What Really Drives ADU Construction Cost
It's the site, not the sink
Investors fixate on finishes. But across projects, the numbers that move an ADU budget most are site and infrastructure, not cabinets:
- Sewer distance and slope — a long or uphill run to the main is expensive.
- Electrical panel capacity — a service upgrade can add five figures.
- Grade and access — hillside lots need engineering and haul-out.
- Soils — expansive or unstable soils change the foundation entirely.
A rough cost stack
For a detached ~1,150 SF ADU in the LA area, a typical breakdown lands near:
| Component | Share |
|---|---|
| Shell & structure | ~45% |
| Site & utilities | ~25% |
| Finishes & fixtures | ~20% |
| Soft costs & permits | ~10% |
The finishes line is the one you control. The site line is the one that surprises people — which is exactly why it belongs in the first feasibility pass, not the last.
The takeaway
Before you model returns on a $/SF rule of thumb, get the site read. A same-day
feasibility check surfaces the expensive surprises early.