An addition is built on three denominators, not one rate

Because the second story shares the two most expensive square feet in the project. A footing and a roof are bought by the footprint, so stacking a second floor on the same footprint doubles the floor area while leaving the excavation, the concrete, the rafters and the covering exactly where they were. What does double is the framing, the finish, the services and the exterior wall. That is why the calculator refuses to price the foundation and the roof on floor area — doing so is the single reason blended rates misprice a two-story job.

Seven distinct pieces of work, and none of them is inside a builder's per-square-foot rate. Where the new roof meets the old there is a valley or a step-flashed junction. The existing exterior wall has to be opened, which means shoring and a header sized for what is above it. Siding that is fifteen years old cannot be patched to match, so an elevation gets re-clad. Windows next to the new ones start to look wrong. The furnace and the panel may not carry the extra load. And two structures built decades apart do not meet flat, so the floors need leveling at the threshold.

On the footprint, always. A crawl space under a two-story addition is the same crawl space it would be under a one-story one, and the excavation, footings and stem walls do not know what is stacked above them. Footings themselves have a floor set by code: IRC R403.1.4 requires bearing at least 12 inches below undisturbed ground and below the frost line where that is deeper, with a minimum thickness of 6 inches under R403.1.1. IRC Table R301.2 leaves the frost depth for your jurisdiction to fill in, which is exactly why the same slab detail costs differently two states apart.

That is a calculation, not a guess, and both calculations are named in code. IRC M1401.3 sends heating and cooling equipment selection to an ACCA Manual S process based on a Manual J load calculation, not to a rule of thumb about square feet per ton — a well-insulated addition with good glazing can need less capacity than an old room half its size. On the electrical side, NEC Article 220 sets out the service load calculation that decides whether your existing service still covers the house plus what you are adding. Both of those lines are on the tie-in list precisely because they are so often assumed away.

Structurally no, visually almost always, and practically it decides the tie-in detail. A matched slope produces a clean valley; a shallower new roof running into a steeper old one produces a saddle and a flashing detail that leaks if it is got wrong. The slope also sets how much roof surface sits over a given footprint, which is not a small factor: a 12-in-12 roof carries about 41 percent more surface than the footprint beneath it, and a 4-in-12 only 5 percent more. Measure the existing rise over a 12 inch run before the new roof is drawn, because the covering is bought on the sloped area rather than the plan area.

Whatever length runs against the existing house, and the page subtracts it before computing the wall area. On a 24 by 16 foot addition attached along its 16 foot side, the perimeter is 80 feet but only 64 of those get sheathing, insulation, windows and siding. Sixteen feet get flashing, a header and a patch instead. Pricing the whole perimeter as new wall inflates the envelope by a fifth on that geometry, and pricing none of it at all misses the most failure-prone joint in the project.

Because a designer's fee, a building department's fee schedule and a builder's margin are three unrelated markets, and a default in any of those boxes would be a number we made up. Permit fees in particular are set by ordinance and often scale off declared valuation, so they are knowable exactly — by asking your building department, not by reading an average. Enter what you were quoted and the percentages are applied to your own construction subtotal, on their own visible lines.