You need to look at the contract you have with the builder. You should have gotten it in writing. You do have one don't you? I can't imagine spending 6 figures ($) for a house without a written agreement with every last detail negotiated at arm's length and being fully read and understood before signing. Don't suppose you had your lawyer help? Contact her or him. Builders usually spell out costs for change orders in their contracts. I can't really comment on a contract that I can't see and read. Change orders are usually more expensive and I bet now you wish you would have planned your selections better with your interior designer when you started.
Ever hear of cost overruns on all these government contracts that run into millions? They usually involve change orders.
Post back with specifics about your contract. If he hasn't started building yet, then you may want to change builders. Good luck.