If the building is impassable terrain, you can
DS on top of it, if there is room.
Ofc the models
DS'ing will have to take a dangerous terrain test also.
If the building is actually able to house models, has removable parts and seperate floors, then I'm not precisely sure.
DS'ing onto the ground floor in that case seems unreasonable and I'd probably say you'd be able to
DS onto the roof. I'd like to hear some more feedback on this personally.
For Ruins, it's definate that you
DS onto the Ground floor, regardless of the number or size of the floors it has.
So..primarily, we need to know the kind of 'building' being discussed in this scenario, to make sure what it falls under specifically.
I'd love to pull the nasty tactic of placing an opponents
DS'ing unit on top of a piece of Impassable terrain, where they could not get down from it

.