 1332 pre-approved plans |
| Los Angeles (custom) | 4–6 weeks | 90–180 days | 2–3 revision cycles typical |
| San Diego | 6–12 weeks | 3–6 months | Varies by complexity |
| San Jose | 4–8 weeks | 2–4 months | Express track available |
| Portland | 6–10 weeks | 3–5 months | Design review may apply |
| Seattle | 4–8 weeks | 2–4 months | Pre-approved plans faster |
| Denver | 4–6 weeks | 2–3 months | Relatively fast approvals |
| Austin | 6–10 weeks | 3–5 months | Expedited review available |
| Sacramento | 4–6 weeks | 2–3 months | Streamlined ADU process |
| Miami-Dade | 6–10 weeks | 3–5 months | Hurricane code compliance |
For detailed city-specific permit fees, see our Permit Fee Database.
Phase 4: Site Preparation
Once the permit is in hand, the site needs to be prepared before vertical construction can begin. This phase often overlaps with contractor mobilization.
Site Work
- •Clear vegetation, debris, and old structures (1–3 days)
- •Grading and leveling (2–5 days)
- •Excavation for foundation (2–5 days)
- •Soil testing if required (1–2 weeks for results)
Foundation
- •Form and pour concrete foundation (3–5 days)
- •Concrete curing time (7–14 days)
- •Foundation inspection (schedule 1–3 days ahead)
- •Utility rough-in below slab if applicable
Speed tip: Schedule your general contractor before the permit is issued. Most GCs book 4–8 weeks out. Starting contractor selection during permitting eliminates the gap between permit and construction start.
Phase 5: Construction
This is the longest and most visible phase. Construction follows a specific sequence — each trade depends on the one before it. Missing one inspection or one delayed subcontractor can push the whole schedule.
Framing & Exterior (4–8 weeks)
Wall framing, roof structure, sheathing, windows, and doors. Your ADU takes shape. Framing inspection required before closing walls.
Rough Mechanical (3–6 weeks)
Plumbing rough-in, electrical wiring, HVAC ductwork, and gas lines. Each system gets a separate inspection before walls close.
Insulation & Drywall (2–4 weeks)
Insulation installation (must pass energy inspection), drywall hanging, taping, and finishing. Texturing and priming follow.
Interior Finishes (4–6 weeks)
Cabinets, countertops, flooring, tile, paint, trim, fixtures, and appliances. This is where material lead times matter most.
Exterior & Landscaping (2–3 weeks)
Siding, roofing finish, exterior paint, walkways, fencing, and landscaping to meet city requirements.
Utility Connections (1–4 weeks)
Final water, sewer, electric, and gas connections. Utility companies set their own schedules — plan for utility hookup costs and lead times early.
Budget tracking matters. Construction is where cost overruns happen. Track your hard and soft costs weekly to catch surprises before they compound.
Phase 6: Final Inspection & Certificate of Occupancy
The city inspector does a final walkthrough to confirm the ADU was built according to approved plans and meets all building codes. Once approved, you receive a Certificate of Occupancy (CO) — your ADU is legally ready for use.
Final Inspection Checklist
If you fail inspection: You'll receive a correction list. Fix the issues and schedule a re-inspection (typically 1–2 weeks). Common failures: missing smoke detectors, improper drainage, electrical grounding issues, and HVAC venting errors.
Timeline by ADU Type
Your ADU type dramatically affects the total timeline. Prefab units and garage conversions are the fastest. Custom detached builds take the longest.
| Custom Build | Prefab / Modular | Garage Conversion | |
|---|---|---|---|
| Design Time | 2–4 months | 1–2 weeks | 2–6 weeks |
| Permitting | 3–9 months | 1–3 months | 2–4 months |
| Construction | 5–10 months | 1–3 months | 2–5 months |
| Total Timeline | 12–18 months | 4–8 months | 6–12 months |
| Typical Cost | $150K–$400K+ | $80K–$250K | $50K–$200K |
Considering prefab? Compare top prefab ADU companies side-by-side in our Prefab ADU Comparison Tool.
Common Delays & How to Avoid Them
Every delay costs money — carrying costs on your loan, extended rental periods, and contractor re-mobilization fees. Here are the most common setbacks and how to prevent them.
How to avoid: Hire an experienced designer who knows local requirements. Submit complete packages the first time.
How to avoid: Expect 1–3 revision rounds. Respond to comments within 48 hours to maintain priority.
How to avoid: Contact utility providers early in the design phase. Schedule hookups before construction ends.
How to avoid: Avoid starting foundation work during rainy season. Build schedule buffers for climate.
How to avoid: Order long-lead items (windows, HVAC, cabinets) early. Have backup suppliers identified.
How to avoid: Use experienced contractors familiar with local codes. Schedule pre-inspection walkthroughs.
How to avoid: Lock in your GC and subs before permits are issued. Maintain a realistic sequencing plan.
How to avoid: Research ADU state preemption laws. In California, HOAs cannot prohibit ADUs (AB 1033).
How to Speed Up Your ADU Project
During Design & Permitting
- Use pre-approved ADU plans (saves 2–4 months)
- Hire a designer with local experience in your jurisdiction
- Submit complete applications — incomplete ones get sent back
- Respond to plan check corrections within 48 hours
- Use expedited review if your city offers it
During Construction
- Lock in your GC during permitting — don't wait
- Order long-lead materials early (windows, cabinets, HVAC)
- Schedule inspections proactively — don't wait for callbacks
- Make all finish selections before construction starts
- Contact utility companies early for hookup scheduling
Frequently Asked Questions
How long does it take to build an ADU from start to finish?
Most ADU projects take 10–18 months from initial planning to move-in. The biggest variable is permitting, which can range from 2 months (pre-approved plans in a fast city) to 9+ months (custom plans with multiple revision cycles). Construction itself typically takes 4–10 months depending on size and complexity.
Can I build an ADU in 6 months?
It's possible with a prefab or modular ADU in a city with fast permitting. If you use pre-approved plans and the city approves within 60 days, plus a prefab unit that installs in days, you could complete the project in 4–6 months. Custom-built ADUs almost never finish in 6 months.
What is the longest part of building an ADU?
Permitting and construction are typically tied as the longest phases. In cities with slow plan review, permitting can take 6–9 months. The construction phase runs 4–10 months for custom builds. Combined, these two phases account for 80–90% of your total project timeline.
Does a garage conversion take less time than new construction?
Yes. Garage conversions typically take 6–12 months total vs. 12–18 months for new detached construction. The existing structure reduces both design complexity and construction time. Permitting is also usually faster because the footprint already exists.
How can I track construction progress on my ADU?
Keep a weekly progress log with photos, maintain a budget tracker for costs vs. estimates, and get regular updates from your general contractor. Tools like TerraLine help owner-builders and developers track loan draws, budgets, and construction milestones in one dashboard.
Your Next Steps
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