J.G.
I always assumed it was the norm everywhere but if the fence is on the property line everyone adds to the existing fence. I would actually think my neighbors were crazy if they put a fence next to mine. For one can you imagine the weeds that would grow in that small gap between the fences and god knows how you would get to them. :-/
So far all sides of my fence have been used by someone. I had always hoped I would be the last fence to go in, you know the lucky person who only has to put the two side bits next to their house.