We have made an offer on a house already under construction. Exterior walls are up and interior studs set. We made the offer with a flooring upgrade and asked that the jacuzzi tub (not installed) be replaced by a closet for more bathroom storage. Builder has responded to the offer but wants payment in advance ($16K) for the "changes". Builder does not have a long history and has only built "a few" houses in the past. I'm reluctant to pay him this money before closing because if he does not complete the house, I'm doubtful I'd get the money back. I also think his upgrade price is way high. Is asking for advance payment on upgrades common?