The relief every married couple assumes they have

A qualified domestic trust satisfies statutory conditions so that property left to a non-citizen spouse still earns the marital deduction. The conditions exist so the Treasury can still collect later what it defers now.

Everything about it follows from that purpose. The requirements are collection mechanisms rather than planning features.

Because the law withholds the deduction where the surviving spouse is not a US citizen. The joint-ownership rule that normally halves an asset's value also stops applying.

The Internal Revenue Service's estate and gift pages set out the general framework the exception sits inside. The disallowance is a carve-out from an otherwise unlimited relief.

The reasoning is collection rather than hostility. A surviving citizen stays inside the system and meets tax on the same property eventually; a non-citizen may leave and never return.

So the rule hits couples whose arrangements are entirely ordinary. A London marriage where one spouse happens to hold a US passport is the standard case, not an aggressive one.

Our note on filing with a non-US spouse covers the income tax side of the same mismatch.

A qualified domestic trust needs at least one trustee who is a US citizen individual or a domestic corporation. That trustee must have the right to withhold tax on any distribution other than income.

The executor elects the treatment on the estate tax return, and nothing undoes that election afterwards. It also fails on a return filed more than a year after the due date, extensions included.

Property must reach the trust, or pass to it under an irrevocable and enforceable assignment, before the estate tax return falls due.

So the drafting and the deadline work together. A trust drafted perfectly but funded late achieves nothing.

A qualified domestic trust pays on distributions of principal to the surviving spouse during their life, and on whatever sits in the trust when that spouse dies. The charge follows the rate that would have applied in the first estate.

Income distributions escape the charge entirely, which is the feature that makes these trusts liveable. A surviving spouse can draw the income without triggering anything.

Distributions on account of hardship are also exempt, as are amounts reimbursing the spouse for income tax on trust income they were not entitled to.

So the design question is how much the spouse will need beyond the income. That figure drives everything else.

Timing matters too. Tax on a distribution falls due in the April following the calendar year it was made, while tax at the survivor's death follows the ordinary estate timetable.

Above it a qualified domestic trust needs real security, and the regulation offers three routes. The rules require one of them to be in place throughout the trust's life, though the trust may switch between them.

The first is a bank trustee, meaning a bank as the Code defines one. A US branch of a foreign bank can serve, provided a US trustee acts alongside it for the whole term.

The second and third are a bond or an irrevocable letter of credit, each for 65% of the trust assets measured without reduction for debt.

So the threshold decides whether this is a document or an institution. Below it a family member can often serve; above it somebody is paying a bank.

A smaller qualified domestic trust faces its own constraint, and it bites exactly where a British couple's wealth usually sits. A trust at or below two million must keep foreign real property to no more than 35% of its value, measured annually, or adopt one of the three security arrangements instead.

A London house inside a modest trust will often breach that proportion on its own. The alternative is the bank, the bond or the letter of credit.

Special rules cover a breach caused by distributions, currency movements or changes in value rather than by design.

So test the proportion before assuming the simpler route is available. Sterling moving against the dollar can breach it without anybody doing anything.

Take an American who dies in London leaving everything to his British wife. The estate includes the family home and a portfolio, and together they exceed the exclusion.

Without a trust the excess meets tax immediately, with no marital deduction to soften it. The widow may have to sell the house to pay a bill triggered by her own passport.

With a qualified domestic trust the deduction applies, the tax waits, and she lives on the income without any charge arising. Principal drawn beyond that carries tax as it comes out.

Notice who the trust protects. It buys time and liquidity for the survivor, not a permanent saving for the family.

Sometimes, and where it is realistic it is much the cleanest answer. The disallowance falls away where the surviving spouse becomes a US citizen before the executor files the estate tax return, provided they lived in the United States at all times between the death and the naturalization.

That residence condition is the obstacle for most British spouses. Somebody who intends to stay in London cannot meet it.

The timetable is also unforgiving, since naturalization is not a quick process and the return has its own deadline.

So treat it as an option for couples already living in America. For a couple settled in Britain it rarely works.

Citizenship carries lifelong tax consequences of its own, which is a separate decision entirely. Nobody should naturalize for an estate tax reason without looking at the income tax side.

The 2026 basic exclusion amount is $15,000,000, with a credit of $5,945,800 behind it. Many estates will sit under that and never reach this question at all.

Where a trust holds assets and unused exclusion exists, the instructions have the executor work out a preliminary figure for the portability election. That figure falls as the trust distributes principal.

The survivor generally cannot set that amount against lifetime gifts while it remains tied up, because nothing settles it until the trust ends or the survivor dies.

So portability still works here, but it works late. Planning that assumes immediate access to the unused exclusion runs into trouble.

A separate convention governs estates between the two countries, and it is not the income tax treaty people usually mean. The 1979 Order carries its text.

Its relief runs the other way from the one most readers need. Where a US-domiciled or US-national decedent leaves UK-taxable property to a spouse not domiciled here, the United Kingdom exempts half the value transferred, subject to conditions.

The convention also carries its own domicile tie-breaker, which settles which country may tax what before anybody reaches the reliefs.

So read it alongside the domestic rules rather than instead of them. Our note on inheritance tax for Americans covers the British charge itself.

A trustee, an annual return, and the security arrangement once the trust crosses the threshold. None of that is free and all of it continues for the survivor's lifetime.

There is also the friction of needing a US trustee's agreement before principal moves. A widow in Surrey asking a trustee in Chicago for money is a real feature of this arrangement.

Valuations are a recurring cost as well, because the proportion tests and the security amounts both depend on current figures rather than on the values at death.

Against that sits the tax deferred, which on a large estate dwarfs the running cost. On a modest one it may not.

So price the administration before drafting. The answer differs sharply by size.

Couples where one spouse holds a US passport and the other does not. That is the whole test, and it catches a great many marriages in London.

Length of marriage does not matter. Nor does where the couple live, or how long the American has been away.

The size of the estate does matter. Below the exclusion the question never arises, so start there.

So check two passports and one total. Those three facts decide whether any of this applies.

The first is assuming the marital deduction simply applies. It is the single most common wrong assumption in cross-border estate planning.

The second is leaving the qualified domestic trust out of the will and hoping to fix it afterwards. The assignment route exists but it is tighter than people expect.

The third is missing the election deadline, which closes a year after the return fell due and never reopens.

A fourth is ignoring the foreign property proportion, which a British family's main asset breaches without anybody noticing.

We start with domicile and citizenship for both spouses, because those two facts decide which rules apply before any planning begins.

Then we size the estate against the exclusion, since many couples discover the question is academic for them.

Where it is not, our treaty relief work covers the convention and the interaction with the British charge, and you can get in touch while the wills still admit changes.

So the time to look at this is while both spouses are alive. Afterwards the options narrow to one.