Resident in both the US and the UK? How the treaty decides, and how to avoid double tax

Spend enough time in both countries, or hold a green card while living in London, and each system can claim you as resident in the same year. The treaty has a procedure for that; it does less for US citizens than people hope.

How each country makes you resident

United States. Citizens are taxed on worldwide income wherever they live. Non-citizens are resident if they hold a green card, or meet the substantial presence test: at least 31 days in the current year and 183 weighted days over three years (all of this year, a third of last year, a sixth of the year before).

United Kingdom. The Statutory Residence Test: automatic overseas tests (very few UK days), automatic UK tests (183 days, an only home in the UK, full-time UK work), then the sufficient-ties test combining day counts with family, accommodation, work, 90-day and country ties. Split-year treatment can apply in the year you arrive or leave.

The treaty tie-breaker

Where both countries treat you as resident, Article 4 decides treaty residence in this order: the country where you have a permanent home; if both, your centre of vital interests; then habitual abode; then nationality; then agreement between the two tax authorities.

For a non-US citizen the result matters: UK treaty residence limits US tax to US-source income under the treaty's articles, with Form 8833 disclosure. For a US citizen the saving clause means the US still taxes worldwide income; the tie-breaker helps with specific articles (pensions, for example) rather than removing the US return. A green card holder who claims UK treaty residence may be treated as having expatriated for exit-tax purposes.

What actually prevents double tax

• Foreign Tax Credit. UK tax on the same income reduces US tax, basket by basket, with carry-forward. For most UK-based Americans it leaves little or no US tax.
• Foreign Earned Income Exclusion. Removes up to US$130,000 (2025) of earned income from US tax; often second-best in the UK.
• Treaty articles. Pensions, Social Security, employment income for short stays and several other items are allocated between the countries, disclosed on Form 8833 where required.

Common situations

• American settled in the UK: UK resident, US citizen; UK return plus US return with the credit.
• Briton with a green card who moved home: still a US resident for tax until the card is formally abandoned, whatever the SRT says.
• Splitting the year between homes in both countries: day counts on both sides, the tie-breaker, and careful treatment of the move year.
• New arrival with foreign income: the four-year foreign income and gains regime in the UK, modelled against the US return before it is claimed.

The practical answer in every case is the same: both returns from one file, prepared in the right order, with the treaty positions written down.