Thanks for your question.
If the extension was completed 22 years ago then it is outside the time limits within which the local authority could take enforcement action in respect of a breach of planning law.
If you did not obtain a buildings regulations completion certificate from the local authority certifying compliance then the purchaser's solicitor will ask for an indemnity policy in respect for the a lack fo building regulation, they will ask that you pay for this and you will probably have to fork out £100.00 approx to obtain one.
Other than that it should not hold up the transaction provided you are not dealing with an overly prudent buyer who gets nervous about the lack of planning regs. You would be unlucky if you did really.
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You will have to disclose it to the seller when you complete the protocol form called Property Information Form, it asks about works and the buyer will be able to sue you if you do answer the question honestly.
It's a very commonly encountered issue in conveyancing and you really should not worry about it too much.
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