Introduction: Additional Medicare Tax and the Cross-Border Blind Spot
The Additional Medicare Tax is a flat 0.9% charge that lands on high-earning Americans long after they have left the United States. Furthermore, it behaves unlike almost every other federal charge these clients meet. It ignores the Foreign Earned Income Exclusion completely. It accepts no foreign tax credit. Moreover, HMRC refuses to relieve it from the other direction.
Consequently, wealthy Americans in Britain regularly overpay it, underpay it, or report it on entirely the wrong basis. At TaxYork we see all three errors in the same week. Notably, the most expensive error runs in the taxpayer's favour to correct: thousands of dollars paid on earnings that never belonged on the form at all.
This guide sets out precisely who owes the charge, who does not, and why the answer turns on the identity of your employer rather than the size of your salary.
Why the Additional Medicare Tax Catches Wealthy Americans Abroad
Most guidance treats the Additional Medicare Tax as an automatic consequence of earning above a threshold. However, that framing is simply wrong for anyone employed in Britain by a British company. The charge attaches to *Medicare wages* and to *self-employment income*, not to gross earnings.
Therefore, the critical question is never "how much did you earn?" Instead, it is "did your earnings enter the US social security system at all?" For a great many Americans on UK payroll, the honest answer is no. Accordingly, the correct entry on the form is zero.
The Thresholds That Have Not Moved Since 2013
The rate sits at 0.9% and the thresholds have stood still for over a decade. Specifically, the IRS confirms in Topic 560 that the charge applies above $250,000 for married couples filing jointly, $200,000 for single and head-of-household filers, and just $125,000 for married taxpayers filing separately.
Importantly, Congress never indexed these figures for inflation. As a result, the real threshold falls every year. A London salary that comfortably cleared the bar in 2013 now sits far above it. Additionally, the IRS questions and answers page confirms there is no employer match, unlike ordinary Medicare tax.
Who Actually Owes the 0.9% Charge on UK Earnings
The answer depends entirely on who signs your employment contract. Furthermore, this single fact separates a nil return from a five-figure liability across a career.
Working for a Genuine UK Employer
Services performed outside the United States by an American for a foreign employer generally fall outside the US social security system altogether. The IRS states plainly on its guidance for persons employed by a foreign employer that such individuals are not generally subject to Social Security and Medicare tax withholding.
Therefore, a US citizen employed in London by a British bank, a British law firm, or a British trading house has no Medicare wages. Consequently, that person owes no Additional Medicare Tax on the salary, regardless of whether it reaches £200,000 or £2 million. The statutory foundation sits in section 3121 of the Internal Revenue Code, which defines FICA "employment" by reference to the employer's nationality.
Nevertheless, several well-known expat guides state the opposite. Specifically, they claim that Americans on UK payroll typically owe both National Insurance and the 0.9% surcharge. That advice costs clients real money every filing season.
Working Abroad for an American Employer
The position reverses when the employer is American. The IRS page on the social security tax consequences of working abroad confirms that US social security and Medicare taxes apply to wages earned abroad when you work for an American employer.
Accordingly, an American seconded to London by a New York investment bank, and kept on US payroll, generates genuine Medicare wages and real Additional Medicare Tax exposure. Furthermore, those wages appear in Box 5 of the Form W-2 even when the Foreign Earned Income Exclusion strips them out of taxable income. Above all, Box 5 is what drives the Additional Medicare Tax calculation.
The Section 3121(l) Agreement That Changes Everything
A third route catches many sophisticated structures. An American parent company may enter a voluntary agreement with the US Treasury extending social security coverage to Americans employed by its foreign affiliates. The IRS defines a foreign affiliate as any foreign entity in which the American employer holds at least a 10% interest.
Consequently, an American working for a London subsidiary of a US group may find the Additional Medicare Tax applies after all. Moreover, the employee often has no idea the agreement exists. Therefore, we always ask clients to obtain written confirmation from group payroll before we complete the form.
Why the Foreign Tax Credit Cannot Touch This Charge
Here the cross-border position turns genuinely punitive. In short, no relief exists in either direction.
National Insurance Is Not a Creditable Income Tax
Americans in Britain pay National Insurance on the same earnings, at rates published by HMRC on GOV.UK. Nevertheless, National Insurance is a social security contribution rather than an income tax. Therefore, it never qualifies for the foreign tax credit under section 901.
Consequently, a client cannot use UK National Insurance to wipe out the Additional Medicare Tax. Furthermore, the reverse is equally true, as the next section shows.
The Treaty Relieves Income Taxes, Not Social Security Taxes
The US-UK double tax treaty relieves income taxes. However, it deliberately excludes social security taxes from its scope. Consequently, no treaty article rescues you here. Instead, coordination between the two social security systems happens exclusively through the separate totalisation agreement administered by the Social Security Administration. Furthermore, the US Treasury treaty library confirms the scope of the income taxes actually covered.
