US UK Estate Tax Treaty Inheritance Planning: 40/40 Trap

The 40/40 problem is deceptively simple: US federal estate tax can charge up to 40% on a worldwide estate, and UK inheritance tax charges 40% on a worldwide estate, so a family straddling both jurisdictions can see the same assets exposed to two 40% death taxes. The US-UK estate and gift tax treaty and disciplined domicile planning are the only reliable defence, and both must be arranged long before death.

For families with a foot on each side of the Atlantic — a US citizen married to a Briton, a London-based founder holding a Green Card, an American executive who has quietly become a long-term UK resident — the arithmetic of two 40% regimes is not theoretical. Left unstructured, an estate that took a lifetime to build can be halved twice over. This guide sets out how the treaty works, why domicile is the master key, and the structures sophisticated families use to keep a transatlantic legacy intact.

Why two countries claim the same estate

The trap exists because the US and the UK each assert taxing rights on a different basis, and those bases overlap.

The United States taxes the worldwide estate of any US citizen or US domiciliary, wherever they live and wherever the assets sit. Citizenship is sticky: a US person carries US estate tax exposure for life unless they formally expatriate. The UK, by contrast, has historically taxed inheritance on the basis of domicile — a common-law concept distinct from residence or nationality — charging 40% on the worldwide estate of a UK-domiciled individual and only on UK situs assets for others. From April 2025 the UK moved toward a long-term-residence test for exposure to worldwide assets, but the core principle is unchanged: cross a threshold of connection to Britain and your global estate is in the net.

A US citizen who has lived in London for fifteen years can therefore sit squarely inside both systems at once. That is the 40/40 problem in one sentence. Understanding your precise status under each regime is the first task of any serious plan, and it is why we begin every engagement with a cross-border tax planning diagnostic before any structure is proposed.

How the US-UK estate tax treaty prevents double 40% exposure

The US-UK Estate and Gift Tax Convention was written precisely to stop the same assets being taxed to death twice. It does three things that matter to wealthy families.

• Domicile tie-breakers. Where both countries would treat an individual as domiciled, the treaty applies tie-breaker rules to assign a single primary domicile, preventing both regimes from asserting full worldwide taxing rights simultaneously.
• Allocation of taxing rights. The treaty assigns primary taxing rights over particular categories of asset — for example, real property is generally taxed where it is situated — so each country's claim is defined rather than overlapping without limit.
• Credit for tax paid. Where both countries retain a claim, the treaty requires one to grant a credit for tax paid in the other. This is the mechanism that turns a potential 80% combined charge into a single effective charge closer to the higher of the two rates.

Crucially, the treaty rarely eliminates tax altogether. Its job is to ensure the same slice of wealth is not taxed twice, not to reduce the headline rate to zero. It also must be actively claimed and evidenced; relief is not automatic, and a poorly documented estate can lose credits it was entitled to. This is technical territory where the interaction of two tax codes rewards specialist US-UK tax accountants who work in both systems daily.

US estate tax versus UK inheritance tax: how they differ

Although both regimes headline at 40%, they are structurally very different animals, and those differences drive planning.

The single most important line in that table is the exemption. Because the US exemption is measured in millions while the UK nil-rate band is modest and frozen, a great many transatlantic families discover they owe substantial UK inheritance tax while owing little or no US estate tax. The instinct to plan around the American rules can therefore be exactly backwards: for many households the binding constraint is HMRC, not the IRS. A candid high-net-worth assessment tests both regimes side by side rather than assuming which one dominates.

Why domicile is the master key

Everything in transatlantic estate planning ultimately turns on domicile, because domicile determines whether the UK reaches your worldwide estate or only your UK assets.

What domicile actually means

Domicile is not the same as residence, nationality, or where you happen to be living. Under UK common law everyone acquires a domicile of origin at birth, usually following their father's domicile. It can be displaced by a domicile of choice — established by residing in a new country with the settled intention of remaining there permanently — but a domicile of origin is remarkably tenacious and can revive if the domicile of choice is abandoned. Layered on top, the UK applies a deemed domicile concept and, following recent reform, a long-term-residence test that pulls long-staying residents into worldwide IHT regardless of common-law domicile.

Why it matters for the 40/40 problem

If you can demonstrate a non-UK domicile, your exposure to UK inheritance tax may be limited to UK situs assets — typically UK real estate and certain UK-based holdings — leaving the bulk of a worldwide estate outside the 40% UK charge. Conversely, sleepwalking into UK domicile or long-term-resident status can expose a global fortune that a US citizen was already reporting to the IRS. Domicile is established and evidenced over years through a pattern of connections, intentions and documentation, which is why it cannot be arranged at the deathbed. Our trusts and estate planning team treats domicile analysis as the foundation on which every other structure is built.

