What is the cost per square foot to build an ADU?
Answered by AskBaily Editorial · Updated
Short answer
Detached ADU cost per square foot in 2026 runs roughly $400-$650/sqft in California coastal metros, $300-$500/sqft in inland California and Seattle/Denver/Austin, and $250-$400/sqft in lower-cost metros. Garage conversions run $200-$400/sqft. Smaller ADUs cost more per sqft because fixed costs (permits, utility hookups, soft costs) spread over less area.
In detail
Per-square-foot figures hide a lot of variation. The actual cost is driven by fixed costs plus variable costs:
Fixed costs (roughly the same whether the ADU is 300 or 800 sqft):
- Permits and plan check: $3,000-$10,000
- Utility hookups (electric meter, sewer lateral, water): $12,000-$40,000
- Soft costs (architectural, structural, Title 24 in California, survey): $12,000-$35,000
- Site preparation: $5,000-$25,000
- Total fixed: $30,000-$110,000
Variable costs (scale roughly with square footage):
- Foundation: $25-$60/sqft
- Framing + roof + weatherproof: $65-$110/sqft
- Plumbing, electrical, mechanical rough-in: $35-$60/sqft
- Finish (drywall, flooring, cabinets, fixtures, paint): $80-$160/sqft
- Total variable: $200-$390/sqft
Why 300 sqft ADUs often cost $500+/sqft while 800 sqft ADUs land at $400/sqft: the $30K-$110K fixed-cost base lands on less area. A 300 sqft ADU at $150,000 is effectively $500/sqft; the same ADU at 700 sqft would be $275,000 ($393/sqft) because only the variable costs scale.
Regional 2026 ranges (detached new-construction):
- Coastal CA: $400-$650/sqft
- Inland CA: $325-$475/sqft
- Seattle / Portland / Denver / Austin: $300-$475/sqft
- Nashville / Atlanta / Charlotte: $225-$375/sqft
- Chicago / Midwest: $250-$400/sqft
Conversions are cheaper per sqft because foundation, roof, and walls are partially reused.
AskBaily's ADU scoping provides a per-sqft estimate anchored to your specific lot and city, including the fixed soft-cost base and the utility separation path.
Sources
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