Take your time.
It is for the Vendor to provide satisfactory information on this.
It is one of the following
(a) exempt from the need for planning but they will need to provide an appropriate certificate confirming that
(b) Unauthorised - in which case they have to advise you of same and you have to make your own decision, taking into account whether you need to borrow from a bank.
(c) Authorised, with planning, and they will need to provide an appropriate certificate confirming that .
Stand back and ask your solicitor or the auctioneer to follow up on this before you make any offer.
mf