Ryan is correct you will have to take your contract and speak with a real estate attorney. It is the wording of the contract that will determine what you are able to do. you may end up having to forfiet some of the deposit. The sooner you act though the better. I am guessing that you had to put so much down because of upgrades. So the sooner you stop the builder the better able he will be to not put in your color choices and upgrades. This will make it easier for them to sell.
Coldwell Banker Triad
If you have anymore questions regarding real estate in the Solano area give me a call. Ryan Vivo, Real Estate Agent, Gateway Realty. 707-384-5894