US-UK Taxes for American Expats: Treaty Relief, Forms and Deadlines (2026)

If you are a US citizen or green card holder living in the United Kingdom, you file in both countries. The United States taxes its citizens on worldwide income wherever they live, and the UK taxes you as a resident. The US-UK tax treaty and foreign tax credits mean very few people pay tax twice on the same income, but nobody escapes filing twice. Here is how the two systems fit together in 2026.

The treaty assigns taxing rights between the two countries and reduces or eliminates withholding on things like pensions, dividends, and interest. It also contains tie-breaker rules if both countries claim you as a resident. In practice, most double taxation relief for Americans in the UK comes from foreign tax credits rather than the treaty itself: UK tax paid on UK income offsets US tax on that same income through Form 1116, and UK rates are usually higher than US rates, so many filers owe little or nothing to the IRS.

The catch is the saving clause. The treaty lets the United States tax its own citizens as if the treaty did not exist, with limited exceptions. That is why treaty elections rarely remove your US filing obligation, and why some UK-favored products, such as ISAs, remain fully taxable in the US. Stocks and shares ISAs holding UK funds can also trigger the punitive PFIC regime.

On the US side, expect some or all of the following:

• Form 1040 reporting worldwide income, every year your income is above the filing threshold.
• FBAR (FinCEN Form 114) if your non-US accounts together exceed $10,000 at any point in the year.
• Form 8938 (FATCA) if foreign financial assets exceed $200,000 on the last day of the year, or $300,000 at any point, for single filers living abroad. Thresholds double for married filing jointly.
• Form 1116 for foreign tax credits, or Form 2555 if you elect the foreign earned income exclusion.
• Form 8621 for each PFIC, which includes most UK and EU domiciled funds and ETFs held outside US wrappers.

On the UK side, you register for self assessment if you have income HMRC does not capture through PAYE, such as US investment income, rental income, or self employment. Most employed Americans with simple finances never file a UK return; most with US brokerage accounts, US rental property, or self employment do.

• UK tax year: April 6 to April 5. Online self assessment returns and payment are due January 31 following the tax year end.
• US tax year: calendar year. Americans abroad get an automatic extension to June 15, with tax still accruing interest from April 15. A further extension to October 15 is available on request.
• FBAR: due April 15 with an automatic extension to October 15, filed separately with FinCEN, not with your return.

From April 2025 the UK abolished the remittance basis for non-doms and replaced it with the foreign income and gains (FIG) regime: new UK arrivals who have been non-resident for the previous 10 years can claim full UK relief on foreign income and gains for their first 4 years of residence. For a newly arrived American, that can mean US investment income stays outside UK tax for 4 years while remaining fully taxable in the US. Sequencing matters: realizing gains or Roth conversions inside the FIG window can permanently change what you owe on each side.

A dual-qualified adviser earns their fee when any of these apply: you hold non-US funds or ISAs (PFIC exposure), you have equity compensation such as RSUs or options spanning the move, you are inside or near the 4-year FIG window, you own a business in either country, or you are behind on US filings and considering the streamlined procedures. The forms are manageable; the interactions between the two systems are where expensive mistakes happen.

Aequify gives Americans in the UK one view of accounts in both countries and produces tax-ready data packs your US and UK advisers can actually use, so the two filings draw on the same numbers.