Fences need to be erected on the common boundary line between two properties. If you aren't sure where that is, you're going to need to have the property surveyed. Otherwise, you could inadvertently encroach on your neighbour's property, which could cost you a lot of money later if your neighbour decides to dispute this with you. In most cases, the cost of the survey will be split between you and your neighbour. It is important to note however, that if your neighbour correctly identifies the property boundary and is proven to be correct, they will not be responsible for a share of the survey cost. Either way, determining where the common boundary line is needs to be done if you want to build a fence.