You should be able to create multiple Rentable Area plans Under the Settings menu select Room and Area Settings. In the Room and Area Settings select the Area Schemes tab and create a new Area Scheme for Leasable/Builtup Areas.
Create a Rentable Area plan for the new scheme, place a new Area Boundary in each space, change the names of the spaces to Leasable or Builtup. Then create a schedule based on this Area scheme.
Mel Persin, AIA
AEC Technology Consultant
Technology to Visualize and Realize Solutions
Modeling the Future/Drafting the Past