Portability vs Credit Shelter Trust for Married Couples

What's the difference between portability and a credit shelter trust?

Portability is a Form 706 election that transfers a deceased spouse's unused federal estate tax exemption (called the Deceased Spousal Unused Exemption Amount, or DSUE) to the surviving spouse. A credit shelter trust (also called a bypass trust) funds an irrevocable trust at the first spouse's death so those assets, and all of their future growth, stay out of the surviving spouse's taxable estate entirely. Both tools aim to make sure a married couple uses both of their exemptions rather than wasting one.

Under the One Big Beautiful Bill Act (OBBBA), signed into law on July 4, 2025, the federal estate tax exemption is $15 million per person for 2026, giving married couples a combined $30 million threshold. Unlike the prior TCJA increase, this provision is permanent with no sunset clause, and it's indexed for inflation starting in 2027 using 2025 as the base year. The top federal estate tax rate remains 40% on amounts above the exemption.

Portability and credit shelter trusts aren't competing strategies so much as complementary ones. The right choice (or combination) depends on your family structure, the size and type of your assets, your state's tax rules, and your goals for future generations. At Neptune, couples work with experienced attorneys, CFPs, and CPAs as a single coordinated team to evaluate both paths together, rather than siloing legal drafting from tax analysis.

How does a credit shelter trust (bypass trust) work?

At the first spouse's death, assets up to the deceased spouse's federal estate tax exemption are placed into an irrevocable trust. The surviving spouse can receive income from the trust (and sometimes limited distributions of principal), but the trust assets are excluded from the survivor's taxable estate when they later pass away. The remainder then flows to the named beneficiaries, typically children or grandchildren.

Here's a simplified walkthrough of the mechanics:

1. Funding. The trust language in the couple's will or revocable trust contains a formula clause directing assets up to the exemption amount (currently $15 million in 2026) into the bypass trust. Any excess typically passes to the surviving spouse outright or into a marital deduction trust (like a QTIP).
2. During the survivor's lifetime. The surviving spouse may receive all trust income, and the trustee (often an independent party) may distribute principal for health, education, maintenance, and support. The surviving spouse does not own the trust assets.
3. At the survivor's death. Because the trust assets are not part of the survivor's estate, they pass to the remainder beneficiaries free of additional estate tax, no matter how much they've appreciated.

The appreciation advantage is one of the biggest differentiators. If $10 million is placed into a credit shelter trust and grows to $18 million by the time the surviving spouse dies 15 years later, that $8 million of growth has escaped estate taxation entirely. Under a pure portability plan, those same assets would sit in the survivor's estate, and the growth would be taxable.

A credit shelter trust also preserves the deceased spouse's generation-skipping transfer (GST) tax exemption, which is not portable. If your plan involves leaving wealth to grandchildren or funding dynasty-style trusts, a bypass trust is the only way to use both spouses' GST exemptions.

The tradeoff is complexity. The trust requires a separate tax identification number, annual fiduciary income tax returns (Form 1041), and prudent trustee management. It's language written into your estate documents during your lifetime but only funded and made irrevocable at the first death.

How does portability and the DSUE election work?

Portability lets the surviving spouse add the deceased spouse's unused exemption to their own, but only if an executor files a timely federal estate tax return (IRS Form 706) for the deceased spouse's estate. If no Form 706 is filed, the unused exemption is simply lost.

The filing deadline is generally nine months after the date of death, with a six-month extension available. For estates that don't otherwise need to file (because they're below the exemption threshold), the IRS offers a simplified procedure for portability-only returns, but the return still must be filed. The cost to prepare a portability-only Form 706 typically runs $2,000 to $5,000, depending on the estate's complexity.

Key limitations of portability

• No inflation adjustment. The DSUE amount is fixed at the value calculated on the Form 706 at the first spouse's death. It never increases with inflation. If the exemption rises from $15 million to $17 million over time, the ported amount stays locked at the original figure.
• No GST portability. The generation-skipping transfer tax exemption cannot be transferred to the surviving spouse under any circumstances.
• Assets pass outright. Unlike a trust, portability doesn't create a structure around the assets. They belong to the surviving spouse outright, which means they're available to the survivor's creditors, could be redirected to a new spouse, and are included in the survivor's estate (appreciation and all).
• Second step-up in basis. One advantage: because assets remain in the surviving spouse's estate, they receive a second stepped-up basis at the survivor's death under IRC § 1014. This can reduce capital gains taxes for the ultimate beneficiaries.

Blended family considerations

In blended families, executor and beneficiary interests may not align. If the deceased spouse's estate beneficiaries are children from a prior marriage and they have no expectation of inheriting from the surviving spouse, the executor may lack motivation to file Form 706 and incur the cost. Including explicit direction in your estate documents to elect portability helps avoid this problem.

