That's obviously a worry, but we'll have an emergency fund ready for unforeseen issues such as a boiler failing or leak in the house, but saving for the extension would still hang over our heads until resolved.
Our mortgage advisor confirmed that HSBC will not allow us to keep the original deal at £274k if the price was dropped to £250k.
As a result of the change to the mortgage deal and the fact we can now use my Help to Buy ISA bonus (as it's £250k), we'll have an extra £11k capital available. Unfortunately this still leaves us a good £20k short of what it will cost to build a new extension and fit a kitchen. So in answer to your question, no we do not have the capital available and it will take some time to gather it (12-24 months at least).
It is a nice house and it's in a lovely area close to local amenities, schools and transport links, but it does not have a USP. It's like any other semi in the area with an extension.
Obviously a question for our solicitor in the morning, but how does that work? I understand the concept of it, but does that not cause issues with the lender? The house isn't worth £274k but we pay £274k regardless, then £24k is held back and paid to us directly after settlement?