ADU Cost Calculator

Estimate accessory dwelling unit cost for garage conversion, detached build, or basement conversion.
ADU Type
sqft
$

Estimated Cost

$120,000.00-$210,000.00

Cost per Sqft

$200-$350

Annual ROI

10.9%

Project Details

Construction cost$165,000.00
Permit estimate (5%)$8,250.00
Total project$173,250.00
Timeline6-12 months
Annual rental income$18,000.00

Use the ADU Cost Calculator above to calculate your results. Enter your values and see instant results — all calculations run in your browser.

Disclaimer: This calculator is for informational purposes only and does not constitute tax, financial, or legal advice. Results are estimates based on the information you provide and current rates. Always consult a qualified tax professional or financial advisor for advice specific to your situation.

How It Works

Our ADU Cost Calculator provides a comprehensive estimate for your accessory dwelling unit project, whether it's a garage conversion, a new detached build, or a basement conversion. Understanding these costs upfront is crucial for budgeting and securing financing, especially as construction material costs are projected to stabilize in 2026 after a period of volatility. This tool helps you avoid unexpected expenses and make informed decisions about your ADU investment.

This calculator utilizes a multi-factor methodology based on average 2026 construction costs per square foot, adjusted for regional variations and project complexity. We factor in base construction, major systems (plumbing, electrical, HVAC), finishes, and soft costs like permits and architectural design, using a weighted average for different ADU types. Our formula accounts for a 5-15% contingency for unforeseen issues, which is critical for accurate budgeting.

When estimating, remember that permits can vary significantly by municipality, so always verify local fees. A common mistake is underestimating the cost of site work for detached ADUs, which can involve extensive excavation and utility trenching. Also, be wary of low-ball bids that don't include all necessary finishes or system upgrades.

Example: Detached 600 sq ft ADU in California

  1. 1 Type of ADU: Detached New Construction. Size: 600 sq ft. Location: California (High Cost). Level of Finish: Mid-Range.
  2. 2 Base Construction Cost: $350/sq ft (CA average for 2026) * 600 sq ft = $210,000. Utility Connections: $25,000. Architectural & Engineering Fees: $18,000. Permits & Impact Fees: $12,000. Contingency (10%): $26,500.
  3. 3 Estimated Total ADU Cost: $291,500.
  4. 4 This estimate reflects a 2026 mid-range detached build in a high-cost area. Expect variations based on specific material choices, site conditions, and local permitting requirements. This figure allows for a comfortable, functional ADU with standard amenities.

Source: HomeAdvisor · Last updated: April 2026

Frequently Asked Questions

How much does it cost to build an ADU?
A garage conversion ADU costs $50,000-$120,000, a basement conversion runs $60,000-$150,000, and a detached new-build ADU costs $150,000-$350,000+. Costs vary significantly by location and finishes.
Do ADUs increase property value?
ADUs typically increase property value by 20-30% of their construction cost. In high-demand rental markets, the rental income can provide a 5-10% annual return on investment.
Do I need a permit to build an ADU?
Yes, virtually all jurisdictions require permits for ADUs. Many states including California, Oregon, and Washington have passed laws making ADU permitting easier. Check your local zoning code for setback and size requirements.

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