Does my Neighbour have a Right to Build on top of my Boundary Wall to facilitate his Extension?

Question

As well as applying to raise his roof, my neighbour has also applied to build a corner conservatory alongside our common boundary. In fact the planning application plans show the rear wall of this lean-to extension as constituting the 6ft garden wall between us – plus another 6ft on top, but only along his half. However, I don’t think he owns any of it. As far as I am aware (and I’m in the process of retrieving my deeds to check) the wall stands entirely on my property and the face on the other side forms the boundary. In this situation, does my neighbour have any right to build on top of the wall? Furthermore, can he even use it as part of a structure on his side?
 
If he concedes the wall is mine and chooses to build an independent structure on his side is he still required to give me notice under the Party Wall Act? It would clearly help, as dissenting – as you suggest for the other work – would be a useful ploy here as well.

Answer

If the existing wall is yours then your neighbour has no right to raise it for his use and would require your consent. If he were to build a new wall alongside it on his own land then it would not be notifiable under Section 1 of the Act as the line of junction has already been built on. It would be notifiable under Section 6 if the foundations to the new wall were deeper than those to your existing wall – it is likely that they will be if the new wall was part of a conservatory rather than just a boundary wall.