Prenuptial and Postnuptial Agreements: What a Financial Advisor Does (and Why You Need One)

A prenuptial or postnuptial agreement is a legal document — your attorney drafts it. But the financial inputs that go into it, and the decisions that follow from it, require someone with a different skill set. A fee-only financial advisor working alongside your attorney helps ensure the legal agreement and your actual financial plan are telling the same story.

The attorney's role and the financial advisor's role are not the same

Your attorney drafts the agreement, ensures it meets your state's legal requirements, and advises on enforceability. What an attorney typically doesn't do:

• Prepare and interpret a detailed net worth statement for both parties, including retirement account balances and unvested equity
• Model how a proposed asset-division formula would work in different economic scenarios
• Identify where the agreement conflicts with existing beneficiary designations or account titling
• Advise on retirement account rules that exist independent of any agreement (ERISA spouse consent rules, IRA beneficiary law)
• Help you understand the long-term financial implications of alimony provisions — now materially different since the Tax Cuts and Jobs Act

The financial advisor fills the gap between legal enforceability and financial coherence. An agreement that is legally sound but financially inconsistent with your actual accounts creates real problems — problems that surface at the worst possible moments (death, disability, divorce).

Financial disclosure: the foundation

Every enforceable prenuptial agreement requires full financial disclosure from both parties. Courts routinely void prenups when one party later demonstrates they weren't given accurate information about the other's finances. What "full disclosure" actually means in practice:

• Net worth statement for each person. Current value of every asset — checking, savings, brokerage, retirement accounts (401k, IRA, Roth IRA, pension), real estate, business interests, unvested equity (RSUs, options), expected inheritance. Liabilities too: mortgage, student loans, credit card debt, business debt.
• Income documentation. W-2s, recent tax returns, business financials if self-employed. For variable-income earners, a 3-year average.
• Business valuations. If either party owns a business — even a young startup — its value (or methodology for valuing it) should be documented at the time of the agreement. Business value tends to be where prenup disputes originate.
• Expected future assets. If a parent is likely to leave a large inheritance, this is typically disclosed (not guaranteed, but disclosed) so both parties understand the full financial picture.

A financial advisor can prepare or review these disclosures, ensuring they're complete and consistent across accounts — rather than leaving it to each party's attorney to argue about completeness later.

What prenups (and postnups) typically address financially

Separate vs. marital property

The core purpose of most prenuptial agreements is to define which assets are "separate property" (owned by one spouse, not subject to division in a divorce) and which become "marital property" (jointly accumulated, subject to division). Without an agreement, state law governs — and state law varies significantly, especially between common-law and community property states.

Common separate-property protections in prenups:

• Pre-marital savings and investment accounts — stated by account and current balance
• Pre-marital real estate — including appreciation during the marriage, or a formula for splitting appreciation
• Business interests held before marriage — typically valued at the time of the agreement, with appreciation split or excluded
• Family inheritances — either received before marriage or expected and named

The prenup can say anything the parties agree to within state-law limits. A financial advisor's value here is modeling what these provisions actually look like in dollar terms at various future points — a provision that seems reasonable today may be severely unfair in 20 years if one party's separately-titled business grew 100x.

Commingling: where separate property becomes marital property

The most common way prenups fail in practice isn't that they're voided by a court — it's that the financial behavior during the marriage quietly contradicts them. Commingling is the culprit.

A financial advisor can set up a system to keep separate-property accounts genuinely separate during the marriage: no joint contributions, clear paper trails, separate cost-basis tracking, and annual documentation. This is ongoing maintenance work, not a one-time agreement signing.

Retirement accounts: what an agreement can and can't control

This is the most technically complex area. A few key rules that exist independent of any agreement:

401(k) and employer-plan ERISA rules. Under ERISA § 205, a married participant's 401(k) is automatically subject to a qualified joint and survivor annuity (QJSA) — the surviving spouse is entitled to at least 50% of the benefit unless they sign a written waiver. A prenup that says "Spouse A's 401(k) is entirely separate property and passes 100% to Spouse A's children from a prior marriage" may be unenforceable against the QJSA rule without a separate, contemporaneous spousal waiver signed when the beneficiary designation is made (not just at prenup signing years earlier). This is a specific legal requirement that an estate attorney and financial advisor need to coordinate on.

IRAs are not governed by ERISA. An IRA beneficiary designation supersedes the will and can supersede the prenup if they conflict. If your prenup says Spouse B gets 50% of your IRA but your IRA beneficiary designation names your estate (or a prior spouse), the beneficiary designation wins. The prenup is not a beneficiary designation.

What a prenup CAN do for retirement accounts: It can specify how retirement accounts accrued before marriage are treated if the marriage ends, and how accounts accumulated during the marriage are split. If it ends in divorce, the mechanism for actually transferring 401(k) funds is a qualified domestic relations order (QDRO) — a court order separate from the divorce decree. IRAs transfer via a process under IRC § 408(d)(6), also court-ordered. The prenup sets the intent; the QDRO/court order implements it at the time of divorce.

