The credit union might be a better option ( assuming your a member ) . They are usually more willing to help with home improvement projects, whereas banks at the moment are a bit like getting blood out of a stone.
Set up a standing order for regular savings into an account for the next few months to build up a good record, and work on that credit card debt . You can usually borrow 5 times your shares, so if you need 25k for extension you'll need 5k saved with them. I find them much more flexible with repayments, you can increase/decrease/clear off without any penalties. Plus you have the option of top-ups in future once you have good repayment history.
Plus I've never been asked for bank statements etc when applying for loans.