US-UK · Journal
US-UK Certificate of Coverage: How to Get One
Pay social security in one country, not both. Which form, which authority, what evidence — and why the self-employed lose 15.3% without a certificate.
Pay social security in one country, not both. Which form, which authority, what evidence — and why the self-employed lose 15.3% without a certificate.
A certificate of coverage is the document that stops you paying social security contributions twice. Under the US-UK social security agreement, contributions are normally due in the country where the work is performed, but a certificate issued by one country's authority proves you remain inside that country's system and are exempt from the other's. Employers apply for posted employees, the self-employed apply for themselves — and for Americans self-employed in the UK, a UK certificate is what removes US self-employment tax of 15.3% stacked on top of National Insurance.
This is the practical how-to. For the wider comparison of the two systems, start with US social security and UK National Insurance: which country do you pay?, then come back here for the mechanics.
One instrument, three vocabularies. The SSA calls it a totalization agreement; HMRC calls it a reciprocal agreement or double contribution convention; the document is a certificate of coverage on gov.uk today and a certificate of continuing liability in HMRC's manuals. Knowing all three names saves time when searching official guidance or reading a letter.
Its scope is narrower than most people assume. SSA's POMS RS 02001.360 confirms that on the US side "the agreement only applies to taxes under the RSDHI program" — FICA for employees and employers, SECA for the self-employed — and "does not include any taxes related to other programs such as Workers' Compensation or Unemployment Insurance." On the UK side it reaches contributions financing old-age, survivors and disability benefits, plus cash sickness, maternity, death grants, unemployment insurance and industrial injuries benefits. It does not reach FUTA or anything resembling income tax.
The general rule is territorial: work in the UK attracts National Insurance, work in the US attracts FICA. The detached-worker exception is what makes assignments workable. POMS RS 02001.370 sets out the conditions. A detached worker is sent by an employer in one country to work temporarily in the other for the same employer or an overseas affiliate, and stays subject only to the home country's laws provided that:
The rule does not apply to officers or crew members of ships or aircraft.
Two of those conditions cause most failures. The pre-existing employment relationship disqualifies the common pattern where someone is recruited locally and then described as a secondee. And the section 3121(l) agreement is a real filing with the IRS that either exists or does not — discovering at month four that it does not is expensive, because the certificate cannot be issued without it. Note too that five years is tested against the expectation at the outset, so an assignment that drifts past five needs an extension request rather than silence.
POMS RS 02001.370 is clear that employment beyond five years stays under the home country's laws only if both countries agree, that the request should be made before the five years expire, and that "neither country grants an extension without the other's concurrence." Neither authority publishes how readily extensions are granted in practice, and SSA's guidance describes them as uncommon — plan around the five years rather than around an extension.
If the worker is to stay in the UK system, the application goes to HMRC. The current route is Apply for a certificate of coverage to pay UK National Insurance whilst working in a country with a social security agreement (CA9107), last updated 7 July 2026. The USA appears on its country list, alongside Canada, India, Japan, South Korea, the Channel Islands and a dozen others.
Who can apply, in gov.uk's own words: "an employer," "an employee," "self-employed," "an agent acting on behalf of an employer," "an agent acting on behalf of an employee." The information requested:
For an extension you also need the expiry date of the existing certificate and any changes in circumstances. Where the combined period exceeds what the relevant agreement allows, gov.uk requires a signed statement explaining why you want to continue paying UK National Insurance and how that is in your best interest.
Two practical points. The page describes an online application only. Older gov.uk publication pages referenced a paper CA9107, so if you cannot apply online, confirm with HMRC whether a postal route still exists. And gov.uk publishes no processing time, pointing instead to its generic "check when you can expect a reply from HMRC" tool — so check the current turnaround there and apply well before the assignment starts.
Do not reach for the A1 family. CA3822 (employees), CA3837 (self-employed) and CA8421 (work in two or more countries) are the EU, EEA, Gibraltar and Switzerland route; using them for a US assignment is a common reason an application comes back.
HMRC's NIM33430 states the effect for someone arriving in the UK from an agreement country: "No Class 1 liability for the period shown on the certificate of continuing liability," with "Class 1 liability would commence from the expiry of the certificate." Without a certificate, "Class 1 liability will exist from the outset of employment in the UK."
One trap for employers: the same page notes that secondary Class 1 liability "would exist where there is a Place of Business or a host in the UK" — so an employer presence can carry its own exposure even where the employee's position is covered. Check this in any specific case: both NIM33430 and NIM33415 carry HMRC's note that the pages are under review and may only be apt for periods beginning before 1 January 2021 and Withdrawal Agreement cases.
To stay in the US system the request goes to the Social Security Administration, and there is no form number. POMS RS 02001.385 directs the employer, or the self-employed person, to contact SSA — with an online request route and a postal address at the International Support Branch in Baltimore, plus a fax line reserved for certificate requests and exemption letters. The request should include:
Where the worker will be employed by a UK affiliate of an American employer, the request must also state whether the employer holds a section 3121(l) agreement with the IRS and, if so, its effective date.
Confirm the live channels before filing. That POMS page was last updated in 2020 and still names the UK authority as the "Inland Revenue National Insurance Contributions Office" — an institution that no longer exists under that name. Treat UK contact details on US pages as out of date by default.
