The answer is in your post:
establishing clear title
The solicitor, who is also acting for the bank, must give them a correct title, this means that extensions have conformed to the planning laws. You ahve to hire an expert to get you one. You will not be able to get the mortgage if you don't.
If you were a cash buyer on the other hand, and you felt you didn't think the extensions would ever cause you a problem, you could buy the property as is, as long as you agreed this with your solicitor.
Your solicitor is acting in your best interest.
( I thought it was 12 years the limit before the co.council could force you to take something down if not in compliance.