Additionally, the foreign tax credit itself only offsets income tax imposed under Chapter 1 of the Internal Revenue Code. Meanwhile, the Additional Medicare Tax sits in Chapter 2 or Chapter 21. Therefore, the two never meet on the same form.
HMRC Refuses Relief From the Other Side
This point rarely appears anywhere online, yet HMRC states it explicitly. Its Double Taxation Relief Manual at DT19851 lists the US taxes admissible for UK credit. Notably, it names contributions under the Federal Insurance Contributions Act as inadmissible, and expressly includes the Additional Medicare tax in that exclusion.
Therefore, the charge is genuinely stranded. The United States will not credit your National Insurance. Likewise, Britain will not credit your Additional Medicare Tax. As a result, the 0.9% represents a real, permanent cost rather than a timing difference. This is precisely why tax treaty planning for US-UK clients must address the charge directly instead of assuming relief exists.
Self-Employment, Partnerships and the Certificate of Coverage
Self-employed Americans face a different mechanism with a far better outcome available.
How a UK Certificate of Coverage Removes the Charge
The totalisation agreement exempts earnings from US social security and Medicare taxes when those earnings already attract contributions in the other country. The IRS totalisation agreements page confirms the exemption covers Federal Insurance Contributions Act taxes, and states that a similar exemption exists from Self-Employment Contributions Act taxes.
Consequently, an American running a consultancy from London and paying UK National Insurance can obtain a certificate of coverage that removes the Additional Medicare Tax entirely. Furthermore, that certificate removes the entire 15.3% self-employment charge. Importantly, it removes the 0.9% element as well, because no US self-employment income subject to the charge then exists.
US Partners in UK Partnerships
Partners in UK partnerships and limited liability partnerships sit in the same analysis. Specifically, their profit share is self-employment income for US purposes unless the totalisation agreement displaces it. Therefore, obtaining and retaining the certificate matters enormously for a partner drawing £500,000.
Moreover, the certificate must cover every year in question. Nevertheless, we routinely meet partners who obtained one certificate years ago and never renewed it. Accordingly, we check coverage year by year during our cross-border tax planning reviews.
The Detached Worker Five-Year Rule
Secondments carry a time limit. Generally, an American sent to Britain for five years or less stays in the US system and pays FICA. However, once the assignment passes five years, UK National Insurance takes over and US coverage ends.
Consequently, the Additional Medicare Tax stops applying at that point for many long-stay secondees. Nevertheless, US payroll departments frequently fail to notice the transition. Therefore, they keep withholding, and the employee keeps overpaying until someone reviews the position.
The Withholding Gap and How the Bill Actually Arrives
Even where the charge genuinely applies, the payment mechanics create their own problems.
Why UK Payroll Never Withholds
A British employer operates PAYE and National Insurance. However, it has no mechanism to withhold US federal charges. Therefore, any Additional Medicare Tax due arrives as a lump sum with the Form 1040 rather than through payroll.
Consequently, clients meet an unexpected balance due each April. Furthermore, the amount grows with every bonus cycle.
Estimated Tax and Underpayment Penalties
The IRS expects taxpayers to fund the charge in advance. Specifically, its guidance directs individuals to make estimated tax payments or request additional withholding on Form W-4. Otherwise, underpayment penalties apply.
Therefore, high earners in Britain should build the Additional Medicare Tax into quarterly estimates. Additionally, the Form 8959 instructions confirm the reconciliation happens on that form, which attaches to the return.
The Married Filing Separately Trap at $125,000
Americans married to British spouses very often file separately. Understandably, they do so to keep a non-US spouse outside the American system. However, that choice halves the threshold to $125,000.
Consequently, the charge starts biting at roughly £94,875 of Medicare wages rather than £189,750. Moreover, an employer still withholds only above $200,000, so a shortfall is almost guaranteed. Therefore, separate filers must plan for a balance due even at modest seniority.
Converting the Thresholds Into Sterling
Thresholds are dollar figures, yet the earnings are sterling. Accordingly, translation matters.
Using the Correct Exchange Rate
The IRS yearly average currency exchange rates give 0.759 for the pound for 2025, against 0.783 for 2024. Therefore, the 2025 thresholds translate to roughly £151,800 for single filers, £189,750 for joint filers, and £94,875 for separate filers.
Additionally, the Treasury reporting rates of exchange differ slightly, which matters for other filings. Furthermore, a weakening pound pushes more sterling earnings above the fixed dollar line, dragging fresh income into the Additional Medicare Tax net. Consequently, currency movement alone can create a liability that did not exist the previous year.
Bonus Timing and Currency Swings
Bonus timing compounds the effect. Specifically, a March bonus and a following January bonus can land in the same US calendar year. Therefore, two bonus cycles occasionally collide inside one threshold test.
Nevertheless, careful planning helps. Moreover, understanding the interaction early allows sensible conversations with employers about payment dates.
