California Code, Civil Code - CIV § 2985.6

(a) A buyer shall be entitled to prepay all or any part of the balance due on any real property sales contract with respect to the sale of land which has been subdivided into a residential lot or lots which contain a dwelling for not more than four families entered into on or after January 1, 1969;  provided, however, that the seller, by an agreement in writing with the buyer, may prohibit prepayment for up to a 12-month period following the sale.

(b) Any waiver by the buyer of the provisions of this section shall be deemed contrary to public policy and shall be unenforceable and void;  provided, however, that any such waiver shall in no way affect the validity of the remainder of the contract.