Back Taxes for Americans in the UK: How Much Will You Really Owe?

Back taxes for Americans in the UK: an introduction

For most people, back taxes for Americans in the UK come to far less than they fear, and often to nothing. UK income tax is usually higher than US tax on the same salary. The foreign tax credit then cancels most or all of the US tax. A bill tends to arise only on income the UK taxes lightly, such as ISA interest, UK funds or a large home sale gain. This article shows where tax arises and how to estimate it before you file.

If you have not filed at all, start with our guide to IRS Streamlined filing. It explains the usual route back into the system and the forms involved.

Why back taxes for Americans in the UK are usually small

The US taxes its citizens on worldwide income wherever they live. However, it gives credit for income tax paid to the UK on the same income. UK rates on employment income are generally high enough that the credit covers the US tax in full. Consequently, the question is rarely whether you owe a large sum. It is which pieces of income slipped past the credit.

Why do most Americans in the UK owe little US tax?

Because UK tax on salary and self-employment income usually exceeds the US tax on it, and the US credits the UK tax. Leftover credits can also be carried to other years.

How the foreign tax credit works

The foreign tax credit is a dollar-for-dollar reduction in US tax for income tax paid abroad. The IRS page on the foreign tax credit says you claim it for foreign taxes imposed on you. You claim it on Form 1116, separately for each category of income. Therefore, UK tax on your salary offsets US tax on your salary, but not necessarily US tax on your savings.

Carrying spare credits

If your UK tax exceeds the US tax in one category, the surplus is not lost. Under section 904(c) of the tax code, excess foreign tax goes back one year and forward up to ten. In a catch-up, spare credits from one year can therefore cover US tax in another. This is one reason catch-up returns are worked out together, not one at a time.

The exclusion as an alternative

The foreign earned income exclusion lets you leave a capped amount of foreign earnings out of US income. The cap is indexed each year. IRS Publication 54 says you cannot also take a credit for tax on the excluded income. For most UK residents the credit gives a better result, because UK tax is high. Nevertheless, the exclusion can help lower earners, so both should be modelled.

Where does US tax actually arise?

US tax arises on income that the UK taxes lightly or not at all. The usual sources are ISAs, UK funds, home sales, investment income above the US surtax threshold and some US-source income.

ISAs

The GOV.UK page on Individual Savings Accounts confirms that ISAs are free of UK tax. The US ignores that status. ISA interest, dividends and gains are taxable on your US return, with no UK tax to credit against them. Our article on ISA US tax reporting covers each type of ISA.

UK funds and PFIC rules

Most UK funds are passive foreign investment companies for US purposes. A passive foreign investment company, or PFIC, is a foreign corporation that mainly earns or holds passive income. The default rules can tax a sale gain at the highest rate with an interest charge. Each fund may also need Form 8621. In a catch-up, PFIC sales are often the largest single source of tax.

Selling your UK home

The UK usually charges no tax on the sale of your main home. In contrast, IRS Topic 701 limits the US exclusion to $250,000 of gain, or $500,000 on a joint return. You must have owned and lived in the home for two of the last five years. The gain is measured in dollars, so a weak pound at purchase and a strong pound at sale can enlarge it.

The net investment income tax

The net investment income tax is a 3.8% US tax on investment income above a threshold. The IRS questions and answers on the tax set the thresholds at $200,000 of modified adjusted gross income for single filers and $250,000 for joint filers. They also say foreign tax credits generally cannot reduce it. Some practitioners argue the US-UK treaty gives relief, but that point is contested. The cautious approach is to include the tax and consider a protective claim.

Income from the United States

Some people still own a rental home or shares in the US. Under the US-UK income tax treaty, the US taxes US-source income first, and the UK gives the credit. So UK tax rarely removes the US tax on it. Rental income is reported net of expenses and depreciation. Consequently, back taxes for Americans in the UK with US property are often larger than for those with only UK income.

How do you estimate what you owe?

You can estimate back taxes for Americans in the UK in five steps before any return is filed. The aim is a working figure, not a final calculation.

Step one: list income by year and type

Take each of the years you must file. For each, list salary, self-employment income, interest, dividends, gains and pension income. Mark which items sat inside an ISA. Convert each item to dollars. The IRS page on yearly average exchange rates says any posted rate used consistently is accepted.

Step two: note the UK tax on each item

Next to each item, note the UK tax actually paid on it. Salary tax comes from your P60. Savings and dividend tax comes from your Self Assessment calculation, if you filed one. ISA income carries no UK tax.

Step three: find the items with no UK tax

The items with little or no UK tax are where US tax will arise. Typically these are ISA income, fund gains and any home sale gain above the exclusion. Add them up for each year.

Step four: apply spare credits

Spare credits only offset tax in the same category. Salary credits generally cannot cover tax on ISA interest, because the two usually sit in different categories on Form 1116. However, they can cover salary tax in a neighbouring year. Your preparer works out the categories precisely. Getting this step right is what keeps back taxes for Americans in the UK low.

