Foundation Height Slider Missing

I just started playing the game, so unsure if it's user error, but I can't get decorative foundations to work.

I've watched tutorials on how to adjust the platform height to add a decorative foundation, and a slider is referenced in the tutorials that I don't seem to have. It should show up on the foundations tab in build mode to the left of the swatches, but it isn't there. If I try to add a platform to a room or vice versa (walls on top of a platform), the walls extend completely down to the ground, removing the foundation entirely. 

I've tried all kinds of different orders of doing things, resetting the game, bulldozing the lot, restarting my computer and the problem still persists. Thanks for any advice!