ADU Cost per Square Foot (2026)
National ADU costs run $200–$700 per square foot in 2026. This guide breaks down per-sf rates by metro, by ADU type, and by build size — with the fixed-cost math that explains why smaller ADUs cost more per sq ft than larger ones.
Last updated: •Reviewed by the aduglossary editorial team
Quick Facts: National ADU Cost per Square Foot
Detached new-build
$300–$700/sf
Most common ADU type
Attached ADU
$280–$550/sf
Saves 10–20% on shared walls
Garage conversion
$150–$350/sf
Reuses foundation + shell
Basement conversion
$100–$250/sf
Lowest-cost path
Permit/soft costs
8–18% of build
Higher in CA, lower in TX
Typical 600 sq ft build
$180K–$420K
All-in including permits
ADU Cost per Square Foot by Metro
2026 per-square-foot ranges for a typical 600 sq ft detached new-build ADU. The Bay Area, Seattle, and Honolulu sit at the top of the cost curve due to dense permitting and high labor costs. Sun Belt and Mountain West metros run 30–50% lower per square foot for comparable builds.
| Metro | $/sf (600 sq ft) | All-in 600 sf cost | Permit fees |
|---|---|---|---|
| San Jose / San Francisco | $450–$700 | $270K–$420K | $15K–$30K |
| Seattle | $420–$620 | $252K–$372K | $12K–$25K |
| Los Angeles | $400–$550 | $240K–$330K | $10K–$22K |
| San Diego | $380–$520 | $228K–$312K | $8K–$18K |
| Portland | $350–$500 | $210K–$300K | $8K–$16K |
| Denver | $400–$550 | $240K–$330K | $7K–$30K |
| Sacramento | $300–$420 | $180K–$252K | $3K–$11K |
| Phoenix / Scottsdale | $280–$400 | $168K–$240K | $4K–$10K |
| Austin / Houston / Dallas | $200–$380 | $120K–$228K | $1K–$6K |
Want a city-specific breakdown? Try the free cost estimator or browse city guides for Sacramento, Denver, Houston, Los Angeles, and San Diego.
Why Cost per Square Foot Drops as ADUs Get Larger
Many homeowners assume a 400 sq ft ADU should be the cheapest option per square foot. The opposite is usually true. ADU builds carry a fixed-cost stack that doesn't scale with size:
- • Permits and plan check — the same fee structure regardless of build size in most jurisdictions.
- • Utility hookups — sewer, water, and electric tap-ins are the same trench and the same connection fee for a 400 or 1,200 sq ft unit.
- • Design fees — an architect typically charges a flat fee or a fee floor that doesn't shrink proportionally.
- • Mobilization and site setup — hauling equipment, erecting fences, and pulling licenses cost the same on day one regardless of footprint.
The result: a 400 sq ft ADU in Sacramento might run $400/sf, while an 800 sq ft ADU on the same lot lands at $340/sf. Total budget is higher for the bigger unit, but value-per-square-foot improves. This is why many homeowners size up to 600–800 sq ft when zoning allows.
Cost per Square Foot by ADU Type
| ADU Type | $/sf range | Why |
|---|---|---|
| Detached new-build | $300–$700 | Full foundation, framing, MEP, finishes from scratch |
| Attached new-build | $280–$550 | Shares one wall and often plumbing stacks |
| Garage conversion (existing slab) | $150–$350 | Reuses foundation, walls, and roof |
| Basement conversion | $100–$250 | Existing footprint; only finishes + egress + MEP |
| Above-garage / second-story add | $350–$600 | Structural reinforcement of existing garage |
| Prefab / modular | $220–$450 | Factory build cuts site labor; site prep still applies |
Finance Your Build
Per-sf math tells you what to budget; financing tells you what you can afford. Compare HELOC, construction-perm, RenoFi, and DSCR loans — or run a payment scenario in the ADU loan calculator.
Frequently Asked Questions
What is a typical ADU cost per square foot in 2026?
Across the US, ADU all-in costs run $250 to $700 per square foot in 2026. The national median is roughly $400/sf for a detached new-build, $300/sf for a garage conversion, and $200/sf for a basement conversion. The biggest swings come from labor markets and permit fees, not materials.
Why does cost per square foot go down as ADUs get larger?
Fixed costs (permits, utility hookups, design fees, mobilization) are roughly the same whether you build 400 or 1,200 sq ft. Spreading those costs over more square footage lowers the effective per-sf number. A 400 sq ft ADU often costs more per sq ft than an 800 sq ft build on the same lot.
Is cost per square foot a reliable budgeting tool?
It is useful for early feasibility but not for final budgeting. Two builds at the same per-sf number can differ by $30K-$50K once site conditions, foundation type, and finish level are factored in. Use per-sf to compare neighborhoods and ADU types; use a contractor bid for the final budget.
Which US cities have the highest ADU cost per square foot?
San Francisco, San Jose, Seattle, and Honolulu top the list at $500-$700+/sf for a 600 sq ft detached build. Los Angeles and San Diego run $400-$550/sf. Sacramento, Portland, and Phoenix are in the $300-$450/sf band. Texas metros (Houston, Austin, Dallas) and the Mountain West run $200-$400/sf.
What is included in the per-square-foot number?
Hard construction (labor + materials), soft costs (design, engineering, project management), and permits. It typically excludes land, demolition, lot-specific work like grading or retaining walls, and furnishings.
Does ADU type affect cost per square foot?
Yes - significantly. Detached new-build is the most expensive at $300-$700/sf because everything is new. Attached ADUs save 10-20% on shared walls. Garage conversions reuse the foundation and shell ($150-$350/sf). Basement conversions are cheapest at $100-$250/sf.
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