American Freelancers in the UK: The Totalization Agreement and Self-Employment Tax

The totalization agreement US UK decides which country collects social security from a self-employed American, and the answer is the country where you ordinarily live. A US citizen freelancing from the UK pays UK National Insurance and, with the right paperwork, no US self-employment tax. Without that paperwork, the IRS can still ask for 15.3%.

"Totalization agreement" is the American name for a bilateral social security agreement. The UK calls the same instrument a reciprocal agreement or social security agreement. Its job is to stop one stream of earnings being charged to both countries, and to let periods of work in each country count towards a pension. This guide is about the first job, for freelancers, sole traders and contractors. It sits alongside our posts on National Insurance for Americans in the UK and self-employed Self Assessment for Americans, and goes further on how the exemption is proved and where it fails. Figures are for 2026 or 2026/27 and are labelled with their currency.

Why does a US citizen freelancer face two social security charges?

A US citizen faces two charges because each country taxes on its own test. The US applies self-employment tax to US citizens and residents wherever they live, once net self-employment earnings reach $400, according to the IRS. The UK applies National Insurance to people self-employed in the UK. A UK-resident American with a design, coding or consulting business is inside both nets.

The US charge is substantial. The IRS describes self-employment tax as 15.3%, made up of 12.4% for Social Security and 2.9% for Medicare, charged on 92.35% of net earnings. You can deduct half of it when working out adjusted gross income. The Social Security part stops at the annual earnings cap, which the Social Security Administration sets at $184,500 for 2026.

The exclusion most expats rely on does not help. The IRS states that the foreign earned income exclusion does not reduce self-employment tax, and its own example charges the tax on the full net profit even where the exclusion is claimed. We cover that choice in FEIE versus the foreign tax credit. For social security, the only lever is the agreement.

What does the US-UK totalization agreement say about self-employed people?

The agreement says a self-employed person covered by both countries is subject only to the country where they ordinarily reside. This is Article 4(3) of Schedule 1 to The Social Security (United States of America) Order 1984, which gave the agreement effect in UK law. The test is residence, not where the work is done and not where the client sits.

Three consequences follow, and the first surprises people.

• Client location is irrelevant. A UK-resident American who bills only US customers is still placed in the UK system.
• Nationality is irrelevant. The US charges its citizens abroad, but the agreement overrides that for social security. It is also a separate agreement from the income tax treaty, so the saving clause we explain in the saving clause post does not decide this question.
• There is no five-year clock. The five-year limit in Article 4(2) applies to an employee sent by an employer to work in the other country. A self-employed person is placed by residence for as long as the residence lasts.

Article 4(4) adds a rule for someone who is employed under one country's system and self-employed under the other's in the same activity: residence decides again. A mix of wages and freelance profits across the border is where we would always take advice before relying on a generic answer.

What do you pay in the UK, and what would the US charge be?

A UK-resident freelancer pays Class 4 National Insurance through Self Assessment. For 2026/27 the GOV.UK rates page gives Class 4 at 6% on profits between £12,570 and £50,270 and 2% above £50,270. Class 2 is £3.65 a week, but HMRC's National Insurance Manual explains that since 6 April 2024 there is no Class 2 liability: anyone with profits at or above the small profits threshold of £7,105 is treated as having paid it, and below that it is voluntary.

The comparison below puts the two systems side by side.

Point	UK (2026/27)	US (2026)
Main charge on profits	Class 4: 6% from £12,570 to £50,270, then 2%	Self-employment tax: 15.3% on 92.35% of net earnings
Where it is reported	Self Assessment return	Schedule SE with Form 1040
Upper cap	None on the 2% rate	Social Security part stops at $184,500 of earnings
Deduction	Not covered in this guide	Half of the tax is deductible when working out adjusted gross income
Can the FEIE reduce it?	Not applicable	No

Illustrative example: a freelancer in Leeds has taxable trading profits of £60,000 for 2026/27. Class 4 is 6% of £37,700 (£2,262.00) plus 2% of £9,730 (£194.60), a total of £2,456.60, and Class 2 is treated as paid. If the same person had to pay US self-employment tax on a net profit of $80,000, the charge would be 15.3% of $73,880 (92.35% of $80,000), or $11,303.64. The two profit figures are in different currencies and are not meant to match; the point is the scale of the gap between the two charges, and why the agreement matters.

