There's usually more than a single 3/8ths inch (less than 0.5 cm) bee-space between the bottom of the lower edge of the lowest frames and the upper surface of the bottom board. My commercially made hives have about twice that, at least so figure perhaps 1.25 cm? I could measure mine exactly, if needed. The lower hive boxes rest on an upstanding lip around three sides of the perimeter of the bottom board. My equipment is from Betterbee, which has excellent pictures in their on-line catalogue to use as examples.
Hope that helps,