Posted: 16/09/2013 at 16:56
Everyone seems to be jumping against the neighbour.
If the extension was built right up to the boundary by the last owners (which is not normally allowed due to issues like this) then it is entirely possible that next doors garden is the level your garden used to be. How would you like to be told to lower part of your garden as next door has built an extension?
Legally they should have been served notice that the extension was being built up to the boundary party wall and would have had the right to veto: if permission was ever sought...so tread carefully.
If we are talking about extension built using part of an old garden wall instead of building a new wall, then there is no wonder that there are water ingress problems as it will not have been designed to be an internal wall.