Our property is separated from our neighbour by an alleyway about 1meter wide owned by the neighbour. He has applied for planning permission to extend out at first floor level over the alley cantilevering from his side. The extension plans to have just a single skin wall and to use our wall as the second skin needed to comply
with building regulations. It will clearly be necessary to seal around the outer and inner skins, thus touching and using our wall.
Your neighbour has a right to cut a flashing in to your wall under section 2(2)(j) of the Act but must serve notice at least 2 months in advance.
If the extension is relying upon your wall to bring it up to current Building Regulation standard then he is making use of your wall. He would not have a right to do that unless it was a party wall. I would advise you to send a polite letter making him aware of these points. If he goes ahead and serves the notice then you will have the opportunity to appoint a surveyor to review the proposals if you wish to do so.