Transmutation Agreements in California: best practices

In California, spouses can change the character of property — from separate to community, or vice versa — through a written agreement known as a transmutation. The relevant statutes make this seem straightforward. Family Code section 850 permits transmutation by agreement or transfer, and section 852 sets out the formal requirements: the change must be made in writing, contain an express declaration, and be accepted or consented to by the spouse whose interest is adversely affected.

In practice, enforceability often turns on a much deeper question:

Did the disadvantaged spouse know exactly what they were giving up, and why?

This question becomes especially important when the transaction results in one party walking away with more money, more property, or a better long-term financial position than the other. The moment one spouse receives a material benefit from the transaction — such as sole title to a home — Family Code section 721 triggers the presumption of undue influence.

At that point, it’s not enough to produce a signed deed or agreement. The advantaged spouse must affirmatively prove that the disadvantaged spouse acted with full knowledge, voluntary intent, and a complete understanding of the legal effect of the change. Without that showing, the transaction may be set aside, even if it otherwise complies with section 852.

What the Case Law Emphasizes

The foundational cases (e.g., Estate of MacDonald, Valli, Benson, Brace) have made clear that:

• A valid transmutation must contain express language showing a clear intent to change ownership or character of the property.

• You can have a valid transmutation but still fail the test of whether there was undue influence.

• Evidence Code section 662 (presumption based on title) does not override the presumption of undue influence under Family Code section 721 in marital transactions.

• When the facts suggest an imbalance — such as a quitclaim deed signed without consideration — the burden shifts to the advantaged spouse to prove that the other party knowingly and voluntarily agreed.

Courts have consistently applied the undue influence presumption in cases where property was transferred without reciprocal benefit. In these cases, the failure to show informed consent, even with a signed deed, was fatal to enforcement.

Drafting and Procedural Safeguards

To protect a transmutation from challenge, it is not enough to meet the statutory elements. Counsel should ensure that the record demonstrates the disadvantaged spouse:

• Had full and accurate disclosure of the property’s nature and value

• Understood the legal and financial consequences of the change

• Was not under pressure to sign (timing around real estate closings is particularly risky)

• Had time to consult with independent legal counsel

For higher-risk transactions, such as interspousal transfers of real property without consideration, it may be wise to include explanatory language directly in the instrument, separate acknowledgments of understanding, and, ideally, documented advice or waiver of separate counsel.

The Practical Takeaway

Transmutations are not mere formalities. When challenged, they are scrutinized for intent, clarity, and fairness — particularly where one spouse gives up a valuable interest and receives little or nothing in return.

To withstand that scrutiny, we must ask the same question the court will ask later:

Did the disadvantaged spouse know what they were giving up, and why?

If the answer is anything less than a well-documented yes, the agreement may not survive.

Why Prenups Have Clear Rules About Debt and Separate Property in Marriage

Under default California law, anything earned during marriage is presumed to be community property. Debts incurred during marriage are also presumed to be shared. It’s easy to assume that if something is in your name, it stays yours. But once loans, shared income, or separate property come into play, things get complicated quickly.

In prenups and postnups, lawyers who specialize in this area include specific language about the use of credit, collateral, and separate property. Without that clarity, we find that couples can be confused about the rules and sometimes in court.

Here’s what that looks like in real life:

Using a loan to improve your own separate property doesn’t mean your spouse now owns part of it.

If someone takes out a loan during marriage in their own name to renovate a home they owed before marriage and uses earnings from work to pay it off, that doesn’t automatically make all of the home community property. Judges often look at what the lender expected when issuing the loan and what income stream was intended to pay it back. If community earnings were used, the loan may not be treated as separate.

Paying for repairs or taxes with joint money doesn’t make the property joint.

Community funds sometimes cover property taxes or maintenance on a separately owned home. That doesn’t turn the home into community property. But if things fall apart, people often feel entitled to equity based on how money was used. A prenup can clarify. 

Being an authorized user on a spouse’s credit card can create confusion about debt.

If one spouse opens a credit card in their name and adds the other as an authorized user, they may assume they are the only one responsible. But if the card is used for joint expenses, California law might treat the debt as community. A prenup can clarify that the cardholder takes responsibility or, alternatively, that the debt is shared. Either way, it’s better to be explicit.

These examples reflect real choices people make in marriage. Helping each other, blending finances, making things easier. But without clear agreements in place, those decisions can lead to misunderstandings and resentment.

These clauses do more than protect people legally. They reduce conflict. They make it easier to trust that decisions made in good faith won’t be misunderstood later. A good prenup just puts into writing what people already believe to be fair. 

Prenups for Startup Founders: How to Share Some, Not All, of the Business

If you own a startup before marriage, you may want to protect your ownership while still recognizing your spouse’s potential contributions to your future success. California law lets you decide what remains separate and what becomes community property. A well-drafted prenuptial agreement can define a middle ground that avoids default rules and future conflict.

Here’s how to structure an agreement when you want to keep most of your business separate but allow some of it to be shared.

Start with a Baseline

Identify what you own before marriage. This includes equity, IP, trademarks, customer lists, and any unvested options. This forms your separate property foundation.

