Hey keen Tis looking good at the mo, things that I would suggest adding or changing would be things like the pipes on the side of the building. If it has been left deralict then the pipes would be broken in places, this would allow you to have water staines pouring down the wall which would then lead to moss growing etc. Same with the telephone cables, perhaps have one or two left hanging or snapped, also you cant forget the random pair of shoes dangling from telephone cables :DFinally the roof sections seem a little over looked, maybe a tv ariel and or satelite dish that had been pulled down or rusted up? and on the flat roof building maybe some air con units or air flu's that you sometimes see...http://images.onesite.com/blogs.telegraph.co.uk/user/kate_day/aircon.jpgHope that helps you, look forward to seeing it finnished :P
keen looking good although drain pipes usually lead to a drain.All it needs to be is a plane. id put mor geometry in there, make it look more clutterd, oo!! i can see roof, have you thought about a chimney?