Tax Residency
Moving between the US and UK doesn’t just change your address — it can change which country taxes your income, when, and on what basis. Here’s what actually shifts, and the planning window most people miss.
The first thing to understand is that the US and the UK each decide who counts as their tax resident independently, using their own rules, on their own timeline. There is no shared definition, and no coordination at the point of assessment — the two tests simply run in parallel. That means it is entirely possible to meet both countries’ residency tests in the same tax year, and to be treated as a tax resident of both at once, at least until a tax treaty is used to resolve the conflict.
This matters because most people assume residency is a single, common-sense question — “where do I live?” — when in practice it’s two separate legal tests that happen to be asking a similar question in different ways.
The United States taxes based on a combination of citizenship and physical presence, which is broader than most other countries’ approach. A US citizen or green card holder is taxed on worldwide income regardless of where they live — moving abroad does not, by itself, end US filing obligations.
For non-citizens, the US applies the Substantial Presence Test: broadly, if you’re present in the US for 31 days or more in the current year, and 183 days or more across a weighted count of the current year and the two preceding years, you’re treated as a US resident for tax purposes for that year. The weighting counts all days in the current year, one-third of days in the prior year, and one-sixth of days in the year before that — which catches people who assume a few months a year, spread across three years, keeps them under the threshold. It often doesn’t.
The UK uses the Statutory Residence Test (SRT), which is more structured than a simple day count, though day counting is still central to it. The SRT works through a sequence: automatic overseas tests (which can make you automatically non-resident), automatic UK tests (which can make you automatically resident), and, if neither applies cleanly, a set of “ties” tests that weigh your UK connections — family, accommodation, work, and prior UK residence — against the number of days you spend in the UK.
The practical effect is that the same number of days in the UK can result in residence or non-residence depending on how many UK ties you have. Someone with a UK spouse, a UK home available to them, and substantive UK work ties will trip into residence at a much lower day count than someone with none of those ties.
Because the two tests are independent, someone who moves from the UK to the US mid-year — or the reverse — can genuinely meet both countries’ residency tests for the same period. When that happens, the US–UK tax treaty provides a tie-breaker: a sequence of tests (permanent home, centre of vital interests, habitual abode, and nationality, in that order) used to determine which country you’re treated as resident in for treaty purposes.
The treaty tie-breaker doesn’t erase either country’s domestic filing requirement — a US citizen is still required to file a US return regardless of the treaty outcome — but it does determine which country has primary taxing rights over specific categories of income, and it’s the mechanism that prevents the same income being taxed twice without relief.
In practice, four things shift when you relocate between the US and UK:
Which return you file, and when — the US tax year runs to 31 December with an April filing deadline; the UK tax year runs to 5 April with a January filing deadline for online returns.
What you’re required to report — US citizens and residents face foreign account and asset reporting (FBAR, FATCA/Form 8938) that has no direct UK equivalent, with meaningful penalties for missing it, even where no tax is ultimately due.
How the year itself is split — the UK’s split-year treatment can, in some circumstances, divide a single tax year into a UK part and a non-UK part, so you’re not taxed as a full-year UK resident simply because you arrived or left partway through it. The US has no equivalent concept for citizens, though it applies differently for non-citizens establishing or ending residency.
What income is taxed where — employment income, rental income, and investment income can each be sourced and taxed differently depending on residency status and the treaty position, which is where most double-taxation risk — and most double-taxation relief — actually sits.
The highest-value planning happens before the move, not after. Once a tax year has closed, the residency position for that year is largely fixed — but in the run-up to a relocation, there is often real scope to time the move itself, structure income recognition, and consider elections (such as claiming split-year treatment in the UK, where the conditions are met) in a way that meaningfully changes the outcome.
The single most common mistake we see is someone who has already moved, and is filing their first return in the new country, discovering a planning opportunity that was only available before the move happened. If a US–UK relocation is on the horizon — in either direction — the conversation is worth having months in advance, not at filing time.
This article is provided for general information only and does not constitute tax, legal, or financial advice specific to your circumstances. Rules referenced are current as of publication and are subject to change. Please talk to a Meridian partner before acting on anything above.
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