Quick Answer: Building an ADU in San Jose takes 8–14 months total: design/planning (1–2 months), permit approval (4–8 weeks for standard ADUs, 2–4 weeks for pre-approved plans), construction (3–6 months for detached ADU, 2–4 months for garage conversion). Pre-approved plans significantly shorten the timeline.
ADU Construction Timeline in San Jose 2025
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One of the most common questions we get from San Jose homeowners considering an ADU is: “How long will this take?” The answer depends on the ADU type, your lot’s permit complexity, and which path you take through the permitting process. Here’s a detailed timeline breakdown.
Phase 1: Design and Planning (4–8 Weeks)
- Week 1–2: Site assessment, zoning review, utility locates, feasibility analysis
- Week 2–4: Architectural design, floor plans, elevation drawings
- Week 4–6: Structural engineering (if required), Title 24 energy calculations
- Week 6–8: Plan finalization, permit application preparation
Time-saving tip: Use San Jose’s Pre-Approved ADU Plans Program — choose from 14 pre-designed ADU plans (400–1,200 sq ft). This can cut design time from 6–8 weeks to 2–3 weeks, and dramatically shortens the permit review period.
Phase 2: Permit Approval (4–12 Weeks)
- Standard ADU (custom plans): 6–12 weeks for plan check + corrections
- Pre-approved ADU plans: 2–4 weeks (San Jose must approve within 30 days by state law)
- Streamlined ADU (conversions under 850 sq ft): 3–5 weeks
California law (SB 13) requires all ADU applications to be approved or denied within 60 days of a complete application submission. San Jose consistently meets this timeline.
Phase 3: Construction (2–6 Months)
| ADU Type | Construction Timeline |
|---|---|
| Garage conversion (attached) | 6–10 weeks |
| Garage conversion (detached) | 8–14 weeks |
| Junior ADU (JADU, within home) | 6–10 weeks |
| Attached ADU (addition) | 10–16 weeks |
| Detached ADU (400–600 sq ft) | 12–18 weeks |
| Detached ADU (800–1,200 sq ft) | 16–24 weeks |
| Two-story detached ADU | 20–30 weeks |
Total ADU Timeline: Fast vs. Standard Track
- Fast track (pre-approved plans + garage conversion): 4–6 months
- Standard track (custom plans + detached ADU): 10–14 months
- Complex track (2-story, hillside lot, custom): 14–20 months
Quartz Construction manages the complete ADU process in San Jose — design, permits, construction, and inspections. We keep projects on schedule through proactive permit management and pre-ordering long-lead materials. CSLB #1003436 | BBB A+ | 4.9⭐. Call (650) 749-7436.
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