The PFIC and ISA Traps for Americans in the UK

For ordinary UK residents the ISA is the simplest tax shelter there is: you can subscribe up to £20,000 in 2026/27 across cash, stocks and shares, innovative finance and Lifetime ISAs, and everything inside is free of UK Income Tax and Capital Gains Tax (GOV.UK). The problem is that the shelter exists only in UK law. US law contains no equivalent recognition of the ISA wrapper, and the US-UK treaty does not create one: its saving clause reserves the right of the US to tax its own citizens and green-card holders largely as if the treaty did not exist.

The result is that, for US purposes, an ISA is just an ordinary taxable account. Interest in a cash ISA is taxable interest on the US return. Dividends and realised gains in a stocks and shares ISA are taxable dividends and gains. The account also counts towards FBAR and FATCA reporting thresholds like any other UK account.

A stocks and shares ISA is often worse than merely unsheltered, because of what typically sits inside it. Most UK platforms populate ISAs with UK unit trusts, OEICs and UK or EU listed ETFs, and for a US person those funds are generally PFICs. That converts a UK tax shelter into a wrapper full of the most awkwardly taxed asset class in the US code, which is the subject of the next section.

A Passive Foreign Investment Company is defined by two tests in the IRS instructions for Form 8621 (irs.gov): a foreign corporation is a PFIC if 75% or more of its gross income for the year is passive income, or if at least 50% of its average assets produce passive income or are held to produce it. An investment fund exists to hold passive assets, so almost any non-US pooled vehicle meets one or both tests. That captures UK unit trusts, OEICs, investment trusts in many cases, and most ETFs domiciled in the UK, Ireland or elsewhere in the EU, whether held inside an ISA, a general investment account or a non-US platform.

The default treatment, known as the section 1291 regime, is deliberately harsh. When a PFIC pays an unusually large distribution, or when the holding is sold at a gain, the amount is treated as an excess distribution and spread over the entire holding period. The slices allocated to earlier years are taxed at the highest rate in force for those years, and an interest charge is added on top, as if the tax had been due all along. Long-term capital gains rates are not available under this default. On a fund held for many years, the combined tax and interest can consume a large share of the gain.

Reporting compounds the pain. Each PFIC generally requires its own Form 8621, the Information Return by a Shareholder of a Passive Foreign Investment Company or Qualified Electing Fund (irs.gov), so a portfolio of a dozen funds can mean a dozen extra forms every year. Two elections, the Qualified Electing Fund (QEF) election and the section 1296 mark-to-market election, can produce less brutal outcomes, but a QEF election needs information most UK funds do not publish, and mark-to-market is only available for regularly traded stock. There is a limited exception to the annual report for smaller holdings, broadly where the aggregate value of all PFIC stock is $25,000 or less ($50,000 for joint filers) with no distributions or gains in the year, but it removes paperwork, not the underlying tax treatment when money eventually comes out. All of this sits on the US return, not the UK one; we set it out here so that the UK side of your affairs is arranged with full knowledge of it.

The PFIC rules target pooled passive vehicles, not investment generally, and it helps to be clear about what usually falls outside them.

• Direct shares in individual operating companies: a trading business normally fails both the income test and the asset test, so holding shares in listed companies directly does not typically create a PFIC issue. Dividends and gains are still taxable in both countries in the ordinary way, with treaty credit relief.
• Cash and bank deposits: a bank account is not a corporation at all. The interest is taxable in the US (and in the UK outside an ISA), and the account counts for FBAR purposes, but there is no PFIC dimension.
• US-domiciled funds and ETFs: a US fund is not a foreign corporation, so it cannot be a PFIC. It has a different problem on the UK side, covered below.
• Most employer pension schemes: funds held inside a UK registered pension are widely treated in practice as outside annual PFIC reporting, but the point is not settled in the US rules, so the position for any particular scheme sits with your US preparer; the pension itself has its own treaty analysis, covered in our guide to US retirement accounts and UK tax.

None of this is a recommendation to hold any particular asset. It is simply the map of where the PFIC regime does and does not reach, which is the starting point for any conversation with your advisers on both sides.

