US/UK Tax Planning for High Net Worth Individuals: Where the Risk Actually Sits

The tax problems that catch high net worth US/UK families aren't the ones ordinary tax planning is built to find. Filing a correct return each year is table stakes. The real exposure sits in three places that only show up at scale: the mismatch between the US estate tax and UK inheritance tax, PFIC exposure hiding inside routine UK investment holdings, and exit-tax consequences that get shaped by decisions made years before anyone considers leaving either country.

Why size changes the planning, not just the numbers

Most cross-border tax guidance assumes a straightforward employment or retirement profile: a salary, a pension, maybe a rental property. High net worth planning is different because assets tend to be more varied — direct investments, trust structures, closely held businesses, multiple properties — and each asset class interacts with both tax systems on its own terms. A mistake that costs a modest estate a few thousand dollars can cost a large one a seven-figure sum, and the fixes are far harder to unwind after the fact.

The estate tax mismatch that catches people off guard

The US estate tax and the UK inheritance tax don't line up the way many families assume. Each system has its own exclusion amount, its own definition of what counts as a taxable estate, and its own rules for what happens when someone isn't domiciled or isn't a citizen of the country in question. Layer in the US/UK estate tax treaty, which provides specific relief mechanisms but requires careful application rather than an assumed equivalence between the two systems, and the planning question becomes genuinely technical:

• US-situs assets (such as US real estate or US company shares) held by a non-US domiciled individual can trigger US estate tax exposure at a much lower exclusion threshold than a US citizen would face.
• UK-situs assets held by a US citizen who is UK domiciled or deemed domiciled fall under UK inheritance tax rules that a US-only estate plan typically never anticipated.
• The treaty's relief provisions can reduce double taxation in defined circumstances, but claiming that relief correctly requires the estate to actually document the treaty position, not simply assume it applies.

This is one of the areas our US/UK trusts and estates service spends the most time on, because the fix has to be built before death or incapacity, not discovered afterward by an executor trying to reconcile two systems under time pressure.

The PFIC trap that scales with portfolio size

PFIC exposure is a well-known problem for any US person holding UK-domiciled funds — unit trusts, investment trusts, and most pooled UK investment vehicles fall under the Passive Foreign Investment Company rules, which impose a punitive excess-distribution tax regime absent a timely QEF or mark-to-market election. What changes at scale isn't the mechanism, it's the dollar impact: the same percentage of unrealized gain in a much larger position produces a much larger tax charge, and the record-keeping burden to support an election retroactively becomes proportionally harder to fix once the position has grown for years without one.

Illustrative example: a family with a large, long-held position in a UK-domiciled global equity fund discovers the PFIC issue after years of growth. Making a late QEF election isn't available retroactively in the way a timely one would have been, and unwinding the position outright can itself trigger the excess-distribution tax it was meant to avoid. The workable fix usually involves a combination of managed disposal and a going-forward election on new contributions — not a single clean move. This is illustrative only; actual outcomes depend on the specific fund, holding period, and elections available.

This is exactly the kind of position our PFIC reporting service is built to untangle: identifying every PFIC holding across a portfolio, modeling the actual cost of different election and disposal strategies, and building the reporting trail an eventual IRS inquiry would expect to see.

The exit-tax question that matters years before anyone leaves

The Section 877A exit tax only applies to someone who becomes a "covered expatriate" — a status defined by net worth, average tax liability, or certification tests. For most people that threshold feels distant. For a high net worth family, it isn't distant at all, and the relevant planning decision usually isn't "should we expatriate" but "how should we structure gifts, trusts, and asset transfers now, in a way that doesn't foreclose options later." A family that never expatriates gets no benefit from this analysis. A family that might, decades from now, benefits enormously from having kept the option clean the whole time. Our expatriation and exit tax service covers this in depth for anyone actively considering the decision, but the earlier structuring question belongs in ordinary estate and trust planning, not as an afterthought.

Trusts with beneficiaries on both sides of the Atlantic

A trust drafted with only one country's tax system in mind routinely creates a problem for beneficiaries in the other. A UK-conventional trust structure can trigger US grantor-trust rules or foreign-trust reporting obligations (Form 3520 and Form 3520-A) for US beneficiaries that weren't contemplated when the trust was drafted. A US-conventional trust can create unanticipated UK inheritance tax exposure on the same logic in reverse. Families with beneficiaries in both countries need the trust instrument itself reviewed against both systems, not just the eventual distributions.

Where to actually start

Restructuring an existing position is not always the right first move — unwinding a PFIC holding, moving a trust's governing terms, or accelerating a gift can each trigger the very tax event the planning was meant to avoid, and each needs its own cost-benefit analysis before acting. The lower-risk starting point is almost always new activity: new contributions structured correctly from the outset, new trusts drafted with both systems in mind, and a documented estate plan that actually names which treaty provisions apply and why. Getting the structure right going forward, while a careful specialist works through what (if anything) is worth unwinding from the past, tends to produce a better outcome than either ignoring the issue or trying to fix everything retroactively at once. Our high net worth planning practice and private client service both start from that same sequencing.

The bottom line

Scale doesn't just make cross-border tax mistakes more expensive — it changes which mistakes are actually worth worrying about. Estate tax mismatches, PFIC exposure at scale, and exit-tax-adjacent structuring decisions rarely show up in generic expat tax guidance, because they only become the dominant risk once the numbers involved make ordinary planning insufficient.