Change Platform Thickness??

[arnolds]

I need to change the thickness of a platform to a smaller size. Cause I need to add some area around the pool. So I make an invisable wall and turn it into a room, then change the floor to river stone. Now when i do this then go to 3d view it show it as being 4" above the ground and i need it to be even with the ground and not be raised like that. Or is there a different way to do this??