Everyone thinks a second-story addition is about building up. They're wrong. The first, most critical, and most expensive part of a second-story addition is about building down.
The Wilsons had a perfect lot in an older Naples neighborhood, but they'd outgrown their 2-bed, 2-bath ranch. They wanted to add a new master suite over the existing living room. They assumed we'd just... take the roof off and start framing.
The first call I made was to my structural engineer. We dug a test pit, and just as we suspected, the original 1970s slab and footers were not designed to hold the weight of another 2,000 square feet, plus furniture, plus people. The house would have cracked in half.
Before we ever touched the roof, we spent the first three weeks of the job inside and outside the home, strategically digging, pouring, and curing new, massive concrete footers and adding new structural columns inside the existing walls. It was a $30,000 "down" job for work the client would never, ever see. But it's the only thing holding up their new addition.
My professional recommendation: If a contractor gives you a quote for a second-story addition before they have done a full structural and foundation analysis, throw that quote in the trash. The real job isn't building the new box; it's seamlessly and safely integrating that box with the old one.