Irremovable Basement Flooring Returns

@jelletinny 

What you have happening is caused by the platform.  When you delete the floor in the room surrounding the platform, the room is no longer recognized by the game as a room.  Just click on a wall and you will see they are no longer connected.  The irremovable floor is the ground which can't be deleted.  This is just the way the game works.

So, if you want a two-story open basement room, then delete the floor before putting in the platform and stairs.  The order in which you do things does matter here.  Tbh, platforms and basements just don't work well together imo, so I'd get rid of the platform and try to get the look you want another way.  If you want an intermediate stair landing, then I'd build a small room in the lower level and let the stairs land on that, then another stair down to the lower lever from there.

Hope this helps.