The walls you can split and adjust in height.
For the cutout in the floor you will have to add your staircase to a Furniture Library. In the Furniture Library you can customize the cutout in the floor. The cutout is defined as an SVG path and you can download a pdf here that explains how that works. It looks difficult but once you understand how to draw a path it's simple.
To create your own Furniture Library you need to download and install the Furniture Library Editor .