http://forums.thesimsresource.com/sh...=1#post3728422
What you do is take a 3 tile wide (any depth) lot and build two walls front to back across the whole property 10 squares from each side (sanity check, should be 10 squares apart).
Then you run LotExpander (there might be a newer version, I'll have to check) and remove a tile from both the right and left sides. The property now has walls right on the property lines.