600 Sq Ft ADU Cost in 2026
A 600 square foot ADU typically costs between $175,000 and $280,000 to build in 2026, or roughly $290–$470 per square foot. Costs vary based on location, finish level, foundation type, and whether you're using pre-approved plans.
Where the money goes
Line-item averages for a typical ADU build. Actual percentages vary by site conditions and finish level.
Most 600 sqft ADU projects in California, Washington, Oregon, and Massachusetts fall within this range. The lower end represents efficient detached ADUs in Sacramento, Portland, or Boise with standard finishes. The higher end represents Bay Area, Los Angeles, or Boston projects with custom design and permit complexity.
Line-item averages for a 600 sqft ADU:
Foundation: $25-45k Framing and shell: $60-120k Mechanical, electrical, plumbing: $35-70k Finishes: $30-90k Permits and impact fees: $10-30k Design and engineering: $15-35k
Garage conversions and prefab options can reduce these figures by 25-40%. Use our cost calculator for a personalized estimate.
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