Which of the following best defines Gross Floor Area (GFA)?

Gross Floor Area (GFA) refers to the total area of all floors within the external walls of a building. This includes not just the usable floor space but also the areas occupied by walls, partitions, mechanical systems, and other structural elements. Defining GFA in this way provides a comprehensive picture of the entire building's footprint, which is important for various regulatory, zoning, and valuation processes.

The other definitions do not encompass the full scope of what GFA represents. For instance, defining GFA as "total usable area in a building" would exclude non-usable spaces such as mechanical rooms, stairs, or corridors, which are critical components of the total building area. Similarly, excluding wall thickness or counting only livable space would significantly misrepresent the actual dimensions and volume of the building. Hence, describing GFA as all floor areas within external walls captures the entirety of the building structure necessary for accurate assessment and compliance with building regulations.