For my city block, I used a piece of 0.06" styrene for the floor on each building. That raised the floor above the sidewalks, which were a single piece of plastic covering the entire block, and let me glue the floor to the sidewalk/base while it was in the building.
Spread some styrene cement on both pieces, put them together, and immediately put the building on. There's no cement around the edges, so the building won't stick, and you have a short time to move it around to the perfect position. Wait until it sets, remove the building, and add some extra cement around the edges, just make sure it doesn't curl.
It also let's you detail the inside of the building, or add lights, without having to do the work in the building.
