Portability vs bypass trusts: the married couple's estate tax architecture

Married couples get two federal estate tax exemptions — but only if they're deliberate about it. For decades, capturing both required a bypass trust (also called a credit shelter or 'B' trust) funded at the first death. Since 2011, 'portability' has offered a simpler path: the survivor can inherit the deceased spouse's unused exemption with an election on an estate tax return. Simpler, though, is not the same as better, and the right choice turns on asset growth, your state's estate tax, remarriage risk, and how much the higher post-2025 exemptions really settle things. Here's the architecture decision, honestly weighed.

Portability: how the DSUE actually works

When the first spouse dies, their unused federal exemption — the Deceased Spousal Unused Exclusion, or DSUE — can transfer to the survivor. With the exemption at $15 million per person (indexed) under current law, a couple can shield $30 million with no trust at all: everything passes outright to the survivor (tax-free under the unlimited marital deduction), and the survivor later applies both exemptions. The catch that snags real families: portability is not automatic. The executor must file a federal estate tax return, Form 706, electing portability — for an estate that owes nothing and would otherwise never file. Miss it, and the first spouse's exemption evaporates.

The bypass trust: what the old design still does better

A bypass trust works differently: at the first death, assets up to the exemption fund an irrevocable trust for the survivor's benefit — income, support, even broad access — but legally outside the survivor's estate. The survivor lives comfortably from it; at their death it passes to the children untaxed, no matter how large it has grown. That last clause is the enduring argument. Portability freezes: the DSUE is fixed at the first spouse's death and never grows. A bypass trust shelters all appreciation. And several other advantages never ported over at all:

• Growth shelter: $15 million in a bypass trust that doubles to $30 million passes entirely tax-free; under portability, the same growth sits in the survivor's estate with only a fixed DSUE against it.
• State estate taxes: most states with estate taxes do not recognize portability. In Massachusetts, Oregon, Washington, New York and others, a bypass-style trust (sized to the state exemption) is still the only way to use both spouses' state exemptions.
• Remarriage protection: DSUE from spouse #1 is lost if the survivor remarries and outlives spouse #2 (you only get your most recent deceased spouse's DSUE). A funded bypass trust can't be lost to remarriage — and it also guarantees the first spouse's assets ultimately reach their intended children, crucial in blended families.
• GST exemption: the generation-skipping transfer tax exemption is not portable, period. Couples planning multigenerational or dynasty trusts must use the first spouse's GST exemption at the first death or waste it.
• Creditor and control protection: trust assets are shielded from the survivor's later creditors, lawsuits, and (bluntly) later bad decisions or exploitation in old age.

What portability does better

The bypass trust's costs are real. It's irrevocable at the first death, requires its own tax returns forever, and — the big one since 2012 — sacrifices the second basis step-up: bypass assets pass to children at the value set at the first death, while assets in the survivor's estate (the portability route) step up again at the second death. Trust income tax rates are also brutally compressed, hitting the top bracket around $16,000 of retained income. For couples safely under the exemptions, portability plus full step-up is usually the better tax outcome, with none of the administration.

The modern answer: build in flexibility, decide later

Because the right answer depends on unknowable futures — asset growth, tax law, which spouse dies first, what state you retire to — modern drafting mostly refuses to hard-code the choice:

1. 1
   Disclaimer trust

   Everything passes outright to the survivor by default, but the will/trust says anything the survivor disclaims within nine months drops into a bypass trust. The survivor decides at the first death, with real numbers and current law in hand. Weakness: it depends on a grieving survivor acting on time, and the survivor can't hold a power of appointment over disclaimed assets.

2. 2
   Clayton election / QTIP-based designs

   The executor elects, on the 706, how much goes to a marital (QTIP) share versus a bypass share — professional-grade flexibility that doesn't rely on the survivor's disclaimer, common in larger or blended-family plans.

3. 3
   File the 706 either way

   Whatever the structure, elect portability at the first death. DSUE stacks with any bypass planning and costs only the return preparation.

4. 4
   Revisit old documents

   Plans drafted before 2011 (or before the exemption soared) often force-fund a bypass trust with 'the maximum exempt amount' — which could now mean the survivor's entire inheritance locked in trust unnecessarily. Formula clauses written for a $2 million exemption world misfire badly in a $15 million one. This is the single most common reason older estate plans need redrafting.

The bottom line

Portability made the double exemption easy to keep, and for most couples under the federal line, outright-to-spouse plus a timely Form 706 wins: simplicity now, full basis step-up later. But the bypass trust never became obsolete — it still beats portability for sheltering growth, for the many states that don't recognize DSUE, for GST planning, and for remarriage and blended-family protection. The professional consensus is to hard-code neither: draft for flexibility, always file the 706, and re-run the decision at the first death with real numbers. And if your documents predate the modern exemption era, have them reviewed — yesterday's careful formula may be today's trap.