On my second floor, above the staircase, there is an empty space (void), which extends from the rear wall of a closet to the exterior wall. I noticed it while in the attic. From the attic looking down, you see the void. It is four feet by four feet (floor), which is really just the ceiling of the 2nd floor staircase.

I wanted to extend my second floor into this void, creating a walk-in closet. However, there is no structural floor there today; just the underside of a plaster ceiling.

So I need to know how do I build a structurally sound floor about this ceiling, so nothing goes crashing into the first floor.

If this does not make sense...I can try and take a picture...