Hi,
I think if you turn all your layers on, select a the thick marquee tool, and drag it around the building and elevate it by the difference you want it should work.
But be careful when you drag the marquee, be sure to zoom out a long way so that you don't miss anything and leave it behind when you elevate. i usually zoom right out until the model is like a dot in the middle of the screen and drag a huge marquee around it! Also make sure it is a thick marquee as apposed to a thin one, as the thick one selects all storeys.
Hope that helps!
Craig