Do I have to file a US tax return if all my income is UK-source and I already pay HMRC?

Yes, if your worldwide gross income reaches the threshold for your filing status. The IRS applies the same filing rules to citizens abroad as to residents, and there is no exemption for having no US-source income. UK tax paid usually generates a foreign tax credit that reduces or eliminates the US liability, but the credit is claimed on a filed return. A nil balance due is not the same as no filing requirement.

What is the US filing threshold if I am married filing separately from a non-US spouse?

For tax year 2025 it is $5 of gross income, at any age, per Chart A in the Instructions for Form 1040. In practice that means a return every year. Filing separately also halves the net investment income tax and Additional Medicare Tax thresholds to $125,000. It does not, however, reduce your Form 8938 threshold to the domestic level, because the higher living-abroad thresholds still apply if you meet the presence test.

Does the foreign earned income exclusion put me below the filing threshold?

No. The IRS requires you to count as gross income any amount you exclude as foreign earned income or as a foreign housing amount when testing whether you must file. The exclusion, capped at $130,000 for tax year 2025, is also an election made on Form 2555 filed with your return, and the initial choice must generally be made on a timely filed return including extensions. Claiming it requires filing, not the reverse.

Does my UK ISA or current account count toward the FBAR threshold?

Yes. The FBAR threshold is $10,000 in aggregate across all foreign financial accounts at any time during the calendar year, and a UK current account, savings account, ISA or brokerage account is a foreign financial account for this purpose. The ISA wrapper is a UK tax shelter with no US effect. Because the test uses maximum value in dollars, exchange rate movement alone can push a static sterling balance over the line.

When is my US tax return due if I live in London?

April 15 is the regular due date and the date tax should be paid. Filers whose main place of business is outside the United States get an automatic two-month extension to June 15 by attaching a statement to the return. Form 4868 extends filing to October 15. Publication 54 also describes a discretionary further two months to December 15, requested by letter to the IRS by October 15. None of these extend time to pay.

Do the net investment income tax thresholds change because I live in the UK?

No. The thresholds stay at $250,000 married filing jointly, $125,000 married filing separately and $200,000 single or head of household, and the IRS confirms they are not indexed for inflation. Living abroad makes the position worse in two ways: excluded foreign earned income is added back when computing modified adjusted gross income, and the foreign tax credit cannot be used against the 3.8 percent charge because it is allowed only against chapter 1 tax.

US Tax Filing Threshold for Expats in the UK

The first US tax filing threshold expat rule you meet as an American living in the UK is the worldwide gross income test on Form 1040, and for tax year 2025 it starts at $15,750 for a single filer under 65, $31,500 for a married couple filing jointly with both spouses under 65, and just $5 of gross income if you are married filing separately. Those figures are set out in Chart A of the Instructions for Form 1040 at https://www.irs.gov/instructions/i1040gi. If your worldwide gross income reaches the figure for your status, you file, whether or not a single dollar of that income came from the United States and whether or not you end up owing any US tax at all.

For the clients we prepare returns for on trading floors and in fund management offices in London, that gross income test is almost never the binding constraint. A managing director clears it in the first week of January. The thresholds that actually generate the work are the information reporting ones, and they are completely independent of the income test and of each other: FBAR at $10,000 of aggregate foreign account value, Form 8938 at $200,000 or $400,000 for people who genuinely live abroad, Form 5471 at 10 percent ownership of a foreign company, Form 8621 with no meaningful dollar floor at all, Form 3520 for large gifts from foreign persons, and two surtaxes that switch on at $200,000 and $250,000 of income and have never been indexed for inflation.

The failure mode we see most often is not a missing return. It is a filer who correctly concluded they had to file, filed a clean Form 1040 with a foreign tax credit, and never tested the six other thresholds sitting alongside it. This guide walks each threshold in the order it bites, with every figure verified against IRS, FinCEN and GOV.UK sources.

What is the US tax filing threshold expat rule for Americans in the UK?

