|Since none of our other Southerners have answered, I'll take this one :-)|
Tenya, a shotgun shack is a small house that is built in such a way that if you stand at the front door you could fire a shotgun into the house and the bullet would go through the back door. I.E., it's long instead of wide. It's common in poorer areas, where houses are crowded together, and since they're usually run-down, it's called a 'shack.'
I don't think it applies to shotgun shacks but another housing style I've heard of in poorer areas is one where there's no hallway. To get from the front of the house to the back you have to go from room to room through adjoining doors. Imagine being a teenager and having your parents have to walk through your room to get to another part of the house. Awful.