Estimate 2026 detached Accessory Dwelling Unit (ADU) construction costs by square footage, bedroom layout, city/state tier (CA, WA, OR, TX, FL), utility trenching, and projected monthly rental income.
In 2026, building a detached ADU (backyard cottage) costs $240 to $420 per square foot in the US. A 600-square-foot 1-bedroom detached ADU averages $156,000 to $252,000 turnkey including architectural plans, permits, utility trenching, full kitchen, and bathroom. Prefab modular ADUs start around $195–$275/sq ft installed.
📐 Engineering Formula Used: Total ADU Cost = (Sq Ft × Build Rate × Metro Multiplier) + Utility Trenching + Plans/Permits + Solar
The table below provides verified 2026 material, labor, and installed turnkey cost ranges for detached ADU cost calculator projects. Compare these figures alongside live industry benchmarks such as ADUZoning.org & ADUVerified.com ↗.
| ADU Size & Layout | Square Footage | 2026 Build Cost (US Avg) | California / High-Cost Metro | Est. Monthly Rent (ROI) |
|---|---|---|---|---|
| Studio Backyard ADU | 350 – 450 sq ft | $105,000 – $145,000 | $135,000 – $185,000 | $1,500 – $2,300/mo |
| 1-Bedroom Detached ADU | 500 – 650 sq ft | $150,000 – $210,000 | $190,000 – $265,000 | $1,900 – $2,900/mo |
| 2-Bedroom Family ADU | 750 – 1,000 sq ft | $215,000 – $310,000 | $270,000 – $385,000 | $2,500 – $3,800/mo |
| 1,200 Sq Ft Max-Size ADU | 1,200 sq ft | $310,000 – $410,000 | $380,000 – $495,000 | $3,200 – $4,600/mo |
Project Scenario: 600 Sq Ft 1-Bedroom Site-Built Detached ADU (National Average Metro)
| Itemized Quote Line Item | 2026 Estimated Cost |
|---|---|
| 600 sq ft Site-Built Construction — Foundation, Framing, Kitchen, Bath, Mini-Split (@ $285) | $171,000 |
| Sewer Tie-In, Water Line & 100A Electrical Sub-Panel Trenching (50 ft run) | $14,500 |
| Pre-Approved ADU Plans, Structural Engineering & City Permit Fees | $11,500 |
| Estimated Total Turnkey Detached ADU Investment | $197,000 ($328.33/sq ft) |
An ADU packs the two most expensive rooms in residential construction—a full kitchen ($22,000–$35,000) and a full bathroom ($15,000–$25,000)—plus its own HVAC system, water heater, electrical sub-panel, and sewer lateral into just 400 to 800 square feet. Expanding an ADU from 500 sq ft to 750 sq ft adds cheap bedroom/living floor space and actually lowers the per-square-foot rate.
Cities across California, Washington, Oregon, and Texas now offer pre-approved standard ADU plan libraries that cut architectural fees by $6,000+ and reduce permit approval timelines from 4 months down to 15–30 days. Additionally, detached ADUs under 750 sq ft are exempt from municipal impact fees in California.
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