Area calculations can be defined using group types.

In the example below: Level is being used to calculate area by the group type. 

Show Group Gross/Net on Project

Shows "sum programmed gross area" and "sum designed gross area" for each group type that has enabled areas in the project property panel.

Total calculated programmed Gross area: Calculated from the sum of the product of the sum of all gross area from rooms in the groups and the group gross/net factors, eg. for a group type "Building" with a building "Building 001" and "Building 002", the total area is (the sum of gross area of all the rooms in Building 001 x gross/net for Building 001) + (sum of gross area of all the rooms in building 001 x gross/net for Building 002) 
Sum <group type> Designed Gross Area: Sum of all the group designed areas of the group type, eg. for a group type "Building", the sum of all designed areas of the buildings.