Basically, bee space is 5/16 to 3/8 inch. That is everywhere. Tops, sides, bottom, doesn't matter. They will be reluctant to build down below the bottom frames when the bottom board is less than one inch below the bottom of the frames, but it can happen if they get crowded enough. Most beeks don't allow bee space at the top, as there is no top space in a natural hive. They will build wild comb in any space 3/4 inch or more, at times, in even smaller spaces.