🌴 COUNTRY A California VS 🇬🇧 COUNTRY B UK

Yes. All US citizens and green card holders must file a US federal tax return regardless of where they live in the world. Moving to the UK does not end your US tax filing obligation. However, the US-UK tax treaty and foreign tax credits allow most Americans to offset UK taxes paid against their US liability — preventing true double taxation. You will typically pay whichever is higher (US or UK rate), not both in full.

California is notorious for auditing people who claim to have left the state. To sever California tax residency, you must establish domicile elsewhere and have minimal ties to California. Simply moving to the UK is not enough — you must also change your driver's license, voter registration, professional licenses, and not spend significant time in California. The Franchise Tax Board may still claim you owe California tax on income earned in your year of departure.

California state income tax at $100,000 is approximately $5,762. UK national income tax at the equivalent income (around £78,000 at current exchange rates) is approximately $23,700 USD — nearly four times higher. However, California residents also pay US federal tax (~$17,344), bringing total CA burden to ~$23,106. The UK national rate and total CA+federal rate are remarkably close at $100,000.

Between £100,000 and £125,140, UK taxpayers lose their personal allowance (£12,570) at a rate of £1 for every £2 earned. This creates an effective 60% marginal tax rate in that band. Californians moving to the UK should be aware this trap applies to income between roughly $128,000 and $160,000 USD.

The Foreign Earned Income Exclusion (FEIE) allows qualifying US expats to exclude up to $126,500 (2024) of foreign earned income from US federal tax. However, if you claim the FEIE, you generally cannot also claim foreign tax credits on the excluded income. For UK-based Americans (where UK rates often exceed US rates), it is often more advantageous to use the Foreign Tax Credit rather than the FEIE.

Yes. UK National Insurance (NI) contributes to the state pension and NHS. Employees pay 8% on earnings between £12,570 and £50,270, and 2% above. US Social Security is 6.2% (capped at $168,600 in 2024) plus 1.45% Medicare. A US-UK totalization agreement means you generally don't pay into both systems simultaneously.