The Postnuptial Agreement After a Funding Round or Exit: Protecting Equity in California
Quick Answer: A postnuptial agreement is a contract spouses sign after they are already married to define how their property, equity, and support will be handled. Founders reach for one when they skipped the prenup and their equity became valuable later, at a funding round, an acquisition, or an IPO. California law allows it, but a postnup is harder to enforce than a prenup, not easier. Because married spouses already owe each other a fiduciary duty of the highest good faith, any agreement that shifts property in one spouse's favor is presumed to result from undue influence, and the spouse who benefits has to prove it was fair. To hold up, a postnup has to be in writing, contain an express declaration changing the character of the property, rest on full financial disclosure, and ideally be negotiated with independent counsel on both sides. If your equity grew after the wedding and you want to protect it, contact The Geller Firm at (415) 840 0570 for a confidential consultation.
What Is a Postnuptial Agreement?
A postnuptial agreement, sometimes called a postnup or a marital agreement, is a written contract between two people who are already married. It covers the same ground a prenup would, how property is characterized, how debts are allocated, and in some cases how spousal support is handled, except it is signed after the wedding rather than before it.
California law expressly allows spouses to contract with each other. Family Code section 1500 provides that "the property rights of spouses prescribed by statute may be altered by a premarital agreement or other marital property agreement." A postnup is that other marital property agreement. There is no single postnuptial agreement statute. Instead, enforceability is built from three sources working together: the authority to contract under section 1500, the transmutation rules under sections 850 through 853, and the fiduciary duty between spouses under section 721.
That last piece is what makes a postnup a different animal from a prenup, and it is the part founders most need to understand.
Why Founders Reach for a Postnup After a Funding Round or Exit
Most founders who need a postnup never set out to skip the prenup. They simply married before the equity was worth talking about. The company was an idea, the cap table was a formality, and a prenup felt premature. Then the business worked.
The moment the equity becomes real is usually a specific event:
A priced funding round. A Series A or later round puts a defensible valuation on the company for the first time, and suddenly the founder's stake has a number attached to it that a court could divide.
An acquisition or a tender offer. A liquidity event converts paper equity into cash or acquirer stock, and the community property character of that money is decided by the marital timeline.
An approaching IPO. As a public offering nears, the equity becomes both valuable and visible, and the window to characterize it cleanly is closing.
At each of these moments the founder realizes the default rules are now expensive. Under California's community property system, the growth in a separate property business during the marriage may belong partly to the community, and equity granted or vested during the marriage is generally community property. A postnup is the tool for getting ahead of that before separation turns it into a forensic accounting fight. For the underlying mechanics, our blog covers how founder equity is treated, how RSUs and stock options are divided, and what happens to equity in an IPO or acquisition.
Why a Postnuptial Agreement Faces More Scrutiny Than a Prenup
This is the part that surprises people. A prenup is signed by two people who are not yet married and are still dealing with each other at arm's length. A postnup is signed by two people who already owe each other the highest duty the law recognizes between private parties.
Under Family Code section 721, spouses stand in a fiduciary relationship. The statute imposes "a duty of the highest good faith and fair dealing on each spouse, and neither shall take any unfair advantage of the other." That same duty that protects a spouse from being cheated also raises the bar on any agreement that moves value from one spouse to the other.
The practical consequence is a presumption. When one spouse gains an advantage from a transaction with the other, California law presumes the transaction resulted from undue influence. The burden then flips to the spouse who benefited to prove the agreement was entered freely and voluntarily, with full knowledge of its effect, and after complete disclosure. A prenup carries no comparable presumption. So the founder, who is usually the spouse the postnup protects, is also the spouse who will have to defend it later. That is not a reason to avoid a postnup. It is the reason to build it carefully.
Note one thing the postnup does not carry: the seven calendar day waiting period that governs prenups under Family Code section 1615 does not apply to postnups. The fiduciary scrutiny under section 721 replaces it, and it is arguably the tougher standard.
What California Law Requires for a Valid Postnuptial Agreement
A postnup that protects equity almost always changes the character of property, which means it has to satisfy the transmutation rules on top of ordinary contract requirements.
It must be in writing with an express declaration. Under Family Code section 852, a transmutation of property "is not valid unless made in writing by an express declaration" accepted by the spouse whose interest is adversely affected. Vague language will not do it. The document has to state clearly and unambiguously that the character of the property is being changed. Oral agreements between spouses about who owns what are not enforceable.
It must rest on full financial disclosure. The section 721 fiduciary duty requires each spouse to give the other true and full information about anything affecting their shared property, including access to the books and records. For a founder, that means disclosing the cap table, the valuation, recent grants, and the real financial picture. As with a prenup, disclosure is not the risk. Concealment is. An undisclosed asset is the fastest way to lose the whole agreement later.
It must be voluntary and fair. Both spouses have to enter the agreement freely, without coercion or duress. Because of the undue influence presumption, the spouse who benefits should be prepared to show the other spouse understood the agreement and signed it of their own free will. Courts also look at whether the terms are unreasonably one sided.
