It is my belief that net square footage should be used in a sale. After all that is what the purchaser is buying, the actual footage that he will own independent of common area.
One of the first things I would do representing a buyer for comparable values, would be to convert the cost per square foot from gross to net usable, which would make the cost per square foot of purchase higher.
Conversely, in leasing such space, representing the Lessor of such space, I would convert from usable to gross square footage.
I suspect to answer your question that an appraiser would use Net square footage when looking at the value of the condo.
Onward and Upward....
CA DRE 544165 (one of the oldest active licenses in the State)
AZ DRE BR 641305000
Mar 8, 2013