Living or working across borders can mean two countries each have a legitimate claim to tax part of your income. Tax residency — not citizenship, not where you hold a passport — is the key concept. Understanding how residency is determined, why two countries can tax the same income, and what international treaties do (and do not) resolve is essential background for anyone in a cross-border situation. The complexity typically warrants input from a tax professional with cross-border experience.
Tax residency is not the same as citizenship
Citizenship and tax residency are separate legal concepts. A US citizen living full-time in the UK is a UK tax resident — but the United States also taxes its citizens on worldwide income regardless of where they live. This combination is relatively uncommon globally; most countries tax based on residency only. The result is that some individuals genuinely owe reporting obligations in two jurisdictions simultaneously.
Tax residency is determined differently by each country:
- United Kingdom: HMRC uses the Statutory Residence Test (SRT), which considers the number of days spent in the UK, ties to the UK (accommodation, work, family), and prior residency history. The test has multiple connecting factors and outcomes that interact in specific ways.
- United States: the IRS taxes US citizens and green card holders on worldwide income regardless of residency. For non-citizens, a Substantial Presence Test (counting days in the US over a three-year period) determines residency.
Other countries — Ireland, Australia, Germany, France, Canada — each have their own tests, usually based on days present, habitual abode, and centre-of-life indicators. Changing residency from one country to another is a formal legal event with its own timing and compliance requirements; it is not simply a matter of moving.
Why two countries can tax the same income
If you are tax resident in two countries, or if one country taxes based on citizenship while you are resident in another, you can face a situation where both countries assert the right to tax the same income. This is called double taxation.
Double taxation can also arise in narrower circumstances without full dual residency — for example, rental income from a property in one country while residing in another, employment income for work performed in a country where you are not resident, or pension income from a former country of employment.
Without a mechanism to relieve double taxation, the same income could be taxed twice at full rates. Most countries address this through foreign tax credits (allowing you to offset tax paid elsewhere against your domestic liability) or through exemption provisions. Which method applies depends on the specific countries involved and whether a treaty is in place.
Double-tax treaties: what they do and do not do
The UK and the United States have a tax treaty (the US-UK Convention). So do most pairs of developed countries. These treaties:
- Allocate taxing rights between the two countries for specific categories of income (employment, dividends, interest, royalties, pensions, capital gains)
- Establish procedures for resolving disputes about residency or allocation
- Provide relief mechanisms — usually a credit or exemption — to prevent full double taxation
What treaties do NOT do:
- They do not eliminate filing obligations. A US citizen in the UK generally still files a US return AND a UK return.
- They do not simplify the paperwork — treaty claims require specific forms and positions to be claimed correctly.
- They do not override domestic anti-avoidance rules in all cases.
- Treaty interpretation is a specialised area; the same treaty provision can apply differently depending on the type of income, the direction of residence, and the individual's specific facts.
Tie-breaker clauses in treaties determine residence for treaty purposes when a person is resident in both countries under each country's domestic rules. These clauses consider factors such as permanent home, centre of vital interests, habitual abode, and nationality — in that order of priority under most OECD-model treaties.
Why specialist input matters
Cross-border tax situations sit at the intersection of two separate legal systems. Practitioners who work primarily in one country's domestic tax code may not have the full picture for a dual-resident or dual-filing situation. Errors or omissions in cross-border filings — missed foreign income disclosures, incorrect treaty positions, failure to file required informational returns (such as the US FBAR or Form 8938 for foreign financial assets) — can result in significant penalties entirely separate from the tax owed.
A tax professional with demonstrated cross-border experience between the relevant countries is best placed to handle these situations accurately.
Where to get help
Search for tax professionals experienced in US-UK cross-border matters through the TaxProsRated US directory. For UK-side practitioners, visit TaxProsRated UK directory.
Sources
- Internal Revenue Service (IRS): irs.gov
- HM Revenue and Customs (HMRC): gov.uk/government/organisations/hm-revenue-customs