You should ask about any changes now, before you are legally committed to the purchase.
I doubt the housebuilder would be prepared to change the driveway until, you have exchanged contracts but you should be talking about it now as it may be 1) Too expensive for you 2) Not permitted under planning.
Whatever you are able to change it will cost you quite a bit more than the work costs the builder and you wont be able to add the cost to your mortgage!