Section-based charts can have subsequent floors created after the first one is drawn.
In order to create a new floor:
Create a new section-based chart, or load an existing section-based chart
Start drawing the first floor
Once you are done, click the + (plus) button on the floor picker at the bottom right corner of the canvas, to add a new floor
Every time you begin drawing on a new floor, the + (plus) button will become available again, all the way up to nine floors.
Converting existing charts to multiple floors
If you are converting an existing floor plan where each floor is a separate section, simply add a floor, then copy/paste the relevant section into that floor, and delete it from the initial floor where you copied it from. You may need to readjust the sections/floors to ensure a good end user view, which you can check by clicking on the Preview button at the top left of the designer.