Most of the stairs on floor plans run upwards to the floor level above, but sometimes you need stairs that run downwards to the level below. Unfortunately, downwards stairs were not yet supported in the editor. But from today, that is history!
It only takes a few simple steps to create downward stairs. First you need stairs, duh. The next step is to let the stairs start at the floor level. Select the stairs and give it a negative Raise from floor value similar to the Height of the stairs. This lets the stairs start at the floor level and run downwards.
Now you have to cut a hole in the floor, otherwise you can't walk down the stairs! You can do this by drawing a surface and marking it as a Cutout. After drawing a surface, select it and check the Cutout box. This will cut a hole in the underlying floor in the shape of the surface, in 2D and in 3D.
That's it. Besides moving stairs (or any other item) under a floor, you can also move the bottom of a wall down. This makes it possible to show a wall next to descending stairs. Just like any other wall, you can apply a color or wallpaper to decorate it.