I think you’d have to create a new property expression and a zone for the entire building or site area to calculate it or have a property which defines the overall area figure then use an expression to calculate the percentage of each zone based on that figure.
Lee Hankins ArchiCAD 4.5 - ARCHICAD 26UKI Intel 3010 and Apple Silicon macOS Monterey (12.5)
To do this correctly, you need to somehow collect all the data about the zones in one object, because formulas (property expressions) work at the individual element level. How can you do it? In this case, it's probably possible to create an property for overall zone to have a total area figure, and then use an expression to calculate the percentage of each zone based on this figure, but this is a semi-automatic method. For complex projects, this is not a very suitable solution.
Unfortunately, the truth is that it's better to export this schedule to Excel, add a percentage column, and then import it back into Archicad. Maybe you know how to do it differently?
In a way that's what I'm currently doing for most projects. Prone to error specially if you are rushing things and might overlook certain items. Am not sure how difficult it is for Graphisoft to incorporate this kind of things in their built in schedules (wishing that there is a button to press with % or show all FFL or Glass symbol for your door and windows schedules.)
In the end what is your Objective? ArchiCAD 9 onwards