If by "surface area" you mean overall exterior-of-walls "building footprint," then maybe:
1) Ensure there's nothing outside the building to impose itself in the operation, or turn off Layers that such things are on, such as for paths/sidewalks/curbs/parking/landscaping.
2) Turn off Layers containing anything like out-swinging doors, and ensure there are Lines or something across the openings, so that the perimeter is "air-tight." Something similar with things like window sills that project from the wall surface, if their area should not be counted as part of the building area. Include things like roof overhangs and canopies if their area needs to be counted.
The rest of this could be automated easily enough:
3) Draw a CIRCLE or a RECTANGLE or something fully outboard of the footprint. [Save that to a variable for later deletion.]
4) Use BOUNDARY or BPOLY, picking in the space between that outer shape and the building. That will make a Polyline for you around the footprint, and one coinciding with the outer shape.
5) [I'm not sure this will always be true, but it seems to be in trials:] ERASE Last will remove the one coinciding with the outer shape.
6) Take the AREA of what is now the Last object.
7) Delete that if you don't need it any more, and the outer shape.
Even parts of steps 1) and 2) could be automated if your plans are drawn with consistent Layer names, door and/or window Blocks are built with header lines on their own Layer, etc.
Search for SHRINKWRAP which could do much of that -- I forget whether it's a routine that's out there or a feature of one of the overlay programs.
Kent Cooper, AIA