Why we draw the section first
Plans are good at showing where rooms are. Sections are better at showing what it will feel like to be in them.
On every project the first drawing on the wall is a long section through the most important room. Plans follow.
A plan answers “where”: which room is next to which, where the doors go. A section answers “what is it like”: how high, where the light comes from, what you see when you look up. Clients rarely read plans well, but almost everyone understands a section once it has a person drawn in it.
The section also exposes problems that plans hide. A stair that works in plan might leave 1.9 metres of headroom; a roof light that looks generous from above might light nothing but a wall. Finding that out on day three is cheaper than finding it out on day ninety.