US UK Tax Accountant: The PFIC Trap and the New FIG Regime Americans in Britain Need to Know

A record 8,790 US nationals applied for British citizenship in 2025, up 42% on the year before — even as citizenship applications across all other nationalities fell. Whatever’s driving that surge, it brings a genuinely complex tax reality with it: the US taxes citizens and green card holders on worldwide income regardless of where they live, and the UK abolished its long-standing Non-Dom regime in April 2025, replacing it with a new Foreign Income and Gains (FIG) system that changes how long-term UK residents — American or otherwise — are taxed on income and gains from outside the UK.

This guide covers what a specialist US-UK tax accountant actually does, the PFIC trap that catches Americans holding ordinary UK investments, why unfiled reporting carries genuinely open-ended risk, and what specialist support typically costs.

Quick Answer

A US-UK tax accountant handles the dual filing obligations facing American citizens, green card holders, and dual nationals living in the UK — a US Form 1040 alongside UK Self Assessment, FBAR and FATCA foreign account reporting, and PFIC analysis for UK investment funds and ISAs, which the IRS doesn’t recognise as tax-free. Since April 2025, the UK’s abolition of the Non-Dom regime and introduction of the new Foreign Income and Gains regime has changed how foreign income and gains are taxed for many UK residents, adding a fresh layer of complexity specifically for Americans navigating both systems. Fees typically run £600–£2,500 a year depending on the complexity of investments, property, and reporting obligations involved.

Key Takeaways

• Over 80,000 Americans live in London alone, part of a UK-wide US-born population exceeding 230,000.
• The UK’s Non-Dom regime was abolished from April 2025, replaced by a new Foreign Income and Gains (FIG) regime that changes how long-term residents are taxed on income and gains from outside the UK.
• UK ISAs offer no US tax benefit — interest, dividends, and gains inside an ISA are fully reportable and taxable on a US return, and most UK-domiciled funds held inside one trigger punitive PFIC rules.
• An unfiled Form 8621 (PFIC reporting) leaves the entire tax year open indefinitely under US law — there’s no statute of limitations until the form is actually filed.
• FATCA data now flows from UK financial institutions to the IRS via HMRC’s Automatic Exchange of Information regime, making it considerably harder for unreported UK accounts to go unnoticed than in the past.
• Typical fees run £600–£2,500 a year, depending on investment complexity and reporting obligations.

Table of Contents

1. What Does a US-UK Tax Accountant Actually Do?
2. The Non-Dom Abolition and New FIG Regime
3. The PFIC Trap: Why UK ISAs and Funds Cause Problems
4. No Statute of Limitations: The Real Cost of an Unfiled Form 8621
5. FATCA and Automatic Exchange of Information
6. FBAR vs FATCA: Two Separate, Overlapping Obligations
7. Americans in London: What We See
8. A Worked Example: The ISA Reporting Gap
9. How Much Does It Cost?
10. Common Mistakes People Make
11. Accountant Insights: What We See in Practice
12. Do You Need a Specialist US-UK Accountant? (Decision Framework)
13. General Accountant vs Cross-Border Specialist
14. Checklists
15. FAQs
16. Sources
17. Final Thoughts

What Does a US-UK Tax Accountant Actually Do?

Beyond preparing a standard UK Self Assessment return, a specialist handles the parallel US Form 1040, applies the Foreign Tax Credit or Foreign Earned Income Exclusion to avoid double taxation, identifies PFIC exposure in UK investment holdings before it becomes a compliance problem, files FBAR and FATCA disclosures for UK accounts, and — since April 2025 — advises on how the new FIG regime interacts with US citizenship-based taxation for anyone who’s been UK resident for a while.

