A lot of what I do involves looking at two-dimensional objects, i.e. maps, plans, surveys, architectural drawings, and trying to make sense of it in three-dimensional space. Trying to explain the concept of a "building envelope", that three dimensional space within which the building is now or is proposed to be within, to a person who does not do this for a living can be a challenge. It is important, though, because the setbacks contained in the Zoning Regulations are not just lines on the ground. They are imaginary vertical walls. So, if your house already has one floor encroaching on a setback, that may be a pre-existing nonconformity. But if you are proposing a *second* floor, over that first floor, even if it is exactly over the old footprint, it breaks more of that imaginary vertical wall and can't be approved, absent a variance.
I'd probably be better off letting my son explain this using Sims4. He's building houses all the time. But zoning never seems to be an issue there. Probably just as well.