You’ll need to come to an agreement with them about building the fence, as well as details such as how high the new fence should be, what building materials it will be made of, and how much to spend on it.
The Fencing Act 1978 provides that in general the occupiers of neighbouring properties that are not divided by an adequate fence have to contribute equally to the cost of work on a fence. If there is no fence or you think the existing fence is inadequate or in need of repair, then you can expect that your neighbour will share the costs of getting the fence built or repaired, however you can agree to share the cost differently.
Once you are in agreement it’s preferable to get it all down in writing, for future reference.
If you change your mind about the type of fence you want, you’ll need to consult with your neighbour to ensure they agree to it. If your neighbour moves before the fencing work begins you’ll need to make a new agreement with the new neighbour.
If you aren’t able to come to an agreement about the details of building a fence, you can serve your neighbour with a Fencing Notice.
Visit the Citizens Advice Bureau website for more information.