We had a similar challenge  - had planning in place to knock down and rebuild but only thought to mention it to the lender (High St supermarket brand) a few months before commencing.   We had signed a 5 year fixed deal with a hefty repayment penalty the year previously.   We'd ruffled a few neighbourhood feathers with our plans (funny how they all love it now it's finished and landscaped) so were worried that someone would dob us in.   Anyway, called the lender and