@paullee Interesting ideas!
I also think managing sites, buildings and levels in one single dialog might make it quickly overcomplicated. But one for "project setup" (sites + buildings) and one for levels, might work well.
You are right that the windows dialog must show which window serves which space(s). That's important information. And yeah a further idea is, from that dialog, produce a spreadsheet exactly like yours.
The idea behind axis/levels/floors would indeed be to merge all this in a more simple structure. But using axes to define levels doesn't seem such a good idea to me anymore. Now I think it might be better to have a more easy to use Floor object (which could display a level mark in the 3D view, exactly like the axes). "Floor" should also be renamed to something else (Storey? Level?) because floor is ambiguous, it could also designate the material you step on. But Storey and Level both refer to a "stacked" situation (one on top of another), I'd like to find a word that is more generic, that would allow for example to have two building parts next to each other. Maybe "Building Part", simply?
Also I want to try to make it from an App::Part instead of a Group, so it can easily be moved around and benefit from all the capabilities of App::Parts.
@bitacovir of course. The idea is ultimately to be able to do it all. But personally I'm more the step-by-step type of person
I don't have much confidence in making so big plans ahead, but go for it, unleash your ideas, the idea here is to experiment.