Shan
You do realise that you would have to advise your current mortgage company of the works as they own 50% of the property currently. They might not give you permission to carry out the works or you to take out any loan, if secured against the property.
So your first step is to talk to your mortgage company about this, so they can inform you of the options.
In regard to obtaining any mortgage for any works, the loan may be staged.
If your mortgage company are not helpful, you should talk to an independent mortgage broker, to see what the most economical way to finance this would be.
Huckster