If the question is how to cut the ceiling/floor above, you can either draw the floor above manually, or export the staircase as .obj and re-import it checking the 'staircase' option. That way the model will make the cut-out (when reaching the ceiling).

If you need a more elaborate cut-out, you need to use the Furniture Library Editor available on the download page. You can find more about how to enter the cut-out shape in the forum or the SVG mini guide.

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