The Non-Dom Abolition and New FIG Regime

From 6 April 2025, the UK abolished its centuries-old “non-domiciled” tax status, replacing it with a residence-based Foreign Income and Gains (FIG) regime. Broadly, new UK residents can access a time-limited window (generally the first four years of UK residence) where qualifying foreign income and gains aren’t taxed in the UK — but once that window closes, or for anyone who’s already been UK resident longer, foreign income and gains generally become taxable in the UK on the same basis as UK income. For Americans, this sits alongside — not instead of — US citizenship-based taxation, meaning the UK side of the equation has genuinely changed in the past year in a way that affects tax planning for anyone recently arrived, or approaching the end of their FIG window, regardless of what the US side of their return requires.

The PFIC Trap: Why UK ISAs and Funds Cause Problems

This is the single most common, and most consequential, mistake Americans in the UK make with their investments. The IRS doesn’t recognise the UK ISA as a tax-advantaged wrapper at all — every pound of interest, dividends, and capital gains generated inside an ISA is fully reportable and taxable on a US return, exactly as if the ISA didn’t exist. Worse, most UK-domiciled investment funds — the kind sold by virtually every major UK platform, including Vanguard UK, Fidelity UK, HSBC UK, and funds available through Hargreaves Lansdown or AJ Bell — are classified by the IRS as Passive Foreign Investment Companies (PFICs). Holding a PFIC as a US person triggers punitive tax treatment: gains taxed at ordinary income rates rather than favourable capital gains rates, IRS interest charges on tax treated as deferred, and a Form 8621 filing requirement for every single PFIC held, every year. Crucially, holding these funds inside an ISA or SIPP wrapper doesn’t change PFIC status at all — US tax law looks straight through the UK wrapper to the underlying investment.

No Statute of Limitations: The Real Cost of an Unfiled Form 8621

This is a detail that makes PFIC compliance genuinely urgent rather than a background administrative task. Under US tax law (IRC Section 6501(c)(8)), failing to file a required Form 8621 for a PFIC holding keeps your entire US tax return for that year open indefinitely — there’s no statute of limitations protection until the missing form is actually filed. For most other tax matters, the IRS generally has a limited window (typically three years) to review and challenge a return. An unfiled PFIC form removes that protection entirely for the whole return, not just the PFIC-related portion, meaning a genuinely old, otherwise-settled tax year can still be reopened years later purely because one investment fund was never properly reported.

FATCA and Automatic Exchange of Information

Since FATCA came into force, UK financial institutions have been required to identify accounts held by US persons and report them — and that data now flows to the IRS through HMRC’s Automatic Exchange of Information regime, giving US tax authorities direct visibility into UK-held accounts without relying on voluntary disclosure. In practice, this means the historic option of simply not mentioning a UK bank account, ISA, or pension on a US return has become considerably riskier than it once was, since the account information itself is often already known to the IRS through this reporting channel — the question isn’t whether the account is visible, but whether the US return correctly reflects it.

FBAR vs FATCA: Two Separate, Overlapping Obligations

Americans with UK accounts often need to file both an FBAR (FinCEN Form 114) and IRS Form 8938 under FATCA for the very same accounts — these are separate information returns with separate thresholds and separate penalty regimes, not alternatives to each other. FBAR applies once the aggregate value of foreign financial accounts exceeds $10,000 at any point in the year; Form 8938 has its own, generally higher thresholds that vary by filing status and UK residency. Current maximum FBAR civil penalties run into the tens of thousands of dollars for non-wilful violations, and considerably higher for wilful non-compliance — genuinely serious consequences for what often started as simply not realising a UK current account or ISA needed reporting at all.

Americans in London: What We See

Over 80,000 Americans call London home, concentrated heavily in Kensington, Chelsea, and Islington, out of a UK-wide US-born population exceeding 230,000 — and that population is growing more engaged with UK residency, not less: a record 8,790 US nationals applied for British citizenship in 2025, up 42% on 2024, even as citizenship grants across other nationalities fell. For London’s American community specifically, that trend means a growing number of people crossing from short-term assignment into genuinely long-term UK residence — exactly the point at which the new FIG regime’s time-limited window matters most, and where PFIC exposure in UK investment accounts, often built up gradually and without realising the US tax consequences, becomes a genuinely material compliance issue rather than a background curiosity.

