It's not clear to me what the problem is though. Is the extension ( even at the, to me, very large area stated) actually exempt? Or is it something which did require planning and obtained it but didn't have an engineer supervise the building? Although to be honest in my opinion neither is insurmountable. If works have been inspected now as built and an engineer is prepared to say that they comply with pp ( if necessary) and building regulations ( with the usual caveat being that they cannot inspect works covered up and didnt inspect during building) and if the bank will agree the qualification then I wouldnt see a problem. Houses with this type of cert are bought and sold every day of the week.