Have you engaged with the person you want to prevent? Are there engineers/architects/lawyers involved? Have you asked the Local Authority to intervene?
Injunctive action should be a very last resort - its expensive , time consuming and you have no guarantee of success in getting an order or, if you get an order, being able to enforce it.
Thanks for your reply mf1.
I wanted to check the cost to gauge the liklihood of somebody applying for an injunction on a small extension. Think it is unlikely if it is that expensive.
Outside the legal issues, your client may wish to consider whether it is good neighbourly practice to build something that is liable to bring on an injunction.
A good way to get a decent estimate is to contact a legal costs accountant. They draw up bills for solicitors and are a great way to get a idea of how much something will costs. Try