Who has to erect a fence?
You are not obliged to erect a fence on your boundary and your neighbour may erect one if you have no objections. Likewise you may erect one on their boundary if they have no objections. Whoever erects the fence is responsible for maintaining it to a reasonable standard. It does not have to be beautiful but it must be safe – ie it should not have any exposed nails and it should not be dangerously flimsy.