Credit Shelter Trust Administration: A Successor Trustee's Guide to the Bypass Trust

When the first spouse dies, a revocable "AB trust" splits into two separate trusts. The survivor's trust (A trust) continues for the surviving spouse. The bypass trust (B trust, credit shelter trust) becomes irrevocable immediately — and you may be its trustee. Here's what that means and what it requires.

What just happened: the AB trust split

Many married couples used revocable living trusts drafted before 2011 with an automatic "AB split" provision. The structure was designed to capture each spouse's estate tax exemption separately, before the federal law allowed one spouse to inherit the other's unused exemption (portability).

When the first spouse dies, the trust document automatically divides into two trusts:¹

• Survivor's Trust (A Trust / Marital Trust): Holds assets allocated to the surviving spouse. Still revocable — the surviving spouse controls it and can amend or revoke it. Assets in the A trust will be included in the surviving spouse's taxable estate and get a second step-up in basis at the surviving spouse's death.
• Bypass Trust (B Trust / Credit Shelter Trust): Holds up to the deceased spouse's estate tax exemption amount, separated from the survivor's estate. Becomes irrevocable the moment the first spouse dies. The surviving spouse typically receives income from it and may have limited access to principal — but cannot amend or revoke it.

As successor trustee, you may be the trustee of the bypass trust, the survivor's trust, or both — depending on how the trust document reads. In many AB trusts, the surviving spouse serves as trustee of the A trust, while an adult child serves as trustee of the B trust to ensure independence.

Why couples created bypass trusts (and why most new ones aren't needed)

Before portability became law in 2011, a married couple that owned $4 million in combined assets faced a problem: if everything passed to the surviving spouse outright, the deceased spouse's estate tax exemption was wasted. By funding the bypass trust with up to the deceased spouse's exemption, both spouses' exemptions were preserved — effectively doubling the tax-free transfer to children.

The tax landscape has changed significantly:

• Portability (IRC § 2010(c)): Since 2011, a surviving spouse can elect to carry over the deceased spouse's unused exclusion (DSUE), preserving it without a bypass trust. A timely estate tax return (Form 706) must be filed to make the election — even if no tax is owed.
• OBBBA (July 2025): The One Big Beautiful Bill Act permanently raised the federal estate and gift tax exemption to $15 million per individual ($30 million combined for a married couple) starting in 2026, indexed for inflation. A bypass trust designed to capture a $600,000 exemption (the 1990s limit) or even a $13 million exemption (2024 limit) is now largely moot for estate tax purposes for most families.²

Your immediate duties as bypass trust trustee

When the first spouse dies and the trust splits, several things need to happen promptly:

1. Obtain the bypass trust's own tax identification number (TIN)

The bypass trust can no longer use the deceased spouse's or the couple's social security number. File IRS Form SS-4 (or apply online at IRS.gov) to get a new EIN for the bypass trust. Every financial account you open or retitle in the trust's name will need this number.

2. Fund the bypass trust according to the formula clause

Most AB trusts have a "formula clause" specifying how much goes into the bypass trust. Common formulas:¹

• Maximum credit shelter formula: "The maximum amount that can pass free of federal estate tax." Under OBBBA, this could direct $15 million into the bypass trust — often far more than the couple intended to lock away from the surviving spouse. Many pre-OBBBA trusts with this formula now have a drafting problem requiring attorney guidance.
• Pecuniary formula: A fixed dollar amount (e.g., "an amount equal to the applicable exclusion amount in effect at the settlor's death"). Often the same issue as above under the $15M regime.
• Fractional share formula: A fraction of each asset rather than specific assets.

Work with the estate attorney to determine what the formula produces given the current exemption amount. If the formula would push most of the estate into the bypass trust and leave the surviving spouse with insufficient assets in the A trust, trust modification may be appropriate and advisable before funding.

3. Open a separate brokerage or trust account for the bypass trust

The bypass trust must maintain completely separate accounts from the survivor's trust and from the surviving spouse's personal accounts. Commingling assets is a significant trustee liability risk. Retitle assets into the bypass trust's name using the new EIN.

