"Also instruct your solicitor to undertake to apply for the planning and forward this to your lender. "
I assume you mean instruct the solicitor to tell the lender that the borrower will undertake to apply for planning permission. The solicitor will not give that kind of undertaking.
"If it more that 6 (or 8?) years old then there should be no problem. That is the 'statute of limitations' on buildings with no planning. We had a similar problem with a house we bought which had a conservatory on the side of the house that had no planning. However, it was more than 6 (or 8 I can't rememebr which) years old so the bank was ok with issuing the mortgage."
This is not great advice. After X years, the local authority can no longer compel someone to pull down an unauthorised development BUT this never makes the development either exempt or legal. It is still an unauthorised development and technically in an insurance claim it is not covered.
To do things right, there should be a signed contract with a clause requiring the vendor to get retention, an application for retention, at the expense of the vendors and the sale should not close until that is done. That is the right way to do things. Anything else ( negotiate loan cheque on terms from lender, close, retain monies and apply for retention) is fraught with potential problems.
If it is the dream home far better to get it right now even if it does mean a few months delay.
mf