Why Small ADUs Cost More Per Square Foot

Here's a number that surprises everyone: a 400 SF ADU often costs more per square foot than a 1,000 SF one — sometimes $450+/SF versus $350/SF. It feels backwards until you see the math.

The expensive rooms come standard

Every ADU — whatever its size — contains one kitchen and at least one bathroom. These are the two most expensive rooms in any home: concentrated plumbing, electrical, ventilation, cabinetry, appliances, tile, fixtures. A kitchen-and-bath package might represent $60K–$80K of cost whether it serves 400 feet or 1,000.

Divide those fixed rooms across 400 SF and they dominate the per-foot price. Divide them across 1,000 SF of comparatively cheap bedroom and living space, and the average falls. The extra square feet in a bigger unit are the cheap feet — drywall, flooring, and air.

The fixed costs pile on

Small units also carry every project's fixed overhead at full price:

• Utility connections — trenching and hookups cost the same for any size
• Permits and fees — largely size-independent below common thresholds
• Design and engineering — a 400 SF plan set isn't half the work of an 800 SF one
• Mobilization — the contractor's setup, temporary facilities, and management cost what they cost

What this means for your decisions

1. Don't multiply naively. If an 800 SF unit runs $300K, a 400 SF unit is not $150K — think $160K–$180K+ for new construction.
2. Marginal square feet are cheap. Going from 600 to 700 SF might add only $25K–$30K, because you're buying the inexpensive kind of space. If the budget is close, the slightly larger unit is often better value per dollar.
3. This is why garage conversions rule the small end. At 400 SF, avoiding foundation and shell costs matters most — exactly what a conversion does.