Decide What to Share

You don’t have to choose all or nothing. You can make specific parts of the business or future value community property. Here are a few ways to do that:

Share appreciation above a threshold

You can agree that the first $1 million of appreciation remains separate. Anything above that becomes community property and gets split equally if you divorce. This protects early risk while sharing long-term upside.

Share liquidity, not ownership

You can keep 100 percent of the business as separate property but agree to share some of the cash if the company is sold or you receive distributions during the marriage. For example, you might agree that 25 percent of any net payout during the marriage becomes community property.

Provide a fixed community reimbursement

If your spouse will support the household while you work long hours, you can include a flat payment in case of divorce. For example, after five years of marriage, your spouse receives $250,000 as recognition for that support. This avoids valuation battles and honors their contribution.

Use a split-growth formula

You can apply a simple formula to divide appreciation. For example, you get a 6 percent annual return on your premarital business value. Any growth beyond that is community property. This mirrors how courts might divide a business but does it upfront, avoiding a fight.

Make future compensation community

If you’re receiving new stock grants or options after marriage, you can agree those are community property, while keeping the original business separate. This works well when equity is tied to continued service.

Plan for clarity and track it

Whatever you agree to, spell it out in plain language and keep good records. Use regular valuations, document distributions, and track equity grants. That way, there’s no guesswork later.

Conclusion

You don’t need to hand over half your startup to be fair. A thoughtful prenup can protect your hard work while acknowledging your spouse’s role in your life and business. When done right, it keeps your intentions clear and your partnership strong.

Consider using a margin loan?

If you’re thinking about using a margin loan during marriage, especially to help buy a house or fund something for both spouses, there are a few family law issues to keep in mind. Margin loans let you borrow against your investment account without selling assets. It might feel like a clean way to use separate property to support your family, but California law may treat that money very differently than you expect.

California presumes that property acquired during marriage is community property. That includes assets bought with borrowed funds. If you take out a margin loan during marriage using your separate brokerage account as collateral, and use the money for something shared like a down payment, that loan—and the asset purchased—may be considered community property. Even if the collateral is clearly separate.

In the Marriage of Grinius case, the court focused on whether the lender relied on community income or joint financials. If so, the loan proceeds were treated as community. This means that using your separate account as collateral might still create a community debt, especially if both parties are involved in the purchase or the asset is titled jointly.

This gets more complicated when the repayment comes from a joint account, or when there’s no written agreement clarifying the character of the funds. The result can be that one spouse takes on risk by using separate collateral, only to lose any separate interest in the property down the line.

If you’re thinking about using a margin loan during marriage, talk with a family law attorney first. We can help document your intent, keep your records clean, and avoid outcomes that feel unfair later. The earlier you clarify, the more flexibility you have.

When Separate Property Becomes Joint: Understanding Moore/Marsden and Family Code §2640 After a Refinance

“If I inherited a home during the marriage, and later we refinanced it into both of our names, do I still get my separate property back?”

The short answer is: Yes, but how we calculate it depends on timing—and two key tools in California law: the Moore/Marsden formula and Family Code §2640.

The Setup: Inherited Property + Refinance + Community Payments

Let’s say one spouse inherits a home during marriage—so it starts off as that spouse’s separate property under California Family Code §770. Over time, the couple pays the mortgage using community funds. Then, they refinance and put the home in both names, changing the character of the property to community property.

So now we’ve got three things going on:

1. The home was separate property by origin (inheritance).

2. The couple used community funds to pay down the mortgage.

3. The couple later transmuted the home to community property by going on title together.

Where Moore/Marsden Comes In

The Moore/Marsden formula is used to determine the community’s interest in a property when community funds were used to pay down the mortgage on separate property. It doesn’t just reimburse the amount of principal paid—it also gives the community a proportional share of the property’s appreciation.

But once the property is transmuted to community property—by adding the non-owner spouse to title—the legal landscape changes.

Where §2640 Steps In

If there was no express waiver of the original owner’s separate property rights (which is common), then Family Code §2640 applies. This statute allows a spouse to reclaim their separate property contributions, even when the asset is now community property—but only dollar-for-dollar, without interest or appreciation.

Here’s the key: To know how much to reimburse under §2640, you first need to calculate what portion of the property was still separate at the time of the refinance. That’s where the Moore/Marsden formula helps.

So, yes—Moore/Marsden is still relevant, but only to establish the separate property value as of the date the title was changed. After that point, the home is treated as community property, and future growth or payments belong to both spouses equally (unless agreed otherwise).

Putting It All Together

If you’re in mediation or litigation involving an inherited property that was later refinanced into joint names, here’s the usual sequence:

1. Use Moore/Marsden to calculate the separate vs. community interests up to the date of refinance.

2. Use that value as the basis for the separate property reimbursement under Family Code §2640.

3. Divide the remainder of the equity equally (assuming no other agreement exists).

Why This Matters

This scenario comes up all the time, especially when couples mix separate and community finances without formal agreements. Being able to trace and distinguish contributions—especially when inheritance and transmutation are involved—can make a major financial difference in divorce outcomes.

If you’re navigating these questions in your own case, working with someone who understands the intersection of family law and property characterization is key. It’s not just about who paid what—it’s about when, how, and what the law says about those payments.