The US tax filing threshold expat rule is simply the domestic rule applied to worldwide income. The IRS states at https://www.irs.gov/individuals/international-taxpayers/us-citizens-and-residents-abroad-filing-requirements that US citizens and resident aliens living outside the United States are generally required to file income tax returns in the same way as those residing in the United States. You file if your gross income from worldwide sources reaches the amount shown for your filing status.

Gross income here is a deliberately wide concept: all income you receive in money, goods, property and services that is not exempt from tax. Two points in the IRS wording do most of the damage in practice. First, in determining whether you must file, you must count as gross income any amount you exclude as foreign earned income or as a foreign housing amount. Second, if you are self-employed, gross income includes the amount on the Gross Income line of Schedule C, not your profit. Both rules push filers over a line they assumed they were under.

The tax year 2025 figures from Chart A in the Instructions for Form 1040 are:

Single: $15,750 under 65, $17,750 if 65 or older
Married filing jointly: $31,500 with both spouses under 65, $33,100 if one spouse is 65 or older, $34,700 if both are
Married filing separately: $5 of gross income, at any age
Head of household: $23,625 under 65, $25,625 if 65 or older
Qualifying surviving spouse: $31,500 under 65, $33,100 if 65 or older
Any filing status, if net earnings from self-employment were $400 or more

These amounts are adjusted for inflation each year. Publication 54, the IRS guide for citizens and resident aliens abroad at https://www.irs.gov/publications/p54, moved to a continuous-use format from tax year 2025 and no longer prints the table itself, instead pointing filers to the Form 1040 instructions and to https://www.irs.gov/inflationadjustment. If you are reading this for a year other than 2025, take the concept from here and take the number from those two pages.

Why is married filing separately the threshold trap for a banker with a non-US spouse?

This is the interaction that competitor guides consistently miss, and it is the single most common profile in our London client base: a US citizen banker married to a British spouse who is not a US person. Unless a specific election is made to treat the non-US spouse as a US resident for income tax purposes, which drags that spouse's worldwide income into the US net permanently until revoked, the American files as married filing separately.

That one status choice moves three separate thresholds in three different directions at the same time:

The Form 1040 gross income threshold collapses from $31,500 to $5. There is effectively no income floor at all, so the return is mandatory in every year in which any income arises
The net investment income tax and Additional Medicare Tax thresholds halve, from $250,000 for a joint filer to $125,000 for a separate filer, so the 3.8 percent and 0.9 percent surtaxes start biting at half the income level
The Form 8938 threshold does not fall to the domestic married-filing-separately level of $50,000 and $75,000. Because the filer lives abroad, the not-married-filing-jointly abroad thresholds of $200,000 at year end and $300,000 at any time apply instead

In other words, filing separately makes the income return unavoidable and the surtaxes harsher, while leaving the FATCA asset reporting threshold at the generous expatriate level. A filer who assumes the three move together will get at least one of them wrong. We model both the separate-filing position and the election position before we choose, because the election is easy to make and painful to unwind.

Does self-employment or partnership income change the filing threshold?

Yes, and it lowers it dramatically. Publication 54 states that if your net earnings from self-employment are $400 or more, you must file a return even if your gross income is below the amount listed for your filing status. That $400 figure is not a typo and it is not indexed. It catches the banker who takes a single consulting engagement after leaving a bank, the investor who sits on an advisory board for a modest fee, and the business owner drawing income through a UK partnership or LLP rather than through PAYE.

The Schedule C rule compounds it. Because gross income for the threshold test is the Gross Income line of Schedule C rather than the net profit, a consultant billing significant fees with equally significant costs is measured on the top line. A person who genuinely made very little money can still be well over the filing threshold on gross receipts alone.

Whether US self-employment tax is actually payable on that income is a separate question, governed by the social security agreement between the United States and the United Kingdom and by which country's system the individual is covered under. That analysis affects the tax due. It does not affect the filing obligation, which is triggered by the $400 figure regardless.

Do the foreign earned income exclusion and foreign tax credit remove the obligation to file?

