ADU cost calculator
Estimate the all-in cost to build an accessory dwelling unit. Pick the type, floor area, your state, and finish level — results are ranges drawn from published 2026 cost guides and scaled by your state's regional cost index. This is a planning anchor, not a bid.
Estimates use national detached new build ranges of $150–$400/sq ft (Angi & HomeGuide, 2026) scaled by California's regional cost index (×1.31). Finish level adjusts ×1. All figures are planning ranges — verify with a local contractor and your city building department.
Estimated build cost
600 sq ft × $196 – $523/sq ft = $117,753 – $314,009
Site work, foundation & structure (30%)
$35,326 – $94,203
Plumbing & electrical (22%)
$25,906 – $69,082
HVAC / mechanical (13%)
$15,308 – $40,821
Interior finishes (35%)
$41,214 – $109,903
Permits & plan-check
$1,500 – $9,000
All-in total (build + permit)
$119,253 – $323,009
Line-item percentages are planning allocations of the build cost, not a contractor bid. Permit range from Angi (2026); actual fees are set by your city/county.
Next steps. Use this estimate as a planning anchor, then: (1) confirm your city allows the ADU type and size on your lot via the California ADU rules page, and (2) model the rental return with the ADU rental ROI calculator.