Unfortunately until you both sign the contract it is either partiesâ€™ choice at anytime to modify and accept or alter the conditions of the contract. One option might be that you put a limit on him say 5% increase. Also, suggest to him that if cost went down would he be willing to reduce the price. Do you have a realtor? If so, he/she should be able to negotiate this issue for you. If not put yourself in the builders position. Trying to understand his point of view will help you negotiate stronger. Know also that to truly negotiate with full power you have to be willing to walk away from the deal. That being saidâ€¦ I still think however it is perfectly reasonable for you to expect him to show you the cost increase. The reason, I am guessing here, that he doesn't want to show you the increase is that would reveal his profit margin. Builders hold this very close to their chest for obvious reasons. Try setting a cap on how much he can increase cost on you as I mentioned above. I personally would walk away if he isn't willing to justify the increase in costs. I am guessing your view is something like this; If you buy a car and it is a factory order, you pay in full and they deliver. If there is a strike in the factory or the railroads explode or whatever other cost occur to the manufacturer, well that is the cost of doing business. BUT with homes, the builder is paying as he goes and more than likely he is caring a construction loan with an aggressive bank asset manager breathing down his neck, so if he agrees to build your home in a year and it is for 1.2 million then all of a sudden there is a weevil attack on the lumber industry and his cost double he could lose everything. Not really a solution but maybe it will help you in seeking an arbitration point. He wants to build the house for you, that is his business, he just doesn't want to lose his shirt doing it. With the current economic fear going on he is just probably trying to protect himself. Suggest that he put a cap on costs and maybe add a clause that if the cost go down he will reduce the price... Let me know what you decide. I will also check with my partner and see what she thinks. She has several years working on the home builderâ€™s side of the deal. She will respond soon I am sure.