Question

My neighbours of some twenty years whom we have a good relationship with have recently approached us about building an extension onto the back of their house. We also have an extension which we completed 10 years ago which is built just inside the existing shared boundary wall dividing both our properties.

My neighbour has said that his builder has recommended that they build their extension on the existing boundary wall as this will prevent dampness and rubbish getting into this area and effecting both properties. We are wondering if this would cause any problems if we were to sell our house.

Answer

The builder is correct in saying that voids / gaps between extensions can can be a source of dampness and also be an area that can collect rubbish that can be difficult to remove.

As you extension is already built you have two main options.

1. Politely inform your neighbour that you want the existing boundary wall to remain unaltered and that your neighbour should build his extension on his side of the boundary line as you have. This will prevent any future disputes on the ownership of the wall.
2. If you wish to allow your neighbour to build his extension on the boundary line you should agree and sign a party wall agreement stating that you can also use this wall at a later stage if you decide to make changes, or whatever to your property.

Also, no moisture should seep into your property or rubbish should collect if the building works are carried out correctly. If you are going to allow your neighbour build on the boundary wall you should request that his engineer furnish you with a  certificate that these works are in compliance with building regulations insofar as they affect your structure.