For a curved staircase you need to include a library (because the model itself does not support the correct cutout (and it is not yet automated).

I put my own test-staircase on the SF SweetHome models forum:
Curved staircase

Which, as I mentioned, you need to import as a FurnitureLibrary.

Note that the staircase is a rough draft, but you can change colours and size. But as it was made specifically for one building, the proportions may not be ideal. I will probably update the model later.

Ola-Kristian

PS - although it is called Circular, it is actually a 180 deg curve.