hey William, sorry for delay, I was just puzzlesd by your question.
The app doesn't really work (games, at least) on stairs/multi-level areas. But your case is just one step, something not super uncommon in different house layouts.
Initially, I would suggest mapping a smaller area and playing around with the games on a single level. But, assuming you start the process on the lower level, I suppose nothing is stopping you from just going to the custom builder and laying out a really big flat cube as the second level floor. You could still model your whole space, but games would likely be unplayable with a big barrier on the floor.