Postnuptial Agreement in California: Why It Is Held to a Higher Standard Than a Prenup

By Aaron Thomas, Esq. · Updated October 28, 2026

California enforces postnuptial agreements, but holds them to a stricter standard than prenups. Spouses owe each other a fiduciary duty under Family Code § 721, and changing the character of property takes the express written declaration that § 852 demands.

Key Takeaways

• California enforces postnuptial agreements, but they are governed by different and stricter rules than prenups: the spousal fiduciary duty in Fam. Code § 721 and the transmutation rules in § 852.
• Because spouses owe each other a fiduciary duty, a postnup that benefits one spouse can face a presumption of undue influence, which makes the fairness of the process and the record behind it central.
• Under § 852, changing property between separate and community generally takes a writing with an express declaration by the spouse whose interest is adversely affected. General language may not be enough.
• Independent counsel for each spouse is strongly advisable, and a postnup can be signed at any point in a marriage.
• At Prenups.com, a California postnup is a flat $6,500, with no hourly billing.

Does California Enforce a Postnuptial Agreement?

Yes. California courts enforce postnuptial agreements, which define what belongs to each spouse and what happens to assets and debts if the marriage ends. But a postnup is not governed by the same rules as a prenup. The premarital agreement statute, Fam. Code § 1600 and following, is written for agreements signed before the wedding. Postnups are governed by two different parts of the Family Code: the fiduciary duty spouses owe each other in § 721, and the transmutation rules in § 852.

A postnup can be signed at any point in a marriage, a few weeks after the wedding or decades in. Couples sign them after an unfinished prenup, when a business grows, when money has become a recurring source of conflict, or to put in writing what they have always assumed. California sets no deadline. It sets a high bar for how the agreement is made.

Why a Postnup Is Held to a Different Standard

The voluntariness rules that make California prenups demanding, such as the independent counsel requirement and the seven-day waiting period, are written for premarital agreements. A postnup does not escape scrutiny by falling outside them. It faces scrutiny of a different kind, one that grows out of the marriage itself.

Two people planning a wedding bargain at arm’s length. Two people who are already married do not. One may handle the finances while the other trusts that things are fine. California’s rules for postnups are built around that reality, and they are stricter for it.

The Fiduciary Duty in Family Code Section 721

Under § 721, spouses owe each other the duties of a fiduciary, a duty of the highest good faith and fair dealing, including full and honest disclosure of material facts in transactions between themselves. A postnup is exactly that kind of transaction.

The practical consequence is a presumption. When a postnup gives one spouse an advantage over the other, a presumption of undue influence can arise, and the burden can shift to the spouse who gained the advantage to show the agreement was fair and freely made. That is why the record matters so much: complete disclosure, time to review, separate counsel, and terms that are fair on their face. An agreement built without that record gives the advantaged spouse nothing to answer the presumption with.

Transmutation: Why Wording Matters Under Section 852

Section 852 governs transmutation, which is the legal term for changing property’s character between separate and community. The rule is demanding. A transmutation generally is not valid unless it is made in writing with an express declaration by the spouse whose interest in the property is adversely affected.

That standard shapes how a postnup is drafted. A sentence like “all property is shared” or “everything stays separate” may not be enough to change anything. Each classification the couple wants should be stated clearly, and the spouse giving something up should be the one making the declaration. Vague language is one of the most common ways a well-intentioned postnup fails to do what the spouses meant.

Why Reclassification Is Often the Point

California is a community property state, which means income either spouse earns during the marriage is shared by default. That default is predictable, but it does not keep separate property separate on its own. A spouse who owns a premarital asset and keeps funding it from a post-wedding paycheck can create a community interest without the other spouse ever contributing.

For many couples, that is exactly what a postnup is for. It can state what each spouse agrees is separate and what is shared, settling the question on purpose instead of by drift. Section 852 is what makes the drafting precise, because reclassification only works when it is stated expressly.

Building a California Postnup That Holds Up

Both fiduciary duty and transmutation point to the same checklist. Each spouse gives full, current financial disclosure, backed by documents. Each has their own attorney, which is not required but is the clearest way to show each person understood the terms. Both have enough time to read, ask questions, and decide, and nobody signs under an ultimatum. Every reclassification is stated expressly, and the terms are fair enough that the advantaged spouse can explain them.

No agreement can promise how a particular court will rule, and anyone who says otherwise is overselling. What a careful California postnup can do is give both spouses the strongest record of a transaction that was honest, informed, and freely chosen. A postnup that is enforceable in court in California is built that way from the first draft.

Getting a California Postnup

The process begins with a consultation to understand your situation and goals, followed by full financial disclosure from both spouses, drafting with express language for every reclassification, a round of review with each spouse’s own counsel, and signing when you are both ready. At Prenups.com, a California postnup is a flat $6,500. No hourly billing. For the full picture of what is included, see the flat-fee pricing. For couples who are still before the wedding, California’s seven-day rule and what a California prenup costs are covered separately.

Want a California postnup on a flat fee? Every California agreement is prepared by our attorneys, including a California-licensed attorney as counsel of record, from the first draft through signing. You can schedule a consultation to talk through whether a postnup fits your situation. For the broader picture of how postnups work, see the full guide to postnups.

Frequently Asked Questions

Sources

• Cal. Fam. Code § 721 (spousal fiduciary duty)
• Cal. Fam. Code § 852 (transmutation)
• Cal. Fam. Code § 1600 et seq. (Premarital Agreement Act)