400 sq ft ADUs
How much does a 400 sq ft ADU cost in 2026?
A 400 sq ft ADU is the entry-level footprint — large enough for a studio or compact one-bedroom layout, small enough to fit on most California lots after setbacks. The 2026 cost range is $60,000–$180,000 depending on whether you're converting a garage or building new, and which state you're in.
Typical 2026 all-in cost
$60,000 – $180,000
150–450 $/sq ft · 400 sq ft, mid-finish, slab foundation.
Kellow Feb 2026; DQB Jan 2026; Angi Mar 2026.
Three sources converge on this range. Kellow Construction (Feb 2026) puts the California cost band for 400 sq ft at $60,000–$120,000. Dynamic Quality Builders' SoCal data (Jan 2026) reports a higher band at $120,000–$180,000 for the same size, reflecting the SoCal premium. Per-sqft cost is actually highest at this size — fixed costs (kitchen, bath, permits, utilities) spread over fewer square feet.
By type and tier
| Type or tier | Range | Notes |
|---|---|---|
| Garage conversion (400 sf) | $100K–$130K basic; $140K–$175K mid-finish | |
| Detached new (400 sf) | $120K–$180K national avg; higher in CA premium metros | |
| JADU (≤500 sf) | $50K–$100K (interior conversion, no new foundation) |
What moves the price most
Fixed costs don't scale
DQB's analysis: a 400 sf ADU has roughly $60K in fixed costs (kitchen, bath, permits, utilities) plus $60K in variable costs = $120K total, $300/sf. The same fixed costs on an 800 sf ADU spread over twice the floor area, dropping per-sqft to $250.
Garage conversion sweet spot
Converting an existing 400 sf garage is the cheapest legal path to a 400 sf ADU in most California markets. Plan for $100K–$180K SoCal (DQB 2026) including the kitchen/bath buildout.
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400 sq ft ADUs FAQ
- Is a 400 sq ft ADU big enough for a one-bedroom?
- Yes — most 400 sf ADUs are designed as compact one-bedrooms or open studios with a defined sleeping area. With thoughtful storage (built-ins, vertical space, multi-use furniture) a 400 sf unit lives larger than the footprint suggests.
- Why does a 400 sq ft ADU cost more per sq ft than a 1,000 sq ft ADU?
- Fixed costs. A kitchen, bath, permits, and utility connections cost roughly the same whether the ADU is 400 or 1,000 square feet. Spread over fewer square feet, those fixed costs push per-sqft pricing higher — typically $300–$450/sf for a 400 sf ADU vs $250–$400/sf for an 800 sf ADU (Dynamic Quality Builders, 2026).
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