I must be visualising the whole thing wrong. If the OP decides to construct a wall on his side of the garden boundary, lets say by about a few inches. Are you saying that the person on the other side of the property has no right to paint his side of the wall? Is he supposed to construct a new wall on the old boundary so that he can paint his side? Does the new wall not create a new boundary or is the OP still claiming the couple of inches on the other side of the wall as his? Does he maintain these couple of inches?