My neighbours have built a dormer 18 months ago, we have a shared chimney stack, they have said that my side of the chimney needs repairing (which I agree with after seeing the photo) and this is causing a leak in their bedroom which is under the dormer but the leak is in the recess to the side of the chimney brest and away from any of the walls? There is no damp on my side at all! Can my chimney be causing their leak, as it makes no sense to me logically! My side is the right of the picture, the damp patch is to the right of their chimney brest. Hope that makes sense!
Both these issues need doing.
My chimney was in a bad state and my neighbours would have water running down their attic wall, had the chimney rebuilt and not heard a peep out of them since so assume the leak has stopped.
Lol ! who in the right mind would attach anything new to that stack and not expect to have issues? Inform the people who constructed it to come back and rectify it, If you don't hold these muppets accountable then they will just keep doing it.