Both meshes need to form closed, 3 dimensional shapes. So in the case you describe, even if your building is a closed mesh, the extruded/duplicated part is not going to be a closed mesh, unless you fill in the open face.
I admit that I can’t picture what you’re trying to do though. It’s probably just my own lack of imagination, but on the off chance that you’re not 100 percent clear on boolean operations, you might take a look at this:
It might help you understand why closed meshes are necessary too (although you have to imagine all this happening in 3D, instead of 2D).
A little note, though. At the moment I think that booleans, although much improved in 2.40, are still not really that good a modeling tool in Blender. Even when they work perfectly they tend to result in messy meshes and I think they should be avoided unless necessary. What you’re after sounds better to do with plain old extrusion and maybe some resizing and moving edges and vertices directly.