"楼面可用面积" can be translated into English as "usable floor area" or "available floor area".
"Usable floor area" emphasizes the area that can actually be utilized for specific purposes.
"Available floor area" highlights the area that is accessible or ready for use.
Both expressions are commonly used in real estate, architecture, and related fields to describe the portion of a building's floor space that is suitable or available for occupation or utilization.