Get the total occupiable floor area represented by a mass instance. 
   Namespace:   Autodesk.Revit.DB  
  Assembly:   RevitAPI  (in RevitAPI.dll) Version: 2015.0.0.0 (2015.0.0.0) 
  Since:  2012 
Syntax
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Parameters
- document
-  Type:  Autodesk.Revit.DB Document  
 The Document.
- massInstanceId
-  Type:  Autodesk.Revit.DB ElementId  
 The ElementId of the mass instance.
Return Value
The gross floor area in square feet.Remarks
 The area is computed from the cross sections that are created by intersecting the associated Levels with the mass instance Geometry. 
 Exceptions
| Exception | Condition | 
|---|---|
| Autodesk.Revit.Exceptions ArgumentException | The ElementId massInstanceId is not a mass instance. | 
| Autodesk.Revit.Exceptions ArgumentNullException | A non-optional argument was NULL |