The UK starts taxing you from your residence start date
UK residence for 2026/27 is decided by the Statutory Residence Test, and the answer is all-or-nothing for the year: 183 or more UK days, an only home in the UK, or full-time UK work each make you resident automatically, with a sufficient-ties test behind them. Taken alone that would expose a September 2026 arriver's US income from 6 April 2026 to UK tax, which is why split-year treatment is the single most valuable relief in the arrival year: where one of the five arriver cases applies, the overseas part of the year comes first and only income and gains from the split date onwards fall fully within the UK net. From that date you are into normal UK taxation, PAYE on UK employment, Income Tax on worldwide income and CGT on worldwide gains.
The FIG regime, and your first UK deadlines
Most arrivers from the US also qualify for the four-year Foreign Income and Gains regime, provided they were non-UK resident for the 10 tax years immediately before arrival. A valid claim relieves qualifying foreign income and gains from UK tax for up to the first four UK-resident years, with no charge for bringing the money into the UK, claimed each year on the SA109 alongside the split-year claim. Two costs matter: each claim year forfeits the £12,570 Personal Allowance and £3,000 CGT exempt amount, and for a US citizen the relief removes the UK tax that would otherwise have been credited on the US return, so whether claiming saves anything is a two-country calculation. For a 2026/27 arrival, register for Self Assessment by 5 October 2027, file on paper by 31 October 2027 or online by 31 January 2028, and note that HMRC's free service does not support the SA109, so you need commercial software or an agent. Our FIG regime guide covers eligibility in detail.
The US side does not stop
Moving to the UK never switches the US side off; it adds a UK layer on top. Your 2026 Form 1040 covers the whole calendar year including post-move UK income, due 15 April 2027 with an automatic extension to 15 June for taxpayers abroad and a further extension to 15 October via Form 4868, and the FBAR runs alongside with its own automatic October extension. Double tax on earnings is then managed through the foreign earned income exclusion (up to $132,900 for 2026, with residence tests a mid-year mover often cannot meet by the normal deadline) or the foreign tax credit, a sticky choice your US preparer models rather than a default. US filings are handled by our US partners (Enrolled Agents and CPAs), whom we coordinate for you, while Horizon prepares the UK side, including the year-of-arrival return with SA109, on fixed fees from £550; our arrival-year playbook maps the whole timeline, and a free clarity call at /book is the natural first step.