The ADU construction process, explained in six phases.

How long does building an ADU take, really?

The honest, sourced answer is 10–18 months from your first serious conversation to a certificate of occupancy. The two California-focused builders publishing the most detailed recent timeline data — SnapADU in San Diego and ACI in the Bay Area Peninsula — both land on roughly the same ranges. Design takes 2–5 months, permitting takes 3–9 months (the widest variance), and construction takes 6–10 months. Most projects cluster around 12 months in California overall, slightly longer outside California where state-level ADU preemption laws don't compress the permit phase.

The single biggest timeline lever is your jurisdiction. Cities with pre-approved ADU plan catalogs — Los Angeles offers 20+ free standard plans through LADBS — can shorten permitting to 50–70 days. San Jose currently runs the fastest permit cycle in California at 35–55 days actual. San Francisco's process, governed by DBI and Planning together, takes longer and costs more — permit fees alone can run 6–9% of construction cost. Outside California, jurisdictions vary widely; expect a 60–90 day baseline in ADU-friendly states like Oregon and Washington and longer elsewhere.

The construction phase tracks closely with ADU type. ACI's 2026 data — consistent with what SnapADU reports for the San Diego market — gives the following construction-phase ranges:

• Garage conversion: 12–20 weeks
• Attached ADU: 16–28 weeks
• Detached on a standard lot: 18–32 weeks
• Hillside or complex foundation: 28–40+ weeks

What an ADU project actually costs in 2026

Across the four cost surveys we re-validated this quarter — Dynamic Quality Builders (Jan 2026), Angi (Mar 2026), Kellow Construction (Feb 2026), and Better Place Design Build citing Terner Center research — the ranges that emerge for California are consistent. Outside California, costs run 20–35% lower depending on the state's construction-cost index.

• Garage conversion: $80,000–$175,000 in California ($100,000–$175,000 in Southern California specifically). Nationally, Angi reports an average of $110,000 with a $60,000–$150,000 typical range. California averages $160,000 vs $102,000 in Texas, $113,000 in Florida.
• Junior ADU (JADU): $50,000–$100,000. Lowest cost because the unit is carved out of existing home space.
• Attached ADU: $150,000–$300,000 statewide; $200,000–$300,000 in SoCal for 500–800 sq ft.
• Detached ADU: $200,000–$400,000+ in California. The Terner Center research cited by Better Place Design Build puts the statewide median at $150,000 or $250 per sq ft — but that median includes smaller and simpler units; mid-finish 800 sq ft detached units typically run $250,000–$350,000.
• Prefab/modular: $200,000–$400,000 for fully installed units. Prefab tends to compress on-site construction to 3–4 months, but site work, foundation, and utilities still run on the normal timeline.

The three decisions that move the most money

Two ADUs of the same square footage can differ by 40–50% in total cost based on three decisions, almost always made in the first month of planning:

1. ADU type. A garage conversion and a detached ADU of the same size differ by 30–40% (DQB 2026). Across California: garage conversion runs $250–$350/sqft, detached runs $350–$500+/sqft.
2. Site complexity. A flat lot with easy access versus a sloped lot with poor access or unusual soil can swing site-prep costs by $20,000–$80,000 (DQB 2026; ACI 2026 notes hillside ADUs extend construction by 12+ weeks beyond standard).
3. Finish level. The same shell with high-end versus builder-grade finishes can move all-in cost by 15–25%. A premium kitchen and bath alone add $40,000–$80,000 to a detached ADU.

Soft costs and fees most homeowners under-budget

Total California permit costs land in a wide range — $12,000 to $77,000 depending on your city, ADU size, and utility connection needs (Andalusia Drafting, 2026 guide). The structure of these costs:

• Direct permit fees: $2,000–$27,000. Inland Empire cities sit at the low end ($12,000–$22,000 all-in); SF DBI sits at the high end ($33,000–$57,000 all-in).
• Professional soft costs: $9,000–$30,000 covers architectural plans, structural engineering, Title 24 energy compliance (California-specific, $1,500–$3,500), soils report ($1,500–$5,000), and survey ($1,200–$3,500).
• Utility connection costs: $0–$35,000. Existing utility runs from main house: $10,000–$40,000+ depending on distance (DQB 2026). Electrical panel upgrade: $2,500–$6,000.
• Impact fees: ADUs under 750 sq ft are partially or fully exempt under SB 13 and follow-on legislation. Above 750 sq ft, expect proportional impact fees — sometimes $10,000–$25,000 in California metros.

Beyond permits, plan for a 10–15% contingency on total project cost (Kellow Construction 2026). 10% is the minimum; 15% is realistic for older homes, hillside lots, or extensive utility runs.

How much you can expect to recoup

The most consistent figure across builder and appraiser reporting is that a permitted ADU adds 50–70% of build cost to appraised property value in the short term, and 80–100% once rental income is documented (DQB 2026; consistent with Fannie Mae's 2024 underwriting guidance that lenders may factor projected ADU rental income into loan qualification). In California rental markets, this can mean meaningful cash flow on top of property appreciation. Run your specific numbers in our ROI calculator.

Ready to start? Begin with Phase 1.

The next page, Phase 1 — Planning, walks you through how to confirm feasibility, set a realistic budget, and decide what kind of ADU best fits your goals.