If it is an unauthorised construction then you must apply for retention.
If it has been in situ for x years (depending on local authority bye-laws I think) then retention is normally "automatic", but, without documented permissions, can you prove when it was built and how long it is there?
A buyer having a survey done will ask to see proof of compliance with planning bye-laws, grants of permission, etc.
How did your surveyor, solicitor, engineer miss this or did you keep them in the dark?
You will need a surveyor / engineer / architect to draw up plans and submit the retention application. Then you just wait for a decision, and wait, and wait....
BTW, if this were a "temporary structure", i.e. a wooden shed on blocks in the back garden, it would be different, but this is attached to the fabric of the dwelling-house.