The single most valuable piece of tax advice for an American moving to the UK is this: the best planning happens before you arrive, not after. Once you become UK tax resident, doors quietly close — gains become UK-taxable, certain US accounts turn into UK headaches, and options that were simple at home become complicated. The weeks or months before your move are a rare window to lock in advantages on both sides of the Atlantic. Here is what to sort out before you land.
Why the pre-move window matters
When you become UK tax resident, the UK gains the right to tax much of your worldwide income and gains, while the US continues to tax you as a citizen. Decisions that are tax-free or simple while you are solely a US taxpayer can trigger UK tax — or fall into the gap between the two systems — once you are resident in Britain. Acting before that switch flips is what separates a smooth move from an expensive one.
The UK also changed its rules for new arrivals from 6 April 2025, replacing the old non-dom remittance basis with a new residence-based regime that can give relief on foreign income and gains for the first few years of UK residence. The detail is technical and personal, but the headline is the same: the rules that apply to you are set by when and how you arrive, so the planning belongs before the move.
Consider realising gains before you become UK resident
If you hold appreciated investments, selling (and potentially repurchasing) before you become UK tax resident can reset your cost base for UK purposes, so future growth — not historic gains accrued while you were in the US — is what the UK might later tax. For US purposes you would recognise the gain anyway, but doing it before the UK has any claim avoids the gain being exposed to UK Capital Gains Tax later. This is highly fact-specific and interacts with the new-arrival rules, so model it carefully rather than assuming.
Roth conversions and retirement accounts
The pre-move window can be a good time to consider Roth conversions while you are still squarely in the US system, because a qualified Roth can stay tax-free on both sides once