Alimony / spousal support provisions

Many prenuptial agreements include a spousal support formula or limit — for example, "alimony capped at X per month for Y years." Understanding the tax consequences of the proposed formula is essential to evaluating whether it's fair to both parties.

Since the Tax Cuts and Jobs Act (TCJA), alimony under agreements executed after December 31, 2018 is no longer deductible by the payer and no longer taxable income to the recipient.¹ This is a complete reversal of the pre-2019 rules. If your attorney is referencing the old deductibility rules, or if you're using an agreement template that predates 2019, the financial analysis is wrong.

The practical effect: post-2018 alimony is effectively taxed at the payer's marginal rate and received tax-free. A $10,000/month alimony provision costs the payer $10,000 in after-tax dollars (the old rules would have cost approximately $7,200 in a 28% bracket). The prenup formula needs to be built using post-TCJA economics, not pre-2019 assumptions.

Postnuptial agreements: why couples get them during marriage

A postnuptial agreement does the same thing as a prenup but is signed after marriage. Courts scrutinize postnups more carefully than prenups (less "arms-length" because the parties are already legally bound), but they are enforceable in most states with full disclosure and independent counsel for each party.

Common scenarios where couples seek a postnup:

• One spouse starts or grows a business. The prenup (or lack of one) didn't anticipate the business. A postnup can define the current value, set a formula for appreciation during the marriage, and clarify whether business equity is separate or marital property.
• Large inheritance received during marriage. One spouse inherits $1M from a parent. If they want to keep it separate, a postnup (combined with careful titling and a strict no-commingling policy) strengthens the argument.
• Career change where one spouse becomes primary caregiver. A spouse who leaves a high-paying career to raise children has given up earning power in a way that a split-at-divorce would undercompensate. A postnup can guarantee a minimum settlement that reflects this sacrifice — whether the lower-earning spouse initiates it or the higher-earning spouse offers it out of fairness.
• Reconciliation after a separation. Couples who reconcile sometimes formalize financial boundaries to rebuild trust. A postnup creates clarity about what's shared and what remains individual.
• Prior marriage children. If you didn't do a prenup and later realize your estate plan is misaligned with how you want to protect children from a prior relationship, a postnup can address this alongside updating your trust and beneficiary designations.

Where the financial plan and the legal agreement must be consistent

The most common failure mode isn't a bad prenup — it's a good prenup surrounded by an inconsistent financial structure. A financial advisor's "consistency audit" checks four things:

1. Beneficiary designations match the intent of the agreement. If the prenup says Spouse A's IRA passes to Spouse B in full, the IRA beneficiary designation should name Spouse B as primary beneficiary. If the intent is a trust for minor children from a prior marriage, the trust (not the individual) should be named — and the trust must be drafted appropriately for retirement accounts (see "see-through trust" rules under IRS Regulation 1.401(a)(9)-4).
2. Account titling matches the separate vs. marital classification. Accounts intended to remain separate should stay in one spouse's name only. Joint accounts should hold only marital property. The titling should be documented and reviewed annually.
3. Estate plan (will + trust) is consistent with the agreement. If the prenup limits one spouse's inheritance rights, the will should not inadvertently give them more. If a trust is intended to protect separate assets for children from a prior relationship, the trust's funding mechanism must be reviewed alongside the prenup.
4. Insurance provisions in the agreement are actually in place. Some prenups require each party to maintain life insurance naming the other as beneficiary for a certain period. If no policy was purchased after signing, the provision is unenforceable.

Annual gift tax and the intra-spousal exception

U.S. citizen spouses can give each other unlimited amounts with no gift tax (the unlimited marital deduction under IRC § 2523²). This is relevant when one spouse has agreed to "equalize" wealth with the other — for example, transferring assets to build up the lower-wealth spouse's separate accounts. No annual limit applies for U.S. citizen spouses.

Exception: if one spouse is not a U.S. citizen, the marital deduction does not apply. Gifts to a non-citizen spouse are limited to $194,000 per year for 2026.³ Couples with one non-citizen spouse need to plan transfers carefully.

Working with both an attorney and a financial advisor

The typical sequence for a financially-informed prenuptial process:

1. Financial disclosure preparation. A financial advisor helps each party compile a complete net worth statement, including retirement account values, unvested equity, and business interests. This becomes the financial exhibit attached to the agreement.
2. Scenario modeling. Before agreeing to any formula (business appreciation split, alimony cap, property division), model what it looks like in 10, 20, and 30 years under different economic assumptions. $50,000 alimony per year sounds reasonable now; over 15 years it's $750,000, and the economic fairness depends entirely on what the business has grown to be worth.
3. Draft review. When the attorney produces a draft, the financial advisor reviews it for financial consistency — retirement account provisions vs. ERISA rules, alimony tax analysis, consistency with current titling.
4. Post-signing implementation. The financial advisor sets up the financial structure to match the agreement: separate-account systems, beneficiary designations, insurance policies named in the agreement, annual documentation protocol to prevent commingling.
5. Annual review. As assets change, the prenup's financial picture diverges from reality. An annual review — especially when a business grows, an inheritance is received, or a major purchase is made — keeps the two aligned.