Is it going to have a lot of impact on your neighbour like blocking up a shared drive or taking all the light out of their front room? Have you talked to your neighbour about it? If not, why not? If you know it is going to upset them are you concerned about that or do you think it is none of their business, how ever much it impacts on them? You sound as if you are going to do it whether they like it or not....... is there a compromise if it will impact them badly? Just a few things to think about as well as the legal things.