This is a Pre Contract issue.
You've already clearly flagged that you expect there is a problem. You should ask the Estate Agent to have the Vendor's provide such planning documentation as they will be relying on in the Contract. Have your solicitor review that before investing any more time or effort in this property.
As a general rule, for extensions, they are either exempt development ( and so certified by a qualified person) or not exempt in which case a purchaser needs to see the Planning Permission and Certificate of Compliance. In either case, you also need a Certificate of Compliance with Building Regulations.
A cash purchaser can "take a view" but a purchaser borrowing must disclose the issue and most banks won't proceed unless the situation is rectified.
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