What happens to my estimate if I want to add a floor later (G+2 to G+3)?

If you already know a future floor is likely, your structural design should account for that load from day one, since retrofitting a stronger foundation after construction is far more disruptive and expensive than designing for it upfront. Re-run the calculator with the higher storey count to see the difference in estimate, then discuss with your structural engineer whether the foundation and columns should be sized for the eventual load now. Building for an unplanned future floor without engineering input is one of the more expensive mistakes we see corrected later.