The rules for planing permission & building regs. are quite different. I don't understand why your enquiry to the Council has anything to do with this because the buyer's solicitor should have asked for building regs. documents anyway. Nor do I understand how it came about that you got planning permission but no building regs. The normal Council procedure would be for the planning dept. to notify building control, who then make sure they do the necessary inspections etc. If this was not done, you might have a case to complain about maladministration.
The fact that you made the enquiry makes me wonder whether you had some reason for thinking there might be a problem.
Unless you have a comeback against the Council as indicated above, I suspect all you can do now - if the buyer's lender won't give a mortgage - is try to get retrospective building regs. That may be difficult/expensive depending on the nature of the extension, & if it doesn't comply you have a serious problem.