Pop has a example of a pop up that I think has gone too far. Normally they show some nice tasteful or at least height appropriate townhouses with new rooms / floors on top. DC is a great city for Pop Ups and going to the rooftops. Just be wary how wide your lot is.
In my neighborhood there is now a second example of someone building as tall as they can despite a lot width of 12 feet. I am sure I would reach both walls in this house if I tried. It is almost dangerously tall for the buildings around it. Plus do you really want to spend that much time on the stairs?