Downstairs count as rough floor/wall in value;= material value. BDs can't destroy buildings who they can't stand next to, so locked covers are safe.
@Bumber: To me it looks like your second room has two tiles of gap, not 0, but better test the hypothesis:
So I made modest quarters with total value 101 (guildhall value 81, as bed is excluded), by using smoothed 2 wall, 8 floor tiles instead of 10 for 99.
Excluding 1 wall from room should drop it to 100, but remain Modest.
Before re-expanding, it is modest (Edit: On further testing, doors remove GR before they're built; without door being marked for building this is meager):
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After re-expanding...It's still modest - if it took 15 value penalty, it'd need more than 10 smooth tiles, in both cases.
So yeah,
it seems this diagonal approach prevent GR - though mind that in case of previous tests for second stage, digging out the corner was used as a last tile, so this isn't always true Nvm, see below.
I also discovered another interesting thing:
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The removed tiles in this shot didn't reduce craft guildhall value. It seems it takes into account the total value in a rectangle, even if the zone doesn't include a particular tile. However, excluding door reduced value properly (but only for door, not tile underneath).
In fact, if I paint max size zone, and then remove tiles, it keeps using original value:
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This Surgeon guildhall is worth 961, despite zone only including 9 tiles.
E: On a further test, the first seems to only take floor penalty as well; digging out a wall and placing a door there turns exact-100 value room from modest to meager to back to modest to back meager on reexpanding.