How do you join floors?

I'm building a retail space that has a ground floor and a basement...i have stairs that go down into the basement - they won't show as if they're going down (when i put it into isometric view) - im hoping i can join the floors so they can go down???

I thought about having the stairs start from the basement and go up...but is there a way to reverse this? and how can I join the two floors?

Thanx a lot!