Our council has a "Planning Portal" where applications for planning permission are published, with detailed plans. Yours probably does too.
I don't think your neighbour could have made the alterations you talk about without permission, so if you do a google search for "planning application" plus the address, you should be able to see just what they applied for - and what was was granted - and if the two don't match he final result, you can contact the planners.
If there doesn't seem to be any application applied for and granted, then they could be in trouble, even up to having to reinstate the house as it was before.