"可用地板面积" can be translated into English as "usable floor area" or "available floor space".
"Usable floor area" emphasizes the area that is actually available for use, excluding any non-usable spaces such as columns, walls, or other structural elements.
"Available floor space" also conveys the idea of the area that is accessible or can be utilized, though it might be slightly more general in its usage.
In most architectural or real estate contexts, "usable floor area" is a precise and commonly used term.