4. File Form 706 (estate tax return) if portability is needed

Even if no estate tax is owed, the deceased spouse's estate should file Form 706 within 9 months of death (with a 6-month extension available) to elect portability of any unused exclusion. If the bypass trust fully uses the deceased spouse's exemption, there may be nothing to port — but confirm with the estate attorney before skipping the filing.

Mandatory income distributions to the surviving spouse

Most bypass trust documents require the trustee to distribute all income to the surviving spouse at least quarterly (often annually at minimum). This is not optional — it is a mandatory provision in the vast majority of bypass trust documents.

What counts as "income" depends on the trust document and applicable state law:

• Traditional accounting income: Interest, dividends, rents, royalties received by the trust. Capital gains are typically allocated to principal, not income, under traditional trust accounting rules.
• Unitrust income: Some trusts use a "total return" unitrust approach, paying the surviving spouse a fixed percentage of the trust's value annually (often 3–5%) regardless of what the trust actually earned in dividends or interest.

Failing to make required income distributions is a breach of your fiduciary duty to the surviving spouse as current income beneficiary. It also creates unnecessary tax cost: income retained in the bypass trust is taxed at compressed trust rates, reaching 37% on income above $16,000 in 2026 — far higher than what most surviving spouses pay individually. Every dollar of required income that is not distributed on time costs the family money.

Principal distributions: HEMS or discretionary?

Beyond required income distributions, the trust document may allow or require the trustee to distribute principal to the surviving spouse. Two common patterns:

• HEMS standard: Trustee may (or must, if needed) distribute principal for the surviving spouse's Health, Education, Maintenance, and Support. This is a broader standard that gives the surviving spouse significant access to principal while preserving the trust's tax effectiveness. See our distribution decisions guide for how to apply HEMS correctly.
• Health and support only (narrower): Some bypass trusts restrict principal distributions to health and support needs — deliberately limiting the surviving spouse's access to protect the principal for remainder beneficiaries (usually the children).

Document every principal distribution decision in writing. The decision should reference the applicable standard, the specific need, the amount, and why it meets the standard. This documentation protects you against claims from remainder beneficiaries who may argue distributions eroded their inheritance.

The impartiality problem

As bypass trust trustee, you owe fiduciary duties to two groups of beneficiaries with different interests:

• The surviving spouse (income beneficiary) wants distributions — both required income and potentially principal for expenses.
• The remainder beneficiaries (usually adult children) want principal preserved and invested for growth.

Under the Uniform Prudent Investor Act and most bypass trust documents, you must be impartial between current and future beneficiaries. This tension drives your investment policy decisions: you generally need a portfolio that generates enough income to satisfy the required distribution obligation while also maintaining real (inflation-adjusted) purchasing power for the remainder beneficiaries.

A total-return investment policy with a distribution policy aligned to the trust's payout obligation is typically the right framework. See our trust investment policy guide for how to structure this. The important point: you cannot simply maximize income at the expense of principal growth (that would harm remainder beneficiaries), nor can you invest purely for growth and withhold required income distributions (that would harm the surviving spouse).

The no-second-step-up trap: the most important thing to understand

This is where many successor trustees — and even some advisors — make costly mistakes.

What happens at the first spouse's death

When the first spouse dies and assets fund the bypass trust, those assets get a stepped-up cost basis under IRC § 1014. An asset that cost $200,000 and is worth $800,000 at the date of death gets a basis of $800,000. No embedded capital gain.³

What happens at the surviving spouse's death

Here is the trap: bypass trust assets do not get a second step-up. The bypass trust was intentionally structured to be outside the surviving spouse's taxable estate. Because those assets are not "included in the gross estate" of the surviving spouse, IRC § 1014 does not apply to them at the second death.

Survivor's trust (A trust) assets are different: they are in the surviving spouse's estate, so they do get a step-up when the surviving spouse dies.

Why this matters

Assets in the bypass trust that appreciated significantly between the first spouse's death and the surviving spouse's death carry embedded capital gains. Those gains pass to the children along with the asset. Example:

• First spouse dies; bypass trust funded with S&P 500 index fund with a fair market value of $500,000. Basis is now $500,000 (step-up received).
• 20 years later, surviving spouse dies. That fund is now worth $2 million. Basis is still $500,000.
• Children inherit the fund and immediately sell: they owe capital gains tax on $1.5 million. At the 20% long-term capital gains rate plus 3.8% NIIT, that's roughly $357,000 in federal tax.
• If the same fund had been in the survivor's trust instead, the children would inherit it with a $2 million basis — zero tax on sale.

