DHD - Level Addition - HeEeELlLlPpPpP!!!!

Oh Geez.... How the freezing llama do I do a level addition? I've tried adding walls, removing walls, adding stairs, removing stairs, adding basements, removing basements, adding and removing furniture.... nothing works. Well, almost nothing. I can add and remove the stairs that are currently on the lot and I can add and remove the roof. And that's it.

How do I do a level addition? Please help! I can't even find any tutorials online for this! Room additions I can find, but not level additions.

I should have stuck to just doing what I knew... *wanders off muttering into my broomstick*