| Category | Estimated Cost Range |
|---|---|
| Construction (hard) | $100,000 – $150,000 |
| Site prep & utilities | $12,000 – $25,000 |
| Design & engineering | $8,000 – $15,000 |
| Permits & city fees | $6,000 – $12,000 |
| Total | $125,000 – $200,000+ |
Most 400 SF ADUs finish in the $140k–$170k range when built with standard finishes and on straightforward sites.
💡 Tip: At 400 sq ft, you can design a simple but efficient floor plan that works well for long-term rentals while keeping costs under control.
Building a 400 square-foot ADU usually takes less time than mid- or large-scale ADUs but still requires full design and permitting. Typical California schedules:
💡 Ataman Studio manages the design, engineering, and permitting process, helping homeowners streamline approvals and avoid costly delays.
A 400-square-foot ADU offers the livability of a self-contained studio with full amenities. With construction costs still on the manageable side, it’s an ideal choice for guest housing, in-law living, or long-term rental income. Request your custom estimate today and see how this size can work for your property.
