Replacing shared garage roof

[lesliemk]

My garage has a shared wall with the neighbour and both roofs meet at the same place. See picture.

My garage roof has leaks and I guess the neighbour has it too. But they are not too bothered about it and dont want to replace it. Can I have just my part replaced? And do I need planning permission to close the front opening and make a door which opens into my garden? Thanks