No, and the logic runs the other way. Both reliefs are claimed on a return, so they presuppose a return. The foreign earned income exclusion for tax year 2025 is a maximum of $130,000, and the Instructions for Form 2555 at https://www.irs.gov/instructions/i2555 state that to choose the exclusion you complete Form 2555 and file it with your Form 1040, 1040-SR or 1040-X, and that the initial choice must usually be made on a timely filed return, including extensions, or on a return amending a timely filed return. An unclaimed year is not a year you were under the threshold. It is a year you failed to make an election.

The foreign tax credit on Form 1116 behaves the same way. UK income tax paid at rates well above the equivalent US rates frequently wipes out the US liability on employment income entirely. That is a computation performed on a filed return. It produces a nil balance due, not an absence of a filing requirement. And because the gross income test explicitly counts amounts excluded under the foreign earned income exclusion, a filer earning $200,000 in London who excludes $130,000 of it is still measured on the full $200,000 for threshold purposes.

This is the second gap most guides skip: there is no de minimis rule for having no US-source income. A US citizen whose entire economic life is in the United Kingdom, whose employer is British, whose bank is British and who has not set foot in the United States for a decade is measured against exactly the same thresholds as a filer in Ohio.

When is the US return actually due if you live in London?

There is a ladder of extensions, and high earners abroad routinely need all of it because UK employment and investment data is not final until well after the US deadline. The IRS sets out the position at https://www.irs.gov/individuals/international-taxpayers/us-citizens-and-resident-aliens-abroad-where-and-when-to-file-and-pay.

April 15: the regular due date, and the date by which tax should be paid regardless of any filing extension. Interest runs on unpaid tax from this date
June 15: an automatic two-month extension for a filer who lives outside the United States and Puerto Rico with their main place of business or post of duty outside the United States and Puerto Rico. It is claimed by attaching a statement to the return explaining which situation qualified you
October 15: an additional four-month extension obtained by filing Form 4868 before the automatic two-month extension date. It extends time to file, not time to pay
December 15: a discretionary further two months for calendar-year filers still out of the country. Publication 54 explains that this is requested by sending the IRS a letter explaining why the additional two months are needed, by October 15, and that the IRS will not notify you unless the request is denied
Form 2350: a separate extension for filers who need more time to satisfy the bona fide residence or physical presence test in order to qualify for the foreign earned income exclusion

Note what none of these do. None of them extend the time to pay, and none of them extend the FBAR, which runs on its own calendar described below.

What is the FBAR threshold and how does it differ from the return threshold?

The FBAR, FinCEN Form 114, is required where the aggregate value of a US person's foreign financial accounts exceeded $10,000 at any time during the calendar year reported. The IRS summarises the rule at https://www.irs.gov/businesses/small-businesses-self-employed/report-of-foreign-bank-and-financial-accounts-fbar and FinCEN publishes the underlying guidance at https://www.fincen.gov/report-foreign-bank-and-financial-accounts. It is filed electronically through FinCEN's BSA E-Filing System and is not filed with your federal tax return. It is due April 15 with an automatic extension to October 15 that requires no request.

Four features of the threshold catch high earners repeatedly:

It is aggregate, not per account. Ten UK accounts holding $1,500 each cross the line even though none of them individually comes close
It is a maximum value test, not a year-end test. A bonus that lands in a UK current account in March and is deployed elsewhere by December still sets the reportable maximum for that account
It covers signature or other authority, not only beneficial ownership. Bankers who are signatories on an employer account, a UK company account or a family business account can have a filing obligation over money that is not theirs
It is a dollar threshold applied to sterling balances. A UK account whose balance never changes can cross $10,000 purely on exchange rate movement, which means the answer to whether you file can differ between two otherwise identical years

If FBARs were missed, they are filed through the BSA E-Filing System with a reason for late filing selected in the system, or brought up to date inside the Streamlined Filing Compliance Procedures where the underlying returns also need correcting. Do not rely on older commentary describing a separately branded IRS route for delinquent FBARs, which is no longer published.