Nothing in US law changes the UK treatment. HMRC continues to regard the ISA as fully tax-free, the £20,000 subscription limit is unaffected by the holder's nationality, and nothing inside the wrapper goes on a UK Self Assessment return. The trap is entirely one-sided: the UK gives the relief, and the US ignores it.

The one-sidedness has a sting in the mechanics of double tax relief. Foreign tax credits work by crediting tax actually paid in one country against tax due in the other on the same income. Because the UK charges nothing on ISA income and gains, there is no UK tax to credit, so the US tax on that income stands in full. A US person can therefore pay more total tax on income inside an ISA than a UK-only saver pays on nothing at all, plus the compliance cost of reporting it.

The UK has its own version of the same instinct: suspicion of foreign pooled funds. Under the offshore funds rules, a gain on a non-UK fund only qualifies for Capital Gains Tax treatment if the fund has HMRC reporting fund status, broadly throughout the period you hold it. HMRC publishes the list of approved reporting funds and updates it monthly (GOV.UK). A gain on a fund without that status is an offshore income gain, taxed at Income Tax rates of up to 45% instead of CGT rates, with no CGT annual exempt amount.

For an American in the UK this is the mirror-image trap. From the UK's perspective a US mutual fund or ETF is an offshore fund, and many US mutual funds have never applied for UK reporting status, since they were built for a domestic US market. Some US-domiciled ETFs do appear on HMRC's list, and the fund's status can be checked against it by ISIN, but it can never be assumed. A couple who moved from the US with a portfolio of US funds can therefore find the US side taxing their UK funds punitively and the UK side taxing their US funds punitively, each country penalising the other's vehicles.

This is a fund-by-fund factual question, not a judgement call: a fund is either on the reporting funds list for the relevant period or it is not, and the answer decides whether a disposal goes on the capital gains pages or the income side of the return. Confirming the status of each holding is part of preparing an accurate UK return, and it is work we do routinely for clients with US portfolios.

Since 6 April 2025 the UK has offered qualifying new residents the 4-year Foreign Income and Gains regime (GOV.UK): anyone becoming UK resident after at least 10 consecutive non-resident tax years can claim full UK tax relief on qualifying foreign income and gains for their first four years, claimed each year on the SA109. Our FIG regime guide and FIG checker cover eligibility in detail.

For an American arriving with a US portfolio, FIG reshapes the UK half of the picture. Dividends, interest and gains on US holdings can be taken out of UK tax entirely during the window, and HMRC's guidance treats offshore income gains as qualifying foreign income, so even the non-reporting-fund trap on US funds can be relieved while a valid FIG claim is in place. The claim has a price, the loss of the Personal Allowance and the CGT annual exempt amount for that year, so it needs arithmetic rather than assumption, and from year five the normal UK rules, including the offshore income gain treatment, apply in full.

FIG does nothing for the US half. The US taxes its citizens on worldwide income regardless of any UK claim, so the PFIC regime and the ISA analysis are untouched, and because FIG removes the UK tax there is no UK credit to soften the US bill on that income. What the window really offers is time: four years in which the UK side is quiet and the mismatch between the two systems is at its most forgiving. Many new arrivals use that window to map every holding with both their UK and US advisers before the full two-system regime applies. Whether anything should change hands is a decision for you and your advisers; our role is to make sure the tax map underneath the decision is right.

We act on the UK side: residence under the SRT, the FIG claim and its arithmetic, checking each offshore fund against HMRC's reporting funds list, and preparing the Self Assessment return with the right split between capital gains and offshore income gains. We then supply your US preparer with the UK figures they need, and flag every UK product with a US consequence, ISAs above all, before it becomes a problem on the other side of the Atlantic. If you are still establishing whether you are a US tax resident in a move year, our Substantial Presence Test checker covers the US day-count test.

Everything is on fixed fees agreed upfront: personal returns from £350, non-resident and expat returns from £550, and complex cross-border work from £750. If you are a US person in the UK with an ISA, a fund portfolio on either side of the Atlantic, or a FIG window running, the earlier the two sides of your file are joined up, the cheaper the answer tends to be.