About the only items that can be figured to have true square foot costs would be a floor, roof, or slab. I say "true" to mean that there is a defineable amount of material per square foot. Even roofing is questionable because it has wall flashing, valley flashing, etc that doesn't cover the whole roof but the shingles could be a square foot cost with additions for the other accessories that make the roof actually function as a roof.
As unregistered mentioned - a base square foot cost can be given but will change with every project. Think of a home and a single square foot column running through the whole house vertically. In that square foot there are a lot of items - but move that square foot to a different location in the hose and it all changes. To bid by the square foot - you take all the materials, labor, overhead, profit etc to complete the job and divide it by the square footage. Therefore, that cost will be different for every project done.