Both spouses should have independent counsel. The law does not always require it, but given the fiduciary scrutiny, a postnup negotiated with separate lawyers on each side is far easier to defend. One lawyer cannot represent both spouses, and a support provision in particular is vulnerable without independent representation.
What a Founder's Postnup Should Address
Like a prenup, a generic postnup template is not built for equity. The agreement that actually protects a founder is specific:
Characterization of the now valuable equity. State expressly that your founder shares and your interest in the company are your separate property, with the language meeting the section 852 express declaration standard.
Treatment of future grants and vesting. Decide how RSUs, options, and additional grants going forward will be characterized, so the next several years of vesting do not reopen the question.
Appreciation of the business. Address the growth in the company's value during the marriage, which the default Pereira and Van Camp formulas would otherwise apportion to the community.
Business control and the cap table. Confirm your spouse will not acquire a voting interest, a management role, or an ownership claim through the marriage, which protects your cofounders and investors as much as you.
Spousal support. If support is addressed, do it with independent counsel on both sides given the heightened scrutiny.
Separate and community debts. Allocate responsibility for debts, including anything tied to the business.
Each of these turns on California's default characterization rules and on the date of separation, which is the line that cuts off the community estate. Getting them right is part legal drafting and part financial modeling.
Common Mistakes That Make a Postnup Unenforceable
Assuming a deed change does the job. Adding your spouse's name to a title, or removing it, does not transmute property on its own. Without an express written declaration meeting section 852, the change in character does not happen, no matter what the deed says.
Skipping disclosure. Understating the equity or leaving an asset off the table breaches the section 721 duty and hands the other spouse a clean argument to void the agreement.
Letting the advantaged spouse run the process. When the spouse who benefits drafts the agreement, picks the lawyer, and presents it for signature, the undue influence presumption gets stronger, not weaker. Independent counsel and an unpressured process are how you rebut it.
Using vague language. A transmutation that does not clearly state the property is changing character can fail even when both spouses meant it to work.
Treating it like a prenup. The seven day rule does not apply, but the fiduciary standard does, and it is less forgiving. A postnup needs more care on disclosure and fairness, not less.
Postnup or Prenup: Which Do You Need?
If you are not yet married, the prenup is the cleaner tool, signed before the fiduciary duty attaches and free of the undue influence presumption. We cover that in our companion post on founder prenuptial agreements. If you are already married and the equity has become valuable, the prenup window is closed and the postnup is your option. It is harder to enforce, but a properly built postnup still does the job, and it is far better than leaving a growing equity stake to the default rules.
Why the Financial Modeling Matters as Much as the Legal Drafting
The clauses that carry the most value in a founder's postnup, characterizing equity that now has a real valuation, handling future grants, and addressing business appreciation, are financial questions before they are legal ones. You cannot draft them well unless you can read the cap table, model the vesting, and see what each version of the agreement actually does to your equity across different outcomes.
That is the lens we bring. With a JD, an MBA, and the ability to work through a cap table and a 409A valuation, we draft the legal terms and model the financial result together, so you understand what you are agreeing to before you sign rather than after a forensic accountant reconstructs it in litigation. You should be able to model your own numbers, and when equity has just become valuable, doing that work now is what protects it.
Frequently Asked Questions
Is a postnup really enforceable in California?
Yes, when it is done right. California allows marital property agreements under Family Code section 1500, and courts enforce postnups that satisfy the transmutation rules, rest on full disclosure, and are entered voluntarily. The catch is the fiduciary scrutiny, which makes careful drafting essential.
Why is a postnup harder to enforce than a prenup?
Because married spouses already owe each other a fiduciary duty under section 721. Any agreement that benefits one spouse is presumed to result from undue influence, and the benefiting spouse has to prove it was fair. A prenup carries no such presumption.
Can a postnup protect equity that became valuable after we married?
Yes, and that is its most common use for founders. The agreement can characterize your equity and your business interest as separate property going forward, provided it meets the express declaration standard and rests on full disclosure.
Does the seven day waiting period apply to a postnup?
No. The seven day rule lives in the premarital agreement statute and applies to prenups. A postnup is governed instead by the fiduciary duty and transmutation rules, which are arguably a higher bar.
What can a postnup not do?
It cannot bargain away child support or dictate child custody, which are decided by the child's best interest regardless of any agreement between the parents. It also cannot include terms that are illegal or against public policy.
Speak With a California Family Law Attorney
If your equity grew after the wedding, a postnuptial agreement is the tool for protecting it, but it has to be built to survive the scrutiny California applies to agreements between spouses. That means an express written declaration, complete disclosure, independent counsel, and equity provisions that are modeled, not just drafted. The Geller Firm represents founders, executives, and high net worth professionals across California in postnuptial and premarital agreements involving equity, business interests, and complex financial planning. We offer confidential virtual and in person consultations from our Walnut Creek office.
Call (415) 840 0570 or contact us online to schedule your consultation.