Did some really quick thumbnails and a layout plan for how the level should look. Also modelled a quick whitebox of the buildings. I'm planning on making it a cloudy day, maybe around 9 in the morning, so the sun isn't too high. I'll find out about how fog works in UDK, since this might work to show that its a cloudy and dull day.
I've noticed that in order to jump over to the last building, it will have to be a lot lower. But I don't want to reduce its height by too much, so I think I'll use assets as a way to gain height and jump off from them.
The layout works that you start on the first building, make your way through the slums, and then jump over to the second building. This one is a bit more open, with more air vents around it, you then make your way over to the fire escape, to gain access to the third roof. Here you can jump on some assets, probably the larger AC units, and reach the final building. Here is where the small room would be to pick up a decent gun.