When making sure no two waps on the same channel are close to each other, also consider waps on adjacent floors. Usually, three different channels will be enough. So, use channels as far as possible from each other. We are using ch. 1, 6, 11. That leaves you room for little adjustments later on.

For example, wap in my office corridor is on CH. 6. Because of the radio weirdness occurring in old buildings, my laptop happens to receive a stronger signal from both the floor above me and the floor below me. Both waps generating such signals happened to be on ch 11 (nobody tought that all concrete, thik walls of a 3 century old building would allow the signal to pass, and we were all wrong) and produced destructive interference. I could move one wap to ch 9 and the other on ch 12, solving the problem without creating interferences with other waps around the two involved.