To export a building site to an ADSK file, at least one area plan must be designated as the gross building area. You can create the gross building area before starting the export process, or you can let Revit guide you through the process of creating one when exporting. The gross building area should be at ground level, because it becomes the building footprint when exported. You can have more than one area plan in the 3D view, but only one can be designated as the gross building area. See Creating an Area Plan.
Optionally, you can sketch, or specify using survey data, one or more property lines for the building site. Only one property line is exported with the site.