Neighbour attached roof to our garage

[Guest]

Our neighbour attached his lean-to roof to our garage wall some years ago, without our permission. The council were not interested even though our garage is built within our boundary. The house has now been sold and is a HMO. We are having issues with damp and the garage is now a downstairs bedroom, the council are still not interested. We think the problem could be with either our or our neighbours guttering.
We never gave permission for this lean-to to be attached to our garage and if we were to replace our guttering the neighbour would have to remove his roof from the garage. Could we deny him permission to replace It?

[Amethyst]

How long ago was the lean to built ?

[CR55]

The lean to was put up in 2010