A Worked Example: The ISA Reporting Gap

Illustrative Example: Say an American professional living in London for six years has steadily contributed to a Stocks & Shares ISA holding several mainstream UK investment funds, treating it — reasonably, from a UK perspective — as a genuinely tax-free wrapper. On their US return, none of this has been reported, since it never occurred to them that a UK tax-free account would need declaring to the IRS at all. In reality, every fund inside that ISA is a PFIC, meaning six years of unfiled Form 8621s leave every one of those tax years open indefinitely under Section 6501(c)(8) — not just the current year, but potentially the entire six-year history, until the missing forms are filed and the position is properly corrected.

Illustrative Example: A different American, newly arrived in the UK in 2026, structures their affairs properly from day one — reviewing which UK investments would trigger PFIC treatment before committing any money, and confirming their position under the new FIG regime’s four-year window. Getting this right from the outset avoids the years-long unwinding process the first example faces, at a fraction of the eventual cost.

How Much Does It Cost?

Common Mistakes People Make

1. Assuming a UK ISA is tax-free for US purposes too
Why it happens: The ISA is genuinely UK tax-free, and it’s a natural (but incorrect) assumption that this extends to US reporting.
Consequence: Years of unreported interest, dividends, and gains, plus unfiled PFIC forms for any funds held inside it.
How to avoid it: Treat every UK ISA as fully reportable on your US return from the outset.

2. Buying UK-domiciled funds without checking PFIC status first
Why it happens: Fund selection is usually driven by UK investment considerations, not US tax classification.
Consequence: Punitive PFIC tax treatment and ongoing Form 8621 filing obligations for every fund held.
How to avoid it: Check PFIC status before purchasing, and consider US-domiciled fund alternatives where available.

3. Treating an unfiled PFIC form as a minor oversight
Why it happens: A single missing form for one investment can feel like a small administrative gap.
Consequence: The entire tax year remains open indefinitely under US law until the form is filed.
How to avoid it: Prioritise PFIC catch-up filing specifically, given the open-ended exposure involved.

4. Assuming an unreported UK account will simply go unnoticed
Why it happens: Older assumptions about account privacy haven’t caught up with current FATCA data-sharing.
Consequence: A discrepancy between IRS-held FATCA data and an incomplete US return.
How to avoid it: Assume any UK financial account is already visible to the IRS, and report accordingly.

5. Not reviewing the new FIG regime’s impact on long-term UK residency plans
Why it happens: The change is recent enough that many haven’t revisited their position since April 2025.
Consequence: Missing planning opportunities within the FIG window, or an unexpected UK tax position once it closes.
How to avoid it: Get your FIG regime position reviewed specifically if you’re approaching four years of UK residence.

Accountant Insights: What We See in Practice

• The PFIC/ISA misunderstanding is, in our experience, the single most common and costly issue we see among Americans in the UK — it’s rarely explained clearly before someone’s already built up years of holdings.
• Clients newly arrived in the UK who get PFIC and FIG regime advice before investing consistently avoid the expensive catch-up process others face later.
• The open-ended Form 8621 exposure is something we flag as genuinely urgent, not routine — the earlier it’s addressed, the smaller the eventual correction.
• London’s growing base of long-term American residents, reflected in the 2025 citizenship application surge, is increasingly asking about FIG regime planning specifically, rather than treating their UK stay as purely temporary.
• Combining UK and US filing through a single coordinated engagement consistently produces fewer inconsistencies than treating the two returns as entirely separate exercises.

Do You Need a Specialist US-UK Accountant?

Step 1: Review your UK investment holdings for PFIC exposure. Check ISAs, SIPPs, and general investment accounts specifically.

Step 2: Confirm your FBAR and FATCA filing status. Both may apply to the same accounts.

Step 3: Review your position under the new FIG regime. Particularly if recently arrived or approaching the four-year window’s end.

