Fenceman I have had several different fencing situations with neighbors. On the section where I bought the materials for my half and the neighbor built the whole fence, that was replacing an old fence. We didn't survey that just put it in the same place as the very old dilapidated one..

On the new boundary fences I have put in in recent years that sometimes go through the woods  with no visible prior fence, I had it surveyed and built the new one right on the surveyors line.

The other situation is on some rented ground where another renter has horses on the other side and his "fence" goes all over the place. I just built my own fence on this long term rented ground in a straight line about 5 ft on my side of his. This is not a legal boundary fence, just between two different renters.  My point there is that a 5 ft space between two fences is a nightmare of weeds etc. I would never have a double fence if I could avoid it. this one is not a legal issue. Same landlord on both sides. Sorry if I was not clear. Just commenting on the weeds and near impossible to maintain the space between.

Jim at Dawn