Which term refers to the total floor space within the perimeter of the permanent external building walls?

The term that refers to the total floor space within the perimeter of the permanent external building walls is known as "Gross floor area." This metric includes all areas of a building, including hallways, mechanical rooms, and other non-usable spaces that are enclosed by the walls of the building. Gross floor area is vital in assessing building size for various purposes, such as zoning, building permits, and overall property valuation.

In contrast, "Usable floor area" typically measures the space that can actually be used for specific purposes, excluding areas like walls and structural elements. "Net floor area" focuses on the usable space within the building, generally excluding common areas, access ways, and core facilities. "Effective floor area" is not a standard term widely used in building analysis, and may refer to differently interpreted measurements depending on specific contexts or definitions, which can create confusion. Thus, gross floor area accurately captures the totality of space within the walls, making it the correct choice.