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He will first need to apply for planning permission if the size of the extension requires it. If planning permission is applied for then you can either put in an objection with the local authority against why it should not be granted. You should first ask him if he has obtained permission, or check your local authority website to see if any planning applications have been made. If your title deeds allow you access that you have a right of way to access the rear then no he cannot stop you. Hope this helps.
You should contact your local authority to advise that works have been carried out without consent. It will then be up to them to take the matter further. You cannot sue because they have no obligation to you unless they have affected your use and enjoyment of your property in some way.