California Code, Probate Code - PROB § 146

(a) As used in this section, “agreement” means a written agreement signed by each spouse or prospective spouse altering, amending, or revoking a waiver under this chapter.

(b) Except as provided in subdivisions (c) and (d) of Section 147, unless the waiver specifically otherwise provides, a waiver under this chapter may not be altered, amended, or revoked except by a subsequent written agreement signed by each spouse or prospective spouse.

(c) Subject to subdivision (d), the agreement is enforceable only if it satisfies the requirements of subdivision (b) and is enforceable under either subdivision (e) or subdivision (f).

(d) Enforcement of the agreement against a party to the agreement is subject to the same defenses as enforcement of any other contract, except that:

(e) Subject to subdivision (d), an agreement is enforceable under this subdivision unless the party to the agreement against whom enforcement is sought proves either of the following:

(f) Subject to subdivisions (d) and (g), an agreement is enforceable under this subdivision if the court determines that the agreement at the time of signing made a fair and reasonable disposition of the rights of the spouses.

(g) If, after considering all relevant facts and circumstances, the court finds that enforcement of the agreement pursuant to subdivision (f) would be unconscionable under the circumstances existing at the time enforcement is sought, the court may refuse to enforce the agreement, enforce the remainder of the agreement without the unconscionable provisions, or limit the application of the unconscionable provisions to avoid an unconscionable result.

(h) Subdivision (b) of Section 721 of the Family Code does not apply if the agreement is enforceable under this section.