Credit shelter trust vs portability comparison table

The table below summarizes the major differences. In many cases, the best plan uses elements of both.

Does portability cover state estate tax?

Usually not. Most of the 12 states (plus Washington, D.C.) that impose their own estate tax do not recognize portability, so you can't transfer a deceased spouse's state-level exemption to the survivor through a Form 706 election. A bypass trust is often the primary tool to use both spouses' state exemptions.

The state exemption gap matters more than many couples realize. For example, Oregon's estate tax exemption is $1 million, and Massachusetts sets its threshold at $2 million (2025 figures). A couple with a $6 million estate living in one of these states might owe zero federal estate tax but face a meaningful state estate tax bill if the first spouse's state exemption isn't preserved through a credit shelter trust.

Consider this illustration: A couple in Massachusetts has a $5 million combined estate. Each spouse owns roughly $2.5 million. If the first spouse dies and leaves everything to the survivor outright (relying on portability), the survivor's estate is now $5 million, well above the $2 million Massachusetts threshold. If instead they fund a credit shelter trust with $2 million at the first death, only $3 million is in the survivor's estate, and the trust's $2 million is excluded from Massachusetts estate tax at the survivor's death.

Neptune's CPAs map both federal and state exposure for couples in every state, so you don't discover a state tax gap after it's too late to plan around it.

When do you need a credit shelter trust instead of relying on portability?

You generally need a credit shelter trust when your situation involves a blended family, expected substantial appreciation, multi-generational (GST) goals, state estate tax exposure, or a desire to control who ultimately receives the assets. Portability fits best for simpler first-marriage estates comfortably below the federal and state thresholds, with no GST concerns and no creditor exposure.

Here's a decision framework:

Credit shelter trust makes the most sense when:

• You or your spouse have children from prior relationships and want to direct where assets end up after both spouses pass
• Your combined estate is large enough that post-death appreciation could push the survivor's estate above the exemption
• You want to allocate both spouses' GST exemptions for grandchildren or dynasty trust planning
• You live in a state with its own estate tax that doesn't recognize portability
• Creditor exposure is a concern for either spouse

Portability is often sufficient when:

• It's a first marriage with no blended-family considerations
• Your combined estate is well below federal and state thresholds
• You don't anticipate significant appreciation in estate assets
• Simplicity and lower ongoing costs are priorities
• There are no GST or creditor concerns

Flexibility tools for uncertain situations

Many couples don't fit neatly into one camp. That's where planning tools like disclaimer planning, Clayton QTIP elections, independent trustee discretion, and powers of appointment come in. A disclaimer-based plan, for example, leaves assets to the surviving spouse outright, but gives the survivor the option to disclaim some or all of the inheritance into a credit shelter trust within nine months of the first death. This lets the survivor (with their attorney's guidance) decide based on the actual tax landscape at that time.

A Clayton QTIP approach works similarly. The executor decides at the time of filing Form 706 how much goes into a QTIP trust versus a bypass trust, based on actual exemption amounts and asset values rather than projections made years earlier.

As Michael C. Cotugno, Esq., Managing Partner of Neptune Legal, puts it: "Understanding a partner's relationship with money, their historical experiences of abundance or scarcity, their anxieties tied to financial stability, or their personal definitions of success, allows for a deeper, more empathetic understanding of them as a whole individual." The financial conversations behind these decisions often matter as much as the technical structure.

How Neptune helps couples choose and structure the right plan

Neptune pairs couples with experienced estate attorneys, CFPs, and CPAs (each with 20+ years of experience) and manages the full process from initial conversation through document execution, tax filings, and ongoing review. This isn't a marketplace or a DIY template. It's coordinated, end-to-end planning.

The classic mistake in estate planning is drafting the trust first and asking tax questions later. When an attorney writes a credit shelter trust without input from a CPA who understands the couple's income tax picture, or without a CFP who models future asset growth, the result can be a beautifully drafted document that creates worse tax outcomes than doing nothing. Neptune's integrated approach avoids that by aligning legal structure, tax strategy, and financial projections from day one.

For couples also working through a prenuptial agreement, Neptune's lawyer-led online prenup process integrates naturally with broader estate planning. Your prenup defines how assets are classified and treated. Your estate plan builds on those definitions. When the same team handles both, nothing falls through the cracks.

Couples who plan together grow together, and that planning doesn't stop after documents are signed. Tax laws change, families grow, and assets shift. Neptune's team stays with you through those changes, reviewing your plan as life evolves so your credit shelter trust, portability election, or combination of both continues to reflect what you've built together.