Help! Can I remortgage for building works?!

Hi everyone,

I was hoping to get some advice from anyone who's 'bought the t-shirt' on this one...

I own a 1 bed flat and want to extend it into a 2 bed flat. I have an 85% mortgage but no money to do the building works. The problem is this...

After doing the work, the increase in value would easily pay for the building work twice over (it's in London), but is it possible to remortgage for more than your home is currently worth? Even though the work you intend to do to it would easily recoup all costs? I estimate after the works done I would only need a 75% mortgage.

Any advice would be much appreciated.

Thanks!