Mortgage top-up for extension

Hi all,
Not sure if I'm posting in the right forum, but thinking about doing an extension next year.
Currently have a Tracker with UB and the loan outstanding is about 270K. Repayments are 1300 a month. My question is, is it possible to apply for a top-up on a mortgage - I gather this would have to be at a variable rate - but I don't want to risk losing my tracker and have to re-mortgage. I want a separate loan to cover costs of extension.

House is probably currently valued at 330K - what would I be likely to borrow? Is the LTV calculated at the current value of the house, or the house value after the extension has been completed?

Thanks in advance!

Max top up usually to 80% LTV, may or may not get it as a separate loan. If you want to go with an increased value based on extension and get 80% of that then pretty sure they will insist on a whole new mortgage, especially if there are construction works that require planning or regulations certification.

Thanks wbbs...in that case, credit union loan could be a better route