Dollars per square foot is a common trap people fall into with all the information available. IF, all other factors are comparable, lot size, age of the home, quality of the finishes, builder, just to name a few, it begins to have relevance.
I sold a luxury home for a lower $perSqFt than a starter home and they were 2 blocks apart. One went for nearly $1,300,000 the other for under $400,000.
When I do a CMA to determine value this is a consideration, but only one of many. Please don't get hung up on this, for your own benefit.
As for the dilemma of how to calculate the square footage, you would need to get the footprint of the home, multiply the number of floors above grade and subtract the garage space. Basement is not included.
Anything that's backed by Federal funds is 100% of the living area ABOVE grade only. Some municipalities MAY include finished space below grade to increase tax revenues.
I hope that helps,
Century 21 Associates
Cell (Preferred): 877.614.1494