A Worked Case Study With Real Numbers
Consider Michael, a 44-year-old US citizen and managing director at a London investment bank. Importantly, his employer is a British-incorporated plc with no US parent and no coverage agreement. His 2025 salary and bonus totalled £420,000. He is married to Emma, a British citizen with no US status, so he files as married filing separately.
The Position His Previous Preparer Took
Michael's former preparer translated £420,000 at 0.759, producing $553,360. Subsequently, that figure went straight onto Form 8959 as Medicare wages. The calculation ran $553,360 less the $125,000 separate-filer threshold, leaving $428,360. At 0.9%, that produced $3,855 of Additional Medicare Tax for the year.
Furthermore, the same treatment had applied for three consecutive years. Consequently, Michael had paid $11,565 in total.
The Correct Analysis
Michael performs all his services in London for a British employer. Therefore, his earnings are not FICA employment and generate no Medicare wages whatsoever. Accordingly, Form 8959 Part I should have shown nil.
Additionally, Michael runs a small advisory practice alongside his employment, producing £62,000 of UK profits. He pays UK National Insurance on it and holds a valid certificate of coverage. Consequently, that income carries no US self-employment charge either, so Part II is also nil.
The Outcome
We filed amended returns on Form 1040-X for the three open years. As a result, Michael recovered the full $11,565 plus statutory interest. Moreover, his ongoing annual liability fell to zero.
Notably, the outcome would have flipped entirely had he been seconded by a New York bank and kept on US payroll. In that structure, the identical £420,000 would have produced $3,855 every year, with no UK relief available under DT19851. Ultimately, the employer's identity was worth $3,855 a year to Michael.
Missed Filings, Amended Returns and Catching Up
Errors run in both directions, and both are correctable.
Recovering Overpaid Amounts
Where the charge was paid in error, an amended return recovers it. Generally, the refund window runs three years from filing or two years from payment. Therefore, clients should review promptly rather than waiting.
Furthermore, we always examine the underlying employment analysis before filing. Otherwise, an amendment simply invites questions the taxpayer cannot answer.
Where the Charge Was Genuinely Missed
Some clients discover they owed the charge and never reported it. Specifically, this happens with US-employer secondees and with self-employed Americans lacking a certificate of coverage. Consequently, back years need correcting.
Where wider non-compliance exists, the IRS Streamlined Filing Compliance Procedures may provide the appropriate route. Additionally, unreported foreign accounts usually accompany these cases, so FBAR and FATCA reporting requires review at the same time. Our IRS Streamlined Filing service handles both elements together.
Record Keeping That Protects the Position
Documentation decides these cases. Therefore, retain your employment contract, your certificate of coverage, and written confirmation that no coverage agreement applies to your employer. Moreover, keep payslips showing National Insurance deducted.
Professional guidance from the AICPA, the ICAEW and the Chartered Institute of Taxation reinforces the value of contemporaneous records. Additionally, Investopedia's explanation of FICA offers a useful plain-English primer for clients new to the charge, while MoneyHelper covers the UK National Insurance side.
How TaxYork Can Help
We prepare US and UK returns for high-net-worth Americans across Britain, including bankers, fund partners, founders and company owners. Furthermore, we analyse the Additional Medicare Tax position properly rather than assuming it applies. That analysis begins with your employment structure, not your income level.
Specifically, we confirm your employer's status, verify whether a coverage agreement exists, check your certificate of coverage year by year, and reconcile every dollar of withholding on Form 8959. Additionally, we identify overpaid years and recover them through amended returns. We also coordinate the position with your US tax return preparation and your UK Self Assessment so both sides agree.
Moreover, our clients typically hold complex remuneration: carried interest, restricted stock, deferred bonuses and partnership profit shares. Therefore, we model the threshold across all of it before the year closes.
Conclusion
The Additional Medicare Tax rewards precision and punishes assumption. Above all, it turns on the identity of your employer rather than the size of your package. Consequently, two Americans on identical London salaries can face completely different outcomes.
Furthermore, no relief exists on either side of the Atlantic. The United States refuses credit for National Insurance. Meanwhile, HMRC expressly refuses credit for the Additional Medicare Tax under DT19851. Therefore, every dollar assessed is a dollar permanently lost, and every dollar assessed in error is a dollar worth recovering.
Ultimately, the answer lies in your contract, your payroll arrangement and your certificate of coverage. In summary, review those three documents before you file, and the Additional Medicare Tax stops being a surprise.
Contact Us
Speak to our cross-border team about your position before the next filing deadline. To review your Additional Medicare Tax exposure, book a consultation with our specialists today.
Email hello@taxyork.com or call 020 3488 8606. Alternatively, contact us through our website and we will respond within one working day.
Disclaimer
This article provides general information only and does not constitute tax or legal advice. Tax rules change frequently, and the correct treatment depends entirely on your individual circumstances. Therefore, you should obtain professional advice before acting on anything set out here. TaxYork accepts no liability for any action taken in reliance on this content. Figures quoted reflect published rules and exchange rates at the date of writing.