Its really not too bad of a fix, but it does require some messing around and probably at least 30 minutes (depending on your familiarity with CC3). What I would do is copy everything you have there onto a new sheet (call it "temp"). Then you need to erase all the little doors and lines that separate each room. The multipoly command doesn't like them for some reason - it only wants one solid, contiguous polygon. Once you've done that, you should be able to multipoly that shape and use it as your new "Floor" or whatever.
Let me know if that helps