One option would be to trace the perimeter of the building using a single continuous polyline. Then use the AREA command. AutoCAD will prompt you to Select first corner point or [Object / Add / Subtract]: Type the letter "O" for the Object option then when prompted next to select objects pick the polylined outline of your building. AutoCAD will display the Area and the Perimeter of the building.
Another option would be to use the Hatch command but it's been a while since I've used the technique so I'd have to refamiliarize myself with the procedure.
I'm sure someone else will tell you how to do it before I return with the answer. If not then I'll post a solution.