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7 Mistakes You're Making (and How to Fix Them)

Living in the UK as an American expat should be exciting — exploring new cities, advancing your career, enjoying a different pace of life. But then tax season rolls around, and suddenly you're wondering: “Do I really need to file two tax returns?” or “What's this FBAR thing everyone keeps mentioning?”

Here’s the truth: american expat tax uk requirements are more complex than most people realise. And making mistakes can cost you — literally. We’re talking penalties, double taxation, and sleepless nights wondering if you’ve done something wrong.

At ClearTaxation, we’re both Enrolled Agents and Chartered Tax Advisers, which means we know the US and UK tax systems inside out. We’ve seen these mistakes dozens of times, and the good news? They’re all fixable.

Let’s walk through the 7 biggest mistakes American expats make in the UK — and exactly how to fix them.

1. Assuming You Don’t Need to File a US Tax Return

This is the big one. You’ve moved to the UK. You’re paying UK taxes. You think, “Job done, right?”

Wrong.

The US taxes its citizens on worldwide income, no matter where you live. Even if you're a full-time UK resident and haven’t set foot in the US for years, you still need to file a US federal tax return annually.

Paying UK taxes doesn’t exempt you from US filing obligations. The US–UK tax treaty helps prevent double taxation, but it doesn’t eliminate your reporting requirements.

How to fix it: File your US tax return every year, even if you owe nothing. If you’ve missed previous years, look into the Streamlined Filing Compliance Procedures to catch up without triggering penalties. A qualified US–UK tax advisor can guide you through this quickly.

2. Ignoring FBAR and FATCA Reporting

If you have a bank account in the UK — even a basic current account — you probably need to report it to the IRS.

What trips people up: FBAR and FATCA aren’t part of your tax return. They’re separate filings with serious deadlines and penalties.

FBAR applies if:

The combined total of all your foreign accounts exceeds $10,000 at any point during the year

FATCA applies if:

Your foreign assets exceed $200,000 on the last day of the year (or $300,000 at any point during the year for expats)

Penalties for not filing start at $10,000 per violation and can go much higher.

How to fix it: Report all your UK bank accounts, ISAs, and investment accounts. File your FBAR UK report by April 15 (automatic extension to October 15). Don’t assume your day-to-day account is too small — if it pushes you over the threshold, it counts.

3. Misunderstanding Residency Rules

Just because you live in the UK doesn’t automatically make you a UK tax resident. Getting residency wrong can lead to underreporting (or overreporting) income on both sides.

The UK uses the Statutory Residence Test (SRT) based on:

Days spent in the UK
UK ties (family, accommodation, work)
Previous residency history

Get it wrong and you may pay tax twice — or face penalties.

How to fix it: Work through the SRT before filing. If your situation is complex (split-year, part-year), don’t guess. ClearTaxation helps expats navigate this because we’re qualified in both systems.

4. Not Using the US–UK Tax Treaty Properly

The US–UK tax treaty exists to prevent double taxation. But the benefits aren’t automatic.

You must actively claim treaty benefits using the correct forms. Without them, the IRS and HMRC could tax the same income.

Many expats also don’t realise they can use both:

FEIE excludes up to $120,000 of foreign earned income (2024 figure)
FTC gives a dollar-for-dollar credit for UK taxes paid

How to fix it: File Form 2555 (FEIE) and Form 1116 (FTC). Review income sources with a US–UK tax advisor to choose the best strategy — not a one-size-fits-all template.

5. Mishandling UK Investment Products (ISAs, Pensions, and PFICs)

This is where things get messy. UK investments that seem tax-efficient locally can create serious problems on your US return.

ISAs:

Tax-free in the UK
Fully taxable on your US return
Not like a Roth IRA (common mistake)

UK pension lump sums:

The UK’s 25% tax-free lump sum is not tax-free for US purposes

PFICs:

Many UK mutual funds/unit trusts trigger punitive US tax treatment
Complex reporting
Can cause more tax than the investment earned

How to fix it: Report ISA earnings properly on your US return. Review UK investments with a cross-border expert. If holding PFICs, consider restructuring. And before opening new accounts, consult a dual-qualified adviser like ClearTaxation.

6. Missing Deadlines and Not Pre-Paying UK Taxes

Here’s the timing trap: US and UK tax years don’t align.

US tax year: Jan 1 – Dec 31
UK tax year: Apr 6 – Apr 5

If you don’t pre-pay UK taxes by December 31, you could face double taxation because you can’t claim the credit in time.

Missing UK Self-Assessment (Jan 31) triggers penalties:

£100 immediately
More penalties after 3, 6, and 12 months

How to fix it: Pre-pay UK tax liability by Dec 31 where possible to align credits. Set reminders for both tax systems. Our proactive approach means we remind you before deadlines sneak up.

7. Overlooking Making Tax Digital (MTD) Requirements

HMRC’s Making Tax Digital (MTD) requires many taxpayers to keep digital records and file quarterly updates using approved software.

If HMRC notifies you, compliance is your responsibility. Ignoring it can lead to penalties.

Also, some expats file US state taxes when they don’t need to. Bona fide UK residents often don’t owe US state taxes — but it depends on the state and prior residency.

How to fix it: Enrol in MTD when required and use HMRC-approved software. Clarify US state tax obligations based on facts, not assumptions. ClearTaxation can handle the digital compliance side for you.

Why Dual Expertise Matters

At ClearTaxation, we’re both:

Enrolled Agents (licensed to represent taxpayers before the IRS)
Chartered Tax Advisers (qualified in the UK)

So you’re not juggling two advisers who don’t coordinate. You get one team that understands the full picture and builds one strategy across both countries.