Per foot is shorthand, and almost always shorthand for hard costs only.
Hard costs are the dollars that turn into physical house: excavation, foundation, framing, roof, plumbing, electrical, HVAC, drywall, cabinets, flooring, and the builder's fee on top of all of it. Soft costs are everything you pay to be allowed to build and to hold the project while it's being built: architectural plans, engineering, survey, permit and impact fees, builder's risk insurance, loan origination, property taxes during construction, and interest on the construction loan.
So when three builders quote 145, 190 and "I don't quote that way," they're usually describing different scopes. Site work is the biggest hidden gap. A flat infill lot on city sewer is a very different site cost from a sloped lot needing retaining walls, a long driveway and a septic system, and site work rarely gets folded into a per-foot number.
The other thing that moves it is size itself. Kitchens and bathrooms are the expensive square footage, and a small house has the same number of them as a big one, so a 1,600 square foot home usually costs more per foot to build than a 3,000 square foot home with the same finishes.
Two terms you'll want in your model. Lot basis is what the dirt cost you, including closing and any carrying cost before you break ground. All-in basis is lot plus hard plus soft plus interest carry, which is the number your sale price has to beat before you've made anything. Build the budget line by line off a real scope and use per foot only as a sanity check afterward.
