Can a boundary wall support both neighbours?
Our neighbour is rebuilding an extension to the back of their house and proposes to rebuild the boundary wall (currently 100mm). The new boundary wall will be 350mm. We are unsure whether we should get them to rebuild the wall and use the extra required space on their side. They have told us that then we would not be able to build up to the boundary wall if we were to do an extension in the future. I don’t understand this because part of the new wall would still be ours.
It is assumed from your query that the existing 100m wall is a party wall and that the property (legal title) boundary runs along the centre line of this wall.
Replacement of a party wall by a different wall or boundary type or building on it requires the consent of the other party.
You are right in stating that part of the proposed wall would be yours and may be used to support a future extension you build as it will be a party wall, albeit not centred on the legal boundary. It will be necessary however to ensure that it is designed with adequate load bearing capacity to support both extensions.
It is however usual practice and more pragmatic to centre the party wall on the legal boundary if it is to provide support to buildings on both sides. In this way the foundation is equally shared and this sense of shared interest facilitates cooperation in providing for adequate load-bearing support and in resolving potential jointing and sealing issues which can be problematic in relation to extensions built at different times.
In the absence of an agreement to use a party wall both parties are obliged to build separate walls on their respective sides of the legal boundary. This reduces the floor areas. Unless they are adequately joined and sealed double walls may cause future maintenance difficulties including dampness if there is insufficient space between the extensions for maintenance and removal of accumulated debris.
Niamh O’Reilly is a Chartered Geomatics Surveyor