The Snag list is normally done, when the builder says the house is finished, i.e. the kitchen, fireplace is in, painting is finished. It would be recommended that you get a professional to do this for you, as it is amazing what you could miss, and you will never get the builder back once money has exchanged hands. Also the builders will nomally not allow snagging until after the contracts have been signed.
In relation to the extras these also will ot normally be completed until contratc are signed, as you and they can pull out at any stage before then.
The best of luck......