I had an extension (a conservatory, actually) added to my house. The work involved the demolition of the existing boundary wall between my neighbour and me. The outside wall of the conservatory nearest to my neighbour then became the new boundary wall.
I had a party wall agreement drawn up by my solicitor and this was agreed by my neighbour. This set out the details of the boundary (allowing me, effectively, to build upon it) and also included a clause which allowed me access to my neighbour�s property for the purposes of maintaining the new wall and my conservatory roof.
The agreement was registered at the Land Registry and is actually an agreement between the two properties rather than the current owners. This means any future owners of our two properties are bound by its conditions.