Surface area is a useful calculation for Architects and UK Planners as it demonstrates whether a building is compact or not. Compactness is essential in urban areas, but is also highlighted by UK planners in Rural & Settlement Edges to prevent expansion of the greenbelt.
The issue with Archicad is that the only way to get this figure is by using slab, separating it from all the useful features of the zone. This is unnecessarily cumbersome and requires a custom label to extract the information. Furthermore, you're unable to takeoff this information in a schedule without it producing a figure for the top or bottom surface, we need all of it!
If you need to change your slab, you need to update the figures on the layouts manually - not as refined as it should be!