Not enough info provided to give you a definite answer. The only thing I can provide is that everything should have been agreed to in a written document that you and the builder each agreed to and signed (such as a sales agreement for new construction). Any changes to the original agreement should be in an addendum as you mentioned. However, the builder may have the ability to do this due to the language used in the agreement. If you did not elect to have a Realtor represent your interests, at the very least I hope you had an attorney review the documents prior to signing them and/or you fully understood the terms. I would recommend that you consult your attorney.

Are you not working with a real estate agent to protect YOUR interest? You will need to get with a real estate attorney for your question.

Terrence Charest, e-Pro