Self-Assessment Tax for US expats in the UK: Step-by-Step Guide

Navigating taxes as an American living in the UK can feel overwhelming. You are required to balance both US tax rules and HMRC obligations at the same time. For many, the self-assessment tax process for the UK expats is the biggest challenge. This guide explains how US citizens in the UK can handle self-assessment, avoid double taxation, and remain fully compliant.

Managing dual tax systems is not optional—it is a legal requirement. While the US taxes its citizens on worldwide income, the UK requires residents to declare their income through a self-assessment process. Many expats assume tax treaties automatically remove the burden, but that is not always the case. Filing properly ensures you avoid penalties, reclaim available credits, and take advantage of cross-border tax reliefs.

For those who want professional support, firms like JungleTax specialise in guiding Americans through this process, ensuring compliance on both sides of the Atlantic.

Why US Expats Must File a UK Self-Assessment

The UK self-assessment system requires individuals to declare income that is not automatically taxed through PAYE. US expats often receive income from US-based clients, investments, or pensions, which fall outside the PAYE framework. If you live in the UK for more than 183 days, you are considered a tax resident, and self-assessment becomes essential.

Even if you file your US return annually, HMRC requires a separate declaration. This is where mistakes often happen. Expats sometimes focus only on their US obligations, forgetting that HMRC also expects complete records. Filing both ensures you meet the requirements of two central tax authorities.

Step 1: Registering for Self-Assessment

Your first task is to register with HMRC. You can do this online, and HMRC will send you a Unique Taxpayer Reference (UTR). For most US expats, initiating this process early is recommended, as UTRs can take some time to arrive. Missing registration deadlines could lead to penalties, even before filing begins.

Step 2: Collecting All Income Records

You must include global income in your UK return. That includes US salary, freelance income, rental earnings, dividends, or capital gains. Many expats overlook small amounts, assuming they are insignificant. However, HMRC requires accurate figures.

At the same time, you must prepare your US return, reporting the same income. To prevent double taxation, you will rely on mechanisms like the Foreign Earned Income Exclusion or the Foreign Tax Credit.

Step 3: Understanding the UK-US Tax Treaty

The US and UK have a tax treaty designed to reduce duplicate taxation. However, the treaty does not exempt you from filing. Instead, it allows you to apply credits and reliefs correctly. For example, if you already paid US tax on certain income, you may reduce your UK liability by claiming foreign tax relief.

This is where professional expertise makes a big difference. JungleTax often helps expats determine whether reliefs apply, saving thousands of pounds annually.

Step 4: Filing Your UK Self-Assessment

HMRC’s self-assessment filing deadline is January 31st for online submissions. Paper returns are due earlier, by October 31st. US expats must ensure they complete the correct forms, including foreign income supplements.

Unlike the US system, where extensions are common, HMRC penalties are strict. Late filing results in automatic fines, regardless of whether you owe tax. Staying on top of deadlines is vital to avoid unnecessary costs.

Step 5: Paying UK Taxes While Staying US Compliant

After filing, you must pay any UK tax owed by January 31st. This is separate from your US liability. To align both, careful tax planning is essential. For instance, if you have already paid US tax, you may apply that against your UK bill using treaty provisions.

Professional accountants familiar with expat issues understand how to balance the two systems without triggering audits or double charges.

Real-Life Example: Avoiding Double Taxation

Consider Sarah, an American marketing consultant living in London. She earns income from both UK and US clients. Without expert help, Sarah initially reported all income in the US only. Months later, HMRC fined her for failing to declare global earnings.

When Sarah worked with an expat-focused firm like JungleTax, she learned how to file properly in both countries, apply for treaty relief, and reduce her UK tax bill. Today, she pays less overall and avoids compliance stress.

Common Mistakes US Expats Make

Many US expats in the UK fall into the same traps:

They forget to register with HMRC on time.
They assume US filing covers UK obligations.
They miss out on tax treaty reliefs.
They file late and pay penalties.

Each mistake costs money and peace of mind. A self-assessment tax for US expats filing in the UK, done right, ensures compliance while protecting income.

Why Professional Help Matters

Tax rules change regularly, and both the IRS and HMRC are monitoring global taxpayers more closely than ever if you are balancing business, family, or multiple income streams, professional guidance can save you time and money.

Firms like JungleTax specialise in helping Americans file dual returns seamlessly. By working with specialists, expats gain confidence that they are compliant, protected, and optimising the reliefs available to them.

Final Thoughts

Filing self-assessment tax returns for UK expats in the US is not something to ignore. It ensures compliance with both IRS and HMRC, prevents double taxation, and helps protect your finances. While the process is complex, it can be managed with proper planning and the right professional support.

If you are a US expat in the UK, don’t wait until deadlines approach. Begin gathering documents early, understand your obligations, and seek guidance to ensure accurate filing.

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