Step five: add interest

Interest runs on unpaid tax from each year's original due date. The IRS announced that the rate on individual underpayments for the fourth quarter of 2026 is 7% a year, compounded daily, unchanged from the third quarter. The IRS lists earlier rates on its quarterly interest rates page. On a small balance, the interest is modest.

A worked example of back taxes for Americans in the UK

Consider a typical case. Laura was born in Boston and moved to Leeds as a child. She learns at 38 that she should have filed US returns. She earns a UK salary of £52,000, holds a cash ISA and a stocks and shares ISA with two UK index funds, and owns a home. This is an illustrative example, not a real client.

Laura's salary

Laura's UK tax and National Insurance are taken through PAYE. Only the income tax counts for the credit. Even so, her UK income tax exceeds the US tax on her salary in each year. The credit therefore removes all US tax on her salary, and spare credits carry forward. Her salary produces no US bill.

Laura's ISAs

Her cash ISA pays about £800 of interest a year. Suppose the US tax on it comes to about $200 a year. She has not sold either fund, so there is no PFIC gain. Her funds stay below the Form 8621 reporting exception, but the funds still need tracking for later. Over three years, her US tax is about $600. Interest at rates near 7% adds roughly $50 to $70, depending on how long each year's tax has been unpaid.

What Laura's case shows

Laura's back taxes for Americans in the UK come to well under $1,000, including interest. Her real exposure was never the tax. It was the penalties she might have faced if she had not used the Streamlined procedures. Additionally, her FBARs had to be filed for six years, because her ISAs and current account together exceeded $10,000.

What penalties apply, and does Streamlined remove them?

Outside the Streamlined procedures, late returns can attract failure-to-file and failure-to-pay penalties. Within the foreign procedures, those penalties are not imposed if you follow the rules.

Penalties without Streamlined

The IRS page on the failure-to-file penalty sets it at 5% of unpaid tax for each month or part month, up to 25%. For returns due after 31 December 2025 that are more than 60 days late, a minimum of $525 or 100% of the unpaid tax applies, whichever is less. A failure-to-pay penalty of 0.5% a month can also apply. Where nothing is owed, these penalties are usually nil.

The Streamlined Foreign Offshore Procedures

The IRS page for US taxpayers residing outside the United States sets the rules. You file three years of returns and six years of FBARs, and certify your conduct was not wilful. You must meet a non-residency test, including 330 full days outside the US in at least one of the three years. If you comply, failure-to-file, failure-to-pay, accuracy-related, information return and FBAR penalties are not imposed. However, you must pay the full tax and interest with the returns.

If you do not qualify as a non-resident

Americans who recently moved to the UK may fail the non-residency test. The Streamlined Domestic Offshore Procedures then apply. The IRS page for US taxpayers residing in the United States sets a miscellaneous offshore penalty of 5% of the highest aggregate value of the relevant foreign assets. Our article on foreign versus domestic Streamlined procedures compares the two.

What if you owe more than expected?

Larger bills usually come from fund sales, a home sale or several years of investment income. Even then, the tax is known before you file, so you can plan the payment.

Paying the tax

Under the Streamlined procedures, the tax and interest are paid with the returns. If you cannot pay in full, the IRS offers payment plans. Interest continues to run on any balance until it is paid.

Late FBARs are not a shortcut

The IRS removed its Delinquent FBAR Submission Procedures page in July 2026. Therefore, filing late FBARs on their own is no longer a published penalty-free route. If your returns also need correcting, the Streamlined procedures deal with both together. Our Streamlined filing service covers the full package.

When the cause was not a mistake

The Streamlined procedures are only for non-wilful failures. If your conduct may have been wilful, speak to a lawyer before filing anything. The IRS points such cases to its voluntary disclosure practice instead.

How US UK Tax Experts can help

US UK Tax Experts prepares Streamlined submissions for Americans in the UK, with the US and UK figures worked from one file. We estimate your US tax and interest before you commit, then prepare the three years of returns, six years of FBARs and the Form 14653 certification for your signature. If you have missed UK returns too, we prepare those alongside. Our article on what to do if you haven't filed US taxes living in the UK covers the first steps.

Conclusion

Back taxes for Americans in the UK are usually small, because UK tax on salary cancels the US tax through the credit. Tax arises mainly on ISAs, UK funds, large home sale gains and the 3.8% investment surtax. You can estimate the bill in five steps, then add interest at the IRS rate. The Streamlined Foreign Offshore Procedures remove penalties for non-wilful filers abroad, though tax and interest remain payable. Most people find the number is far smaller than the worry.

Contact us

If you want an estimate of your back taxes for Americans in the UK, email hello@usuktaxexperts.com or use our contact form. Tell us your income, your accounts and the last year you filed, if any. We reply within one working day.