What are the Form 8938 FATCA thresholds for taxpayers living abroad?

Form 8938 reports specified foreign financial assets and attaches to the Form 1040. The thresholds for taxpayers living abroad are materially higher than the domestic ones, and the IRS sets them out at https://www.irs.gov/businesses/corporations/summary-of-fatca-reporting-for-us-taxpayers.

Living abroad, married filing jointly: more than $400,000 on the last day of the tax year, or more than $600,000 at any time during the year
Living abroad, not married filing jointly: more than $200,000 on the last day of the tax year, or more than $300,000 at any time during the year
Living in the United States, unmarried or married filing separately: more than $50,000 at year end or more than $75,000 at any time
Living in the United States, married filing jointly: more than $100,000 at year end or more than $150,000 at any time

Do not assume the higher figures apply just because you have a London address. The IRS conditions the abroad thresholds on a presence test built on the same bona fide residence and physical presence concepts used elsewhere in the international rules, and the definition is set out on that same IRS page. A filer who moved mid-year, or who spent an unusual number of days in the United States on deal work, can find themselves measured against the $50,000 figure instead of the $200,000 one.

Two further points. Filing Form 8938 does not relieve the FBAR obligation, and filing the FBAR does not relieve Form 8938. They are separate filings to separate agencies with separate thresholds and separate penalty regimes. And specified foreign financial assets are broader than accounts: they include stock issued by a non-US corporation, an interest in a non-US partnership, and non-US-issued financial instruments held for investment, which is precisely the asset profile of a senior banker with carried interest, private company stock and a UK brokerage account.

Which ownership stakes trigger Form 5471?

Form 5471 has no dollar threshold at all. It is triggered by ownership and status, which is why business owners and bankers with side investments cross it without noticing. The Instructions for Form 5471 at https://www.irs.gov/instructions/i5471 set out the categories of filer. The tests that matter most in a UK context are:

Acquiring stock which, when added to stock already owned, meets the 10 percent threshold of total combined voting power or value of a foreign corporation, or acquiring an additional 10 percent or more
Disposing of enough stock to fall below the 10 percent level
Control, defined as owning more than 50 percent of the total combined voting power or value of shares at any time during the tax year
Being a US shareholder of a controlled foreign corporation, being a foreign corporation in which US shareholders own more than 50 percent of voting power or value on any day of the tax year

A UK private limited company used as a personal services vehicle, a stake in a friend's UK startup, or a co-founded consultancy all sit inside these tests. The ownership percentages count indirect and constructive ownership, so shares held through another entity or attributed from a family member count too. Form 5471 attaches to the income tax return and is filed by its due date including extensions. The penalty for failure is $10,000 for each annual accounting period of each foreign corporation, with a further $10,000 for each 30-day period beginning 90 days after IRS notice, capped at $50,000 for each failure. A dormant UK company with no profit and no distributions generates the same penalty exposure as a trading one.

When does a UK fund holding trigger Form 8621 as a PFIC?

A passive foreign investment company is a foreign corporation meeting either of two tests set out in the Instructions for Form 8621 at https://www.irs.gov/instructions/i8621: 75 percent or more of its gross income for the tax year is passive income, or at least 50 percent of the average percentage of assets it held during the year are assets that produce passive income. Most pooled investment vehicles domiciled outside the United States meet at least one of these by design.

For an American investor in the UK, that description covers a very large share of the retail and platform fund universe, including UK-domiciled open-ended funds and non-US exchange traded funds bought through a UK broker or held inside a stocks and shares ISA. The ISA wrapper is a UK tax wrapper. It has no effect on the US characterisation of what is inside it.