How do you prove it to the IRS? A certificate of coverage in five steps

You prove it with a certificate of coverage. Article 3 of the Administrative Agreement attached to the 1984 Order says the relevant agency must issue a certificate on request, and that the certificate is proof of exemption from the other country's compulsory coverage. The IRS then expects it on your return.

1. Confirm you ordinarily reside in the UK. Your home, family, bank and day-to-day life should all point to the UK. A freelancer who spends long periods living in the US is a harder case.
2. Request the certificate from the UK side. The HMRC manual says a self-employed person paying UK National Insurance can ask for a certificate confirming UK liability, meant to prevent a demand for US Social Security contributions, and directs requests to HMRC's international National Insurance caseworkers. The manual says the page is under review, so check GOV.UK for the current contact route before you write.
3. Ask for the right dates. The certificate should start from the day you began self-employment as a UK resident, including any year in which you moved.
4. Attach a photocopy to Form 1040 every year. The IRS says to attach the certificate for each year you are exempt and write "Exempt, see attached statement" on the self-employment tax line.
5. Use the SSA statement as the fallback. If the UK will not issue a certificate, the IRS says you can request a statement from the Social Security Administration that your income is not covered by US Social Security, and attach that instead.

Keep the certificate with your records. A UK-resident American who has paid Class 4 for years but never attached anything to Form 1040 has, from the IRS's point of view, simply not filed Schedule SE. Our treaty relief service and UK Self Assessment service handle both ends of that paperwork.

When does the totalization agreement US UK not help?

The agreement does not help when you do not genuinely reside in the UK, when the work is not self-employment, or when the other country is not the UK. Four cases come up repeatedly.

You are still a US resident

If you ordinarily reside in the United States, the same rule places you in the US system. A US-based freelancer with UK clients pays US self-employment tax and is not brought into UK National Insurance by those clients. The same is true if you spend more of the year in the US than the UK and your life is really there.

You move during the year

Coverage follows residence, so a move changes it. The year you arrive in or leave the UK may need two periods of cover, one in each country, and the certificate needs dates that match. The same year is also tricky for income tax; see split-year treatment.

You are an employee of your own company

A freelancer who runs a limited company and pays themselves a salary is an employee for social security purposes, and the employee rules differ from the self-employed rule. That is a different analysis from the one here. Our post on contractors and remote work covers the structure choice.

You are a freelancer and also on a US payroll

Mixed employment and self-employment is the situation Article 4(4) was drafted for. Do not assume the self-employed rule alone settles it.

What does the agreement do for your pension?

The agreement also lets periods in each system be combined when you claim a pension, which is why paying into the UK system during UK freelancing years is not wasted for an American. We explain what that means for benefits in UK State Pension and US Social Security. For the day-to-day question of how much National Insurance to pay and what it earns, see our National Insurance guide.

What do American freelancers in the UK get wrong?

Most mistakes are about paperwork rather than the rule itself. The agreement is clear that residence decides; the practical failures come afterwards.

• Assuming the exemption is automatic. HMRC taxes you under UK rules whether or not you tell the IRS. The IRS only treats you as exempt when the certificate or the SSA statement is attached to the return, and it asks for it every year.
• Treating Form 2555 as the fix. The exclusion applies to income tax. The IRS is explicit that it leaves self-employment tax untouched, so a return that claims the exclusion but attaches no certificate can still carry a large self-employment tax bill.
• Forgetting the year of the move. Many freelancers start in the UK part-way through a year. If the certificate begins later than the first UK trading day, the gap is exposed.
• Mixing up the two tests. UK income tax residence under the Statutory Residence Test and "ordinary residence" for the agreement are different questions, though they usually point the same way. Read our statutory residence test guide for the first.
• Ignoring state and local rules. The agreement deals with federal Social Security and Medicare only. It says nothing about US state income tax, which can follow you if you keep ties to a state.

None of these is hard to fix once spotted. If earlier years went out without the certificate or without a US return at all, our post on what to do if you never filed US taxes abroad explains how to catch up.

A short checklist for a UK-resident American freelancer

• Register as self-employed with HMRC and file Self Assessment on time; see getting a UTR as an American.
• Request a certificate of coverage from the UK side, with the right start date, and keep a copy.
• File Schedule SE or the exempt statement with Form 1040 each year.
• Do not rely on Form 2555 to deal with self-employment tax.
• If your residence changes, ask for the certificate to be reissued with new dates.

If you want a US CPA and a UK tax adviser to check the residence position and the certificate together, our Americans in the UK team works on this every week, and you can start from the contact page.