It is the Staircase option that should make the model cut an opening in the ceiling and floor above. (Movable is not relevant for the cut-out, but should be checked by default and be greyed out if you check Staircase or Door/window).

So if Staircase is checked for the model you should get a rectangular opening above the entire staircase. But that is not always what you want in the case of an L-shaped staricase.

Now, if you want a custom cut-out, e.g. one that follows the L shape of the stairs, you have three options:
- Draw floor/room manually to fit staircase (staircase option should not be checked when importing model).
- Find a model with cut-out already defined for SH3D.
- Add the cut-out shape yourself.

The latter option will require you to use the Furniture Library Editor (available on the Download page) and enter the cut-out shape yourself. On page 6 of the SVG guide, there is an example of what the SVG cut-out could look like for an L shaped staircase (probably something like [font=courier new] M0,0 v1 h1 v-0.5 h-0.5 v-0.5 z [/font] depending on the rotation of the model - this example assumes orientation similar to an L cut anti-clockwise).

You can see the result of L shaped cut-outs in SF 3D Models 329 example file. And BTW, please feel free to use any staircase in the file if that can be of help.


ok

PS - it is not possible to define the cut-out definition directly in SH3D or in Sketchup. As for model export from Sketchup, using the .DAE format is a good idea (.KMZ is basically the same, only zipped). I find it is the format with least surprises and it is also available in the free version of Sketchup.