Freelancing Across Borders: UK Self Assessment, US Self-Employment Tax and the Rules In Between

The freelancer's stack, in two countries

A UK-based freelancer with a US passport runs on four layers: UK income tax and National Insurance through Self Assessment; a US return every year regardless of where they live; social security charges, which both countries will claim unless coordinated; and possibly VAT. None is difficult alone. The cost comes from setting them up in the wrong order — usually by filing the first US return and discovering self-employment tax after a year of decisions that could have prevented it.

The UK side: Self Assessment and National Insurance

Working for yourself in the UK means registering with HMRC for Self Assessment (by 5 October after the end of your first tax year of trading) and filing annually by 31 January. You are taxed on trading profits — income less allowable expenses — with a small trading allowance as an alternative for very modest income. Self-employed National Insurance now runs mainly through Class 4 contributions, calculated on profits through the same return.

Payments on account deserve early attention: once your bill is big enough, HMRC collects two advance instalments for the following year, and the first 31 January after a good year can demand roughly one and a half years of tax at once. Freelancers with growing income should forecast this, not discover it.

The US side: income tax and self-employment tax

A US citizen or Green Card holder files a US return on worldwide income wherever they live, and self-employment profits from roughly $400 upward trigger self-employment tax — the US social security and Medicare charge — on top of income tax. Crucially, the foreign earned income exclusion and foreign tax credits reduce income tax, not self-employment tax. UK income tax paid does not offset it either. Left unmanaged, this is how a London freelancer ends up paying UK NIC and US self-employment tax on the same profits.

Totalization: the fix for double social security

The US–UK totalization agreement exists for exactly this. Broadly, it assigns a worker to one country's social security system — for someone self-employed and resident in the UK, normally the UK — and exempts them from the other's charges. The exemption is evidenced by a certificate of coverage: a UK-based freelancer requests one from HMRC and uses it to support not paying US self-employment tax.

Two practical notes. First, the certificate is the proof — claiming the exemption with nothing behind it invites correspondence. Second, which system covers you also shapes which country's benefits you accrue; the agreement can let contributions in both countries be combined for benefit entitlement. The wider picture is in our social security guide.

IR35 and umbrella companies

UK contracting has a regime freelancers hear about constantly: off-payroll working (IR35). It targets people who supply their labour through an intermediary — usually their own limited company — to clients for whom they would, in substance, be employees. Where an engagement is inside the rules, employment taxes apply to it, and for most medium and large clients the client decides the status. Many contractors in that position are instead paid through umbrella companies, becoming employees of the umbrella for tax purposes.

For a US-person contractor the choice of channel echoes on the US return: an umbrella makes you an employee (simpler US treatment, though the totalization analysis shifts with it), while a personal limited company makes you the American owner of a UK company — with the Form 5471 and GILTI consequences that brings. Do not let a recruiter's default choose your US filing profile.

VAT: the threshold and the cross-border wrinkle

UK VAT registration is generally required once taxable turnover exceeds £90,000 over a rolling twelve-month period; voluntary registration is available below it. For freelancers serving US businesses there is a large wrinkle in your favour: under place-of-supply rules, many business-to-business services supplied to overseas customers are outside the scope of UK VAT — which affects both whether you charge VAT and how close you actually are to the threshold. The rules are category-specific, so get the analysis done once, correctly, and build it into your invoicing.