By default, Revit LT computes the room area using the wall finish as the boundary. You can change the boundary, specifying the wall center, wall core layer, or wall core center. 
Note: You cannot change boundary locations separately for each room. All rooms in the project use the new boundary location. 
- Click Architecture tab
Room & Area panel drop-down
 (Area and Volume Computations).  
- On the Computations tab of the Area and Volume Computations dialog, for Room Area Computation, select one of the following options: 
- At wall finish: The room boundary is located at the finish face inside the room. 
 
- At wall center: Locates the room boundary at the centerline of the wall. 
 
- At wall core layer: Locates the room boundary at the interior or exterior layer of the core closest to the room. 
 
- At wall core center: Locates the room boundary at the centerline of the wall core. 
 
  
- Click OK. 
 
Note: These settings do not affect IFC-based walls. Walls from linked IFC files use the finish face as the room boundary.