The spouse trap: why the marital deduction can fail

Many wealthy couples assume that whatever the death taxes might be, transfers between spouses are exempt. Across the Atlantic, that assumption is dangerous.

In the US, the unlimited marital deduction — which lets one spouse leave an unlimited amount to the other free of estate tax — is only available where the surviving spouse is a US citizen. A US citizen leaving assets to a non-citizen British spouse loses that shelter, and the estate can face US estate tax on the first death rather than deferring it. The standard solution is a Qualified Domestic Trust (QDOT), which allows the marital deferral to be preserved by routing assets through a trust with a US trustee and strict conditions. QDOTs are powerful but unforgiving: the drafting, funding and ongoing administration must be precise, and they are best established as part of the will rather than retrofitted.

On the UK side, the spouse exemption is similarly restricted where one spouse is UK domiciled and the other is not, subject to a capped exemption or an election to be treated as UK domiciled — an election with its own worldwide-IHT consequences. Mixed-nationality couples therefore need both regimes modelled together. Getting this wrong is one of the most common and expensive errors we see, and it is a core focus of our private client tax services.

Structures that defend a transatlantic estate

Once domicile is understood and the treaty position mapped, a defence is built from a small set of well-worn but carefully coordinated tools.

Trusts — used with caution

Trusts remain central to UK estate planning and can remove assets from the taxable estate, but for a US person they must be designed around the punitive US treatment of foreign trusts. A structure that is efficient for a purely UK family can trigger onerous US reporting and anti-deferral rules for an American settlor or beneficiary. The art is choosing trust structures that satisfy both codes rather than optimising for one and being penalised by the other.

Lifetime gifting across two clocks

The UK's seven-year potentially exempt transfer rule means gifts fall out of the estate entirely if the donor survives seven years, making early gifting one of the most effective UK IHT tools. The US instead operates a unified lifetime gift and estate exemption with annual exclusions. A coordinated programme uses both — timing and sizing gifts so they are efficient under each regime — but a gift that is benign in one country can consume exemption or trigger tax in the other, so sequencing matters.

Situs planning and asset location

Because each regime treats real property and certain assets by location, where assets are held can change the taxing outcome. Holding structures for UK real estate, the location of investment portfolios, and the ownership of businesses can all be arranged to align with the treaty's allocation of taxing rights.

Life assurance for liquidity

Even a perfectly structured estate can face a death-tax bill that must be paid before assets can be sold. Appropriately owned life assurance — often written in trust — provides the liquidity to pay the charge without forcing a fire sale of illiquid holdings such as a family business or property. For families with significant US and UK filing obligations, coordinating this with ongoing compliance through our US tax services keeps the whole picture consistent.

A worked illustration of the trap

Consider an American founder, long resident in London, married to a British spouse, holding a worldwide estate of company equity, a London home and US brokerage accounts. As a US citizen, her entire worldwide estate is within US estate tax. As a long-term UK resident, the same worldwide estate is within UK inheritance tax. Her non-citizen spouse means the US marital deduction is unavailable without a QDOT. Absent planning, the London home and global assets could be assessed to UK IHT at 40%, while the same assets are assessed to US estate tax at up to 40%, with only the treaty credit standing between her heirs and a catastrophic combined charge.

With planning — a QDOT to preserve marital deferral, a domicile position argued and documented over years, a seven-year gifting programme begun early, treaty credits claimed with proper evidence, and life assurance in trust for liquidity — the same estate can pass largely intact. The difference between the two outcomes is not luck. It is time and specialist structuring.

Common mistakes that trigger the 40/40 charge

• Assuming citizenship or residence equals domicile. They are three different tests, and conflating them produces the wrong plan.
• Relying on the marital deduction with a non-citizen spouse. Without a QDOT the deferral can simply vanish.
• Using a UK-standard trust for a US person. Foreign-trust rules can convert an efficient structure into a compliance nightmare.
• Leaving the treaty unclaimed or undocumented. Relief is not automatic; credits must be evidenced.
• Planning too late. Domicile is built over years and the seven-year clock only rewards early action.

For families who also have historic US filing gaps, resolving those through the IRS streamlined filing route often runs in parallel with estate structuring, since a clean compliance record underpins every treaty position.

Book a confidential transatlantic estate review

The 40/40 problem is real, but it is also solvable — provided the work begins early and is done by advisers fluent in both the US and UK codes and the treaty that binds them. If your family, your marriage or your assets straddle the Atlantic, the most valuable step you can take is a confidential review that maps your domicile position, tests both regimes, and sets out a structure to keep your legacy where it belongs: with the next generation. Contact Jungle Tax to arrange a discreet, no-obligation consultation with our cross-border estate specialists, and turn a source of quiet anxiety into a settled, defensible plan.