There is no general dollar exemption. A US person files Form 8621 as a direct or indirect shareholder in circumstances including receiving distributions from a PFIC, recognising gain on a disposition of PFIC stock, making or reporting a qualified electing fund or mark-to-market election, and being required to file the annual report under section 1298(f). There is a narrow de minimis rule, and it is narrower than most readers assume: a shareholder is not required to complete Part I in respect of a specific section 1291 fund where the aggregate value test of $25,000, or $50,000 on a joint return, is met on the last day of the shareholder's tax year, and only where the shareholder received no excess distribution and recognised no gain on that fund during the year. It relieves part of one form in limited circumstances. It is not a general exemption from PFIC reporting.

What is the reporting threshold for gifts from foreign persons?

This one catches people who did nothing wrong and owe nothing. The IRS explains at https://www.irs.gov/businesses/gifts-from-foreign-person that a US person must report on Form 3520 gifts from a nonresident alien individual where the aggregate received during the taxable year exceeds $100,000, separately identifying each gift in excess of $5,000. A separate and much lower threshold applies to gifts received from foreign corporations or foreign partnerships: the inflation-adjusted section 6039F amount, which the IRS gives as $20,116 for 2025 and $20,573 for 2026.

A foreign gift is generally not taxable income to the recipient. The obligation is pure reporting, but the penalty is not trivial: 5 percent of the value of the gift for each month it is not reported, capped at 25 percent, unless there is reasonable cause. The Instructions for Form 3520 at https://www.irs.gov/instructions/i3520 confirm the form is due by the 15th day of the 4th month following the end of the person's tax year and is filed separately, to the IRS service centre in Ogden, Utah, rather than with the Form 1040. In a UK context the usual trigger is family support: British parents or parents-in-law helping with a property deposit in London, where a six-figure sterling transfer converts to well over $100,000.

Which surtax thresholds catch high earners specifically?

Two, and neither has ever been indexed for inflation, which is why they capture more people every year. The net investment income tax is 3.8 percent and applies where modified adjusted gross income exceeds $250,000 for married filing jointly, $125,000 for married filing separately, and $200,000 for single and head of household filers. The IRS confirms at https://www.irs.gov/newsroom/questions-and-answers-on-the-net-investment-income-tax that taxpayers should be aware that these threshold amounts are not indexed for inflation.

Two features of the NIIT make it the most expensive threshold on this list for an American in the UK, and this is the fifth gap most guides skip. First, the foreign earned income exclusion works against you here: for filers claiming it, modified adjusted gross income is computed by increasing adjusted gross income by the amount excluded under section 911(a)(1), net of the deductions disallowed under section 911(d)(6). The exclusion that reduced your regular tax pushes your MAGI back up for NIIT purposes. Second, credits allowed only against the tax imposed by chapter 1 of the Code, which includes the foreign tax credit, may not be used to reduce NIIT liability, although foreign taxes taken as a deduction rather than a credit may reduce net investment income. The practical result is that a London banker can pay UK tax at high rates on investment income, owe no regular US tax after credits, and still write a cheque to the IRS for 3.8 percent.

The Additional Medicare Tax is 0.9 percent, with the same threshold pattern of $250,000 married filing jointly, $125,000 married filing separately and $200,000 for single and head of household filers, as set out at https://www.irs.gov/businesses/small-businesses-self-employed/questions-and-answers-for-the-additional-medicare-tax. It applies to wages and self-employment income that are subject to US Medicare tax, so the exposure depends on whether the individual is covered by the US or the UK social security system. Where an American remains on a US payroll or on a US-employer secondment, an employer must withhold the tax on wages over $200,000 in a calendar year without regard to the individual's filing status, which routinely under-withholds or over-withholds relative to the actual $125,000 or $250,000 threshold and shows up as a balancing item on the return.

A worked example: one London banker, eight filings

The following is an illustration only. The figures are invented for the purpose of showing how the thresholds interact, and a single exchange rate of 1.25 US dollars to the pound is assumed throughout purely to keep the arithmetic legible. It is not a rate to use on a return, and the numbers are not a client's.