This is the section that matters most to Americans in Britain, and it works differently from postings.
For self-employment the agreement allocates coverage by residence. POMS RS 02001.371 states that "a self-employed U.S. citizen who is a resident of the United Kingdom is covered only under U.K. law." That person is exempt from US SECA tax — but still files a US return, notes the exemption on Schedule SE, and attaches a copy of the UK certificate of coverage as proof. The mirror applies to a self-employed UK national resident in the US.
Without that certificate, the default is US self-employment tax. IRS Topic No. 554 confirms the rate "consists of 12.4% for Social Security and 2.9% for Medicare taxes" — 15.3% — generally applying from $400 of net earnings. For a freelancer billing from a London flat, that is 15.3% of profit charged a second time on earnings that have already carried UK Class 2 and Class 4. Our longer treatment is in self-employed in the UK and paying US self-employment tax.
There is a wrinkle in the route. A self-employed American living and working in the UK is not "going to work abroad," so the CA9107 framing does not fit the facts neatly. HMRC's NIM33415 is the relevant internal guidance: the NIC&EO International Caseworker issues certificates for self-employed people in relation to agreement countries, including a certificate confirming UK liability for US purposes — a document that, in HMRC's words, "prevents a demand for payment" from the US authorities. Confirm the exact current route with HMRC for a UK-resident self-employed person who needs a certificate purely for US purposes, and expect to have to explain the facts rather than tick a box.
One point to check rather than assume: gov.uk's voluntary Class 2 guidance indicates that certificate holders with profits at or above the relevant threshold have Class 2 treated as paid, subject to sending HMRC accounts — confirm the threshold and the current wording with HMRC before relying on it. Separately, voluntary Class 2 contributions for time abroad were abolished from 6 April 2026, a different rule with its own transitional deadline — see voluntary National Insurance for expats.
| Position | Consequence |
|---|---|
| Employee posted to the UK, no certificate | UK Class 1 "from the outset of employment in the UK" per NIM33430, alongside home-country contributions |
| Employee posted to the US, no certificate | FICA on US earnings alongside continuing UK liability |
| American self-employed in the UK, no UK certificate | UK Class 2 and Class 4 plus US self-employment tax at 15.3% |
| Certificate obtained late | Can generally cover a past period, but payroll and filings must be unwound |
A certificate can usually be issued for a period that has already run. The difficulty is everything downstream: an employer correction and repayment of employee contributions on one side, a refund claim on the other, each with its own evidence requirements and time limits, in payroll systems that have already closed the year. The IRS points to Revenue Procedure 80-56, Revenue Procedure 84-54 and Revenue Ruling 92-9 for the certificate procedure, including the position of people who cannot obtain one. Confirm the applicable refund route and the limitation periods in any specific case. Contributions paid to the wrong country are often recoverable in principle, and reliably more expensive to recover than to avoid.
The agreement allocates social security contributions and says nothing about income tax, which is the job of the separate US-UK income tax treaty with its own residence tie-breakers, credits and exclusions. So a certificate of coverage will not reduce an income tax bill, and an income tax treaty position will not stop a National Insurance demand; the two run in parallel.
And unlike income taxes, contributions have no general cross-border credit mechanism — which is exactly why the agreement works by exclusive allocation rather than by relief. Take advice on the US foreign tax credit treatment of National Insurance before relying on it in any specific case.
The second half of a totalization agreement concerns benefits, and it is widely misread as a way to increase a pension. It is not.
US side. POMS GN 01701.110 requires, for a US totalization benefit, that the worker is not insured on US coverage alone and holds at least six US quarters of coverage. Those six must come from US work; foreign credits cannot be used to reach the floor, and nothing is payable before the quarter in which the sixth is acquired. UK periods then convert to US quarters under set formulas, capped at four quarters a calendar year.
UK side. gov.uk's new State Pension guidance for living and working overseas says you "usually need 10 qualifying years on your National Insurance record to be eligible," and that time contributing in an agreement country can be added to your qualifying years for eligibility — but "the amount you actually get paid will only be based on the qualifying years in your UK National Insurance record."
So aggregation buys eligibility, not quantum. A five-year certificate keeping someone in the US system is five years not accruing UK qualifying years, and vice versa — worth modelling rather than defaulting where one record is thin.
The certificate is cheap and the absence of one is not. If an assignment is being planned, the useful work happens before anybody moves: confirming the pre-existing employment relationship, checking the affiliate and the section 3121(l) position, deciding which system the person should build entitlement in for the next five years, and getting the application in early enough that the first payroll run is correct. If contributions have already gone to the wrong country, the work is recovery — and worth starting quickly, because the limitation periods are not generous.
We prepare the application pack — facts, evidence, the structural checks on the affiliate and the section 3121(l) position, and the Schedule SE treatment for the self-employed — to a ready-for-submission state. The firm does not submit anything to HMRC, the SSA or the IRS on a client's behalf: a licensed professional reviews the pack, and the client or their employer files it. Our reviewer is an IRS Enrolled Agent who is also ACCA-qualified, covering both sides of the position.
This article is general information about the mechanics, not advice on your facts; residence, citizenship, employer structure and the shape of an assignment all change the answer. We act as tax specialists for US and UK positions. If that is where you are, book a consultation.