Something doesn't make sense here.
Could you have afforded the repayments for the additional loan amount within the landers parameters for borrower's?
If you cold have then I would like to know more, not least to advise the Financial Darwinists on AAM and elsewhere of what the financial institutions in this country are doing to existing potential borrowers.
This nonsense of them not lending money to borrowers who can repay has GOT to stop!
If you were able to pay more, there are other forums to which you can post on AMM.
If you feel you were unfairly refused the additional amount please let's get this information out there.
If you post elsewhere on AAM use a different wording with a different query so it won't be deleted for doubling posts.
=========================================
That having been said...
The first matter is paperwork - Contract and Certificates and Planning.
Unless your bedroom is wholly to the rear you cannot begin to claim that it is exempted development.
Even if it is to the rear, there may be other issues which mean you require permission for the new bedroom.
Also, your Conditions may say something like "the development to be carried out in accordance with the plans and particulars lodged".
If they do, you are obliged to finish the house substantially before you commence work on the foundations of the new bedroom - most people don't.
You appear to have had a good working relationship with your contractor.
If he understands your position and is paid to date, he may be willing to terminate the contract mutually.
He may also be willing to offer you a builder's certificate confirming that the work is in substantially compliance with the building regulations, as well as plumbers certs and the electricians cert.
What I don't understand is why the builder is leaving such an unfinished house.
ONQ.
[broken link removed]
All advice on AAM is remote from the situation and cannot be relied upon as a defence or support - in and of itself - should legal action be taken.
Competent legal and building professionals should be asked to advise in Real Life with rights to inspect and issue reports on the matters at hand.