The practical implication: coordinate with the surviving spouse's estate planner to consider whether highly appreciated bypass trust assets should be distributed to the surviving spouse during their lifetime (consuming their HEMS allowance or other distribution right), harvested for gains while rates are favorable, or addressed through trust modification. The decision is not simple — it involves income tax, estate tax, and the trust document's constraints. But ignoring it is worse.

Form 1041: the bypass trust files its own tax return

The bypass trust is a separate taxable entity. It files its own Form 1041 (U.S. Income Tax Return for Estates and Trusts) each year using its own EIN. Key points:

• Income distributed to the surviving spouse flows through on Schedule K-1 and is taxed on the surviving spouse's personal return. The bypass trust deducts these distributions (DNI / distribution deduction).
• Income retained in the bypass trust is taxed at the highly compressed trust rates — 37% above $16,000 of taxable income in 2026.⁴
• Due date: April 15, with a 5.5-month extension available to September 30 (not October 15 as for individuals).
• The 65-day election (IRC § 663(b)): Distributions made within 65 days after year-end can be treated as made in the prior year, giving the trustee flexibility to reduce the prior year's trust-level tax after the year has closed.

See our detailed Form 1041 trustee guide for filing requirements, the distribution deduction mechanics, and estimated payment obligations. A CPA experienced with trust tax returns should prepare the bypass trust's Form 1041 — trust tax returns are meaningfully different from individual returns and the consequences of errors (missed K-1s, incorrect DNI calculations) directly affect beneficiary tax obligations.

Is it time to terminate or modify the bypass trust?

With the OBBBA permanently raising the estate tax exemption to $15 million per person, many bypass trusts created for couples with under $15 million in combined assets no longer serve their original estate tax purpose. Continuing to administer a trust that was created to solve a problem that no longer exists — while incurring administrative costs, tax complexity, and the no-step-up trap — may not be in the beneficiaries' interest.

Options to consider, in consultation with an estate attorney:

• Trust modification (UTC § 411): If all "qualified beneficiaries" consent and the modification is not inconsistent with a material purpose of the trust, the trust can be modified. In most states this requires consent of all current and reasonably ascertainable remainder beneficiaries.
• Judicial modification: If unanimous beneficiary consent is not possible or the trust has minor or unborn beneficiaries, a court can approve modification upon a showing that circumstances have changed in a way the settlor did not anticipate (here: the OBBBA making the bypass structure counterproductive).
• Trust decanting: In states that permit it, the trustee can "pour" the bypass trust assets into a new trust with different terms — effectively rewriting the trust document within certain limits.
• Continue as-is: If the bypass trust is small or the administrative cost of modification exceeds the benefit, simply administering it as written may be the right answer. The no-step-up cost only materializes on the appreciation that occurs after the first spouse's death.

None of these decisions should be made unilaterally by the trustee. All require legal counsel and, in most cases, at least beneficiary notice and often formal consent or court approval.

Common bypass trust administration mistakes

• Failing to get a separate EIN immediately. Running bypass trust transactions through the wrong tax ID creates IRS problems and blurs the trustee's accounting obligation.
• Missing required income distributions. Required distributions to the surviving spouse are not optional. Failure to pay required income on schedule is a fiduciary breach — and runs up a tax bill at trust rates that should have been taxed at individual rates.
• Commingling with the survivor's trust. These are two legally separate trusts. They need separate accounts, separate records, and separate tax returns.
• Ignoring the no-step-up issue until it's too late. The best time to address the embedded gains problem in the bypass trust is before the surviving spouse dies, not after. A fee-only advisor can model the tradeoffs.
• Applying the wrong distribution standard. Confusing what the bypass trust allows with what the survivor's trust allows. The documents may have different distribution standards for each trust.
• Assuming the bypass trust is worthless to administer. Even if the estate tax purpose is moot, the trust holds real assets and has real income that must be distributed correctly. Neglecting it creates liability.