This is something that used to bug the hell out of me until I figured what caused it, and found out how to fix it. Maybe you all know this already, but maybe there's some people out there that'll be helped by this :)

You've probably come along some houses, that in the lotview look perfectly fine, like this:



But when you look at them in the neighbourhood view, they seem to have a wall missing, like so:



Annoying, right? It looks fine in the lotview, then why does the neighbourhoodview think you left in a big damn gap?

The problem is fairly simple, and thankfully fairly simple to fix as well. What you see in the lotview are actually two walls overlapping eachother: the wall of your main structure, and the wall that makes up the back of the segment of roof that borders it. When in lotview, the game shows you the structure wall. While in neighbourhoodview however, for some reason the game has trouble calculating the two, and chooses to show the wall that's part of the roof segment (which isn't there, seeing as the structure wall overlaps it, hence the gap.)

How to fix this? Just make sure that your structure walls are never overlapped by roof segment walls. When adding in the roof segments, always make sure that the back wall is one or more tiles away from the structure wall, like this:



After you've constructed the roof, you can just delete these walls without any problems. Your roof will stay in place no matter what happens with the back wall.



Get rid of that wall...



... and the neighbourhoodview no longer thinks that the structure wall is missing!

Hope this was of some help to someone out there :)