Value of commercial planning permission

I am considering buying a terraced house to live in without using an auctioneer as I know the guy selling the property. We will obviously get the property independently valued so as to agree a selling price but I just wanted to get some opinion as to what people think of the situation.

There is a house for sale not too far away (on the same terrace) but it has planning permission for a GP surgery. How much extra (in % terms) do buildings with this type of planning sell for over and above a normal residential property?

You are not comparing like with like. If it has planning for a GP surgery it cannot be used for residential purposes. Supply and demand will always determine prices so it really depends where the building is located.