Assume a US citizen managing director on a London trading floor, married to a British spouse who is not a US person and who has not been made subject to a residency election. Assume salary and bonus of 400,000 pounds, giving 500,000 dollars of gross income. Assume UK current, savings and brokerage accounts with a combined peak value during the year of 520,000 pounds, or 650,000 dollars, driven by a bonus that landed in March. Assume a 12 percent stake acquired during the year in a UK fintech limited company, two UK-domiciled funds held in a general investment account, and a 90,000 pound gift from the spouse's parents toward a house deposit, being 112,500 dollars.

Form 1040: filing separately, so the gross income threshold is $5. The return is mandatory. Foreign earned income exclusion is available up to $130,000 for 2025, with a foreign tax credit on the balance
Form 2555 and Form 1116: both needed, and both only work because a return is filed on time
FBAR: the $10,000 aggregate threshold was cleared many times over, and the reportable maximum is set by the March peak, not the 31 December balance
Form 8938: living abroad and not filing jointly, so the test is more than $200,000 at year end or more than $300,000 at any time. Comfortably exceeded, and the fintech shareholding is a specified foreign financial asset in its own right
Form 5471: acquiring a stake that crossed the 10 percent level triggers a reporting category on acquisition. Whether a filing is required in later years depends on whether the company is a controlled foreign corporation, which turns on whether US shareholders together hold more than 50 percent
Form 8621: each UK-domiciled fund is separately tested as a PFIC, and the $25,000 de minimis rule offers no help once distributions or gains arise
Form 3520: the gift converts to $112,500, above the $100,000 threshold for gifts from a nonresident alien individual, so a separate form goes to Ogden
Form 8960: married filing separately halves the NIIT threshold to $125,000, and the foreign earned income exclusion is added back in the MAGI computation, so 3.8 percent applies to the investment income with no foreign tax credit relief against it

The regular US income tax due in this illustration, after foreign tax credits on UK tax paid at higher UK rates, is small. The compliance is eight filings, two agencies and three separate penalty regimes. That gap between tax due and work required is the defining feature of high-earner US compliance in the United Kingdom, and it is why threshold testing has to be done asset by asset rather than by looking at the tax bill.

How do the UK deadlines interact with the US thresholds?

The two systems do not share a calendar, which creates real work on the foreign tax credit. GOV.UK confirms at https://www.gov.uk/self-assessment-tax-returns/deadlines that the UK tax year runs from 6 April to 5 April, that a paper Self Assessment return is due by 31 October following the end of the tax year, and that the online return and the balancing payment are due by 31 January, with payments on account falling due on 31 July. The US year is the calendar year. Every UK figure that feeds a US return therefore has to be re-cut across a different twelve months.

On the UK side there is no equivalent of the Form 1040 gross income chart. Self Assessment is triggered by circumstances rather than by a single universal income figure, and GOV.UK explains at https://www.gov.uk/tax-foreign-income that UK residents generally pay UK tax on worldwide income and report foreign income through Self Assessment unless relief applies, with the treatment of individuals with a permanent home abroad having changed from 6 April 2025. The practical sequencing point for anyone with both obligations is to settle the UK position first where the timetable allows, because the final UK liability is the input to the US foreign tax credit, and the 31 January UK deadline sits ahead of the extended US deadlines described earlier.

What we test before signing off a return

In the returns we prepare, threshold testing is a fixed annual sweep rather than a judgement call, because most of these thresholds are binary and most of them are cheap to clear and expensive to miss. The sweep runs across the gross income position by filing status, net earnings from self-employment against the $400 figure, aggregate peak foreign account value in dollars for FBAR, specified foreign financial assets against both the year-end and any-time Form 8938 figures on the correct abroad or domestic basis, every non-US corporate interest against the 10 percent and 50 percent Form 5471 tests including indirect and constructive holdings, every pooled investment for PFIC status, any gift received from a foreign person against both the $100,000 and the section 6039F figures, and modified adjusted gross income against the fixed $200,000, $250,000 and $125,000 surtax lines.

If you are an American in the United Kingdom and you have only ever tested the first of those, you have tested one threshold out of eight. The gross income figure decides whether you file. Everything after it decides what you file, and that is where the exposure sits.