It would be highly unusual to offer to buy a house subject to planning permission to extend a kitchen. More often, you buy the house and you take the risk. Unless it is a big extension, it is likely to be exempt from pp anyway.
However, in saying all that, and if pp would be needed, if there is no-one else interested, then a vendor may consider doing so. It would however be a matter of negotiation as to who would make the application. There are risks for both sides in the matter and you should talk to your solicitor as to how best to achieve the result you want.
mf