The price per square foot that you see in various data bases is based upon square feet under air. Garages, porches, etc. would not be included.
Are you building or buying or selling? If your building then a builder's calculation will be much more complicated. They key basic cost factors would be number of floors and number of corners.
Weichert, Realtors - Hallmark Properties
Please let me know if I can answer any further questions for you,
Maryann K. Lawler, REALTOR
The Lawler Group
Keller Williams On the Water
Direct: (941) 586-8257