"If I want to build a 3000 sq ft home, is it more economical to design a 1500 sq ft foundation and a 1500 sq ft main floor or design with a 1000 sq ft foundation with two stories of 1000 sq ft each?"
On my web site page "How to Read House Plans " I have a tip in the second paragraph that reads as follows:
"Here’s a tip: Two-story construction is cheaper than one-story. Two of the most expensive parts of a house are the roof and the foundation. A two-story home with the same square footage as a one-story has half the roofing costs and half the foundation costs. There are also economies in plumbing and heating in two stories."
Hope this helps,
Carl Heldmann, byoh.com