Quick Answer: A postnuptial agreement is a written contract between spouses, signed after the wedding, that changes how their property is owned or will be divided. California allows it: Family Code section 1500 says spouses can alter their statutory property rights by a "marital property agreement." But a postnup is judged more strictly than a prenup. Spouses owe each other fiduciary duties under section 721, so if the agreement gives one spouse an advantage, courts presume undue influence and the advantaged spouse must prove the agreement was signed freely and with full knowledge. Any change in the character of property must also be in writing with an express declaration (section 852). Full disclosure and a separate lawyer for each spouse are the best protection.
Couples come to postnups for many reasons. Some never signed a prenup and later acquired a business, an inheritance, or a home bought with one spouse's savings. Some want to settle questions after a rough patch. Others signed a prenup that no longer fits their lives. Whatever the reason, the rules that apply after the wedding are different from the rules before it. This post is the general guide. For the founder and equity version, see our post on the postnuptial agreement after a funding round or exit.
What a Postnup Can Cover
Most postnups deal with property. Common terms include:
Characterizing specific assets as separate or community, such as a business, a rental property, or an inheritance.
Keeping future earnings or growth separate, for example the appreciation in a business one spouse owned before marriage.
Allocating debts, including debts tied to one spouse's business or student loans.
Converting separate property to community property, or the reverse, which is called a transmutation.
Setting out how an asset will be divided or valued if the marriage ends.
Family Code section 2550 requires an equal division of community property at divorce "except upon the written agreement of the parties," so a valid postnup can change the default 50/50 result.
What a Postnup Cannot Do
Some limits come straight from the statutes and case law:
Child support. Parents cannot bargain away a child's right to support. Section 3585 says a child support order based on an agreement is still imposed by law, and section 4065 bars the court from approving below-guideline child support unless the parents make specific declarations.
Penalties for misconduct. In Diosdado v. Diosdado (2002) 97 Cal.App.4th 470, the Court of Appeal refused to enforce an agreement between spouses requiring a $50,000 payment if one of them was unfaithful. The court held that a fault-based penalty conflicts with California's no-fault divorce policy.
Changing marital duties other than property. Section 1620 provides that, except as otherwise provided by law, spouses cannot by contract with each other alter their legal relations, except as to property.
Spousal support terms need particular care. Section 3580 expressly allows spouses who agree to an immediate separation to provide for support, and our post on buying out spousal support covers support terms in settlement. A support waiver or limit in a postnup signed while the spouses intend to stay married sits closer to the section 1620 line and should be drafted and reviewed with that in mind.
Why Postnups Get More Scrutiny Than Prenups
Before the wedding, two people negotiate at arm's length. After the wedding, they are fiduciaries. Section 721 imposes "a duty of the highest good faith and fair dealing on each spouse," and neither may "take any unfair advantage of the other." The same section requires each spouse, on request, to give the other true and full information about anything affecting a transaction that concerns community property.
That duty drives the key rule. In In re Marriage of Haines (1995) 33 Cal.App.4th 277, the Court of Appeal held that when a transaction between spouses gives one of them an advantage, a presumption of undue influence arises. The spouse who benefits then has to show that the other spouse acted freely and voluntarily, with full knowledge of the facts and a complete understanding of the effect of the transaction. Our post on the fiduciary duties spouses owe each other explains section 721 in more depth.
The Writing Requirement for Changing Property
A postnup that changes the character of property is a transmutation. Section 852(a) says a transmutation "is not valid unless made in writing by an express declaration" that is made, joined in, consented to, or accepted by the spouse whose interest is adversely affected. In In re Marriage of Valli (2014) 58 Cal.4th 1396, the California Supreme Court held that putting a life insurance policy bought with community funds in one spouse's name did not make it that spouse's separate property, because there was no express written declaration. A transmutation of real property also has to be recorded to be effective against third parties without notice (section 852(b)). Our post on Family Code section 852 covers the transmutation rules.
The practical lesson: a postnup should say in plain words which property changes character and from what to what. Vague language about "keeping things separate" invites a dispute.
Postnup or Prenup: Key Differences
Timing rules. The seven-day review rule in Family Code section 1615 applies to premarital agreements. It does not apply to postnups. See our post on the 7-day rule for California prenups.
Burden of proof. A spouse challenging a prenup generally carries the burden under section 1615. With a postnup that favors one spouse, the burden can shift to the spouse who benefits.
Disclosure. Both need honest financial disclosure in practice, but a postnup is measured against the fiduciary duty to disclose that already exists during marriage.
If you are not yet married, a prenup is usually the stronger tool. Our prenuptial agreements page explains the process.
How to Make a Postnup Hold Up
Exchange full financial disclosure in writing, with account statements, valuations, and tax returns attached or referenced.
Give each spouse a separate lawyer. No statute requires it for postnups, but independent advice is the most direct way to answer a presumption of undue influence.
Allow real time to review. Pressure and short deadlines undercut the showing that the agreement was signed freely.
State the transmutation expressly for each asset whose character changes, and record any deed that changes title.
Explain why the agreement is fair to both spouses, especially if one spouse gives up more than the other.
Update the estate plan so titles, trusts, and beneficiary designations match the agreement.
What a Postnup Costs
There is no standard fee. The cost depends mostly on how many assets need to be characterized or valued, whether a business valuation is involved, and how much negotiation the terms require. Each spouse paying for their own lawyer adds cost up front, but it is also what makes the agreement defensible later. A postnup that a court sets aside can cost far more than the agreement itself.
Key Takeaways
Postnups Are Allowed
Family Code section 1500 lets spouses change their statutory property rights by a written marital property agreement.
Advantage Triggers a Presumption
If a postnup favors one spouse, courts presume undue influence and that spouse must prove the agreement was freely made.
Use an Express Declaration
Any change in the character of property must be in writing and expressly say so under Family Code section 852.
Child Support Stays Protected
Parents cannot bargain away a child's right to support, and fault-based penalty clauses are unenforceable.
Frequently Asked Questions
Are postnuptial agreements enforceable in California? Yes, if they meet the requirements. A postnup must be in writing, any transmutation must be expressed clearly under section 852, and if the agreement favors one spouse, that spouse must be able to rebut the presumption of undue influence.
Does a postnup that changes title to a house have to be recorded? Between the spouses, the written express declaration is what makes the transmutation valid. Under section 852(b), a transmutation of real property is not effective against third parties without notice unless it is recorded, so the deed or agreement should be recorded promptly.
Can a postnup include an infidelity clause? A clause that imposes a financial penalty for infidelity is unlikely to be enforced. In Diosdado v. Diosdado, the Court of Appeal held that a $50,000 infidelity payment between spouses violated California's no-fault policy.
Can we sign a postnup if we already have a prenup? Yes. Under Family Code section 1614, a prenup can be amended or revoked after marriage only by a written agreement signed by both spouses. Because the later agreement is signed during marriage, the fiduciary duty and presumption rules for postnups apply to it.
A postnup is a contract between fiduciaries, and it is only as strong as the disclosure and process behind it. To talk with Attorney Michael Geller about a postnuptial agreement, call The Geller Firm at 415-840-0570 to schedule a consultation.