Step 4: Address any unfiled PFIC reporting urgently. The open-ended statute of limitations makes early correction genuinely valuable.

Step 5: Choose based on genuine cross-border experience. UK Self Assessment and US 1040 filing together need specialist, current knowledge of both systems.

General Accountant vs Cross-Border Specialist

Checklists

Checklist 1: For Americans Newly Arrived in the UK

• ✓ Review your position under the new FIG regime’s four-year window
• ✓ Check PFIC status before purchasing any UK-domiciled fund
• ✓ Set up FBAR/FATCA tracking for UK accounts from the outset
• ✓ Confirm US filing obligations continue despite UK residency

Checklist 2: For Long-Term American UK Residents

• ✓ Review existing ISA and investment holdings for PFIC exposure
• ✓ Confirm all required FBAR and Form 8938 filings are current
• ✓ Address any unfiled Form 8621s as a priority, given the open-ended risk
• ✓ Review FIG regime status if approaching the end of the four-year window

FAQs

Do Americans living in the UK still need to file US taxes?
Yes — the US taxes citizens and green card holders on worldwide income regardless of where they live, so UK residence doesn’t remove the US filing obligation.

Is a UK ISA tax-free for US tax purposes?
No — the IRS doesn’t recognise the ISA wrapper, so interest, dividends, and gains inside it are fully reportable and taxable on a US return.

What is a PFIC?
A Passive Foreign Investment Company — most UK-domiciled investment funds fall into this category, triggering punitive US tax treatment and annual Form 8621 filing requirements.

What changed with the UK’s Non-Dom regime?
It was abolished from April 2025 and replaced with a residence-based Foreign Income and Gains (FIG) regime, generally offering a four-year window for new UK residents before foreign income and gains become fully taxable in the UK.

What happens if I never filed a required Form 8621?
Under US law, the entire tax year remains open indefinitely until the form is filed — there’s no statute of limitations protection in the meantime.

What is the difference between FBAR and FATCA reporting?
They’re separate obligations with separate thresholds and penalties — FBAR (FinCEN Form 114) applies once foreign accounts exceed $10,000 aggregate, while FATCA’s Form 8938 has its own, generally higher thresholds.

Does HMRC share UK account information with the IRS?
Yes — UK financial institutions report US-person accounts, and this data flows to the IRS through HMRC’s Automatic Exchange of Information regime.

How much does a US-UK tax accountant cost?
Typically £600–£2,500 a year depending on investment complexity and reporting obligations, with PFIC analysis often priced separately per fund.

Can I avoid PFIC treatment by using US-domiciled funds instead?
Often yes — US-domiciled fund alternatives, even those accessible in the UK, generally avoid PFIC classification entirely.

Should I get advice before or after investing in a UK fund?
Before — checking PFIC status ahead of purchase avoids years of complicated, costly reporting obligations later.

Sources

• IRS — Publication 54: Tax Guide for US Citizens and Resident Aliens Abroad
• IRS — Form 8621: Information Return by a Shareholder of a PFIC
• GOV.UK — The new Foreign Income and Gains regime
• GOV.UK — FATCA and Automatic Exchange of Information
• FinCEN — Report of Foreign Bank and Financial Accounts (FBAR)

US and UK tax rules, thresholds, and reporting requirements are complex and subject to change — always confirm your specific position with a qualified cross-border specialist and official IRS/GOV.UK guidance before relying on this information.

Final Thoughts

Americans in the UK face a genuinely distinct tax picture — US citizenship-based taxation running alongside a UK system that’s just undergone its own major reform with the Non-Dom abolition and new FIG regime. The PFIC trap inside ordinary UK ISAs catches out even financially engaged people, and the open-ended exposure from an unfiled Form 8621 makes early, proper advice genuinely valuable rather than a nice-to-have. A specialist US-UK tax accountant should be reviewing both sides of this picture together, not treating them as two unrelated returns.

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