How a house is built
Adding a story
Adding a story loads the walls and foundation that are already there, and it opens the roof. This drawing is already two stories.
Arrow keys turn the house. Plus and minus, or the zoom buttons, change the zoom. A horizontal drag turns it after you tap the drawing. Reset view returns to the step camera.
The drawing needs WebGL. The notes on the page still stand.
A second story puts a new floor and a new roof on walls and a foundation that were built for the old roof. Someone has to check that those walls, the lateral bracing, and the footings can take the added weight and the taller wind and seismic load. Bedrooms need a legal way out. The new stair has to fit, with headroom, often by giving up part of a room below.
The roof of the existing house is opened to build the new floor. Until the new roof sheds water, the house below is exposed. Weather protection of that opening is the first site problem, ahead of finishes. The new floor has to tie into the old frame, not merely sit on it.
This model is already a two-story house. It shows what stacking a floor looks like. It is not a design for adding a story to a different house, and it is not a cost.