You can always alter your structural hull...after all, it may impose arbitrary limits on the extent of the map, but those arbitrary limits are set by you
. Also, if you're under the impression that any brush that doesn't touch the void should be detail, RichDiesal was flat-out wrong about that. This is a simplified explanation, but struct brushes block VIS and detail brushes don't: the game acts like you can see through any and all detail brushes, but not structural ones (mostly). Walls of rooms should be structural, even if they don't touch the void...detail brushes are things like furniture, consoles, pillars, light fixtures, machines, railings, loose rocks, etc. It isn't just the "outer hull" of a map that should be structural. So if you're going to go through your map building all the detail brushes first, you'll have a map with "details" but no walls, floors, or cielings. That'll confuse you real fast.
Basically, you should do what you're trying to explain in this
image of yours, but make sure you use structural/detail brushes properly and stop after part 1.