You didn't get an answer because it was already answered. In fact, it's been answered in the first post.
-Yes, you can build an exterior all by yourself using objects (that's the point of this contest)
-You can't edit/improve already an existing buildings (like military base, police department, CJ's house, etc.)
-You can't make shootouts
It remembers me of GalacticXp1's mission for contest #6, where theme was something like that (only 1 action objective I think). Then he designed an awesome puzzle with just objects!
That was XghostX not Galactic, Galactic designed a mission related to robbery or something like that.