Neighbours building across shared boundary - do they need planning permission?

[Emm.chell]

Our neighbours ripped up hedges on the shared boundary and then replaced them with fence panels without asking us or getting our permission (I think this is a breach of the party wall act) but then began to build a building that rests up against the fence posts and overlapping them, making the fence posts part of the building structure. They are saying as the building doesn’t come over 50% of the fence posts that it’s their half and therefore it is fine. Our argument is that the fence/fence posts are on the shared boundary and therefore are shared entirely so no part of the building should touch/overlap the fence posts. Who is right?
we

[atticus]

Planning permission is not the issue; rather it is ownership of the land on which the building is constructed. There is no such thing as a shared boundary of the type you describe. The boundary line is a very thin notional line. Items marking a boundary such as hedge or fence may belong to one side or both.

So the question is whether your neighbours are building on their side of the boundary, even if they are going right up to the boundary.

[Emm.chell]

Thank you for your reply. In the title plans it states “the walls and fences separating the land herby conveyed from the adjoining property are and shall be deemed party walls and fences” so where do we stand? What would be the next steps?

[atticus]

Which are those walls and fences. Didn't you say that there had been a hedge?

[Emm.chell]

There was a hedge with a wire fence running down the middle that was removed and replaced with another fence

[Emm.chell]

Is there anything we can do?

[atticus]

Do you have legal expenses insurance cover?

[Emm.chell]

No unfortunately not