Mortgage increase for extension

[edition]

Hi,

We are wanting to do an extension on our current house and would need to borrow the majority additionally to our current mortgage.

At the moment we have roughly 71% L.T.V and the house is maybe worth 195k. The extension has a rough cost of 35k, meaning roughly 90% L.T.V. if based on pre extension price. Do they do it on pre or post extension?

Thanks in advance for any help, I will ring our provider up but with it being Sunday they are shut.

[edition]

Thanks for the reply. Yes I'm hoping the extension will add at worst the money we put into it so we could keep our L.T.V. figure that I'm happy with....