Mortgage on property with planning permission

MWT

se2020 said:

A 20% deposit would be ideal

I'd talk to the EA and 100% make sure there is no intention to add an overage clause to the contract, then make a rational assessment of the likely current valuation without the planning permission then see how big the gap is...

It doesn't sound like there is any practical value to you in having the planning permission, so if the vendor has used this to bump the price up you need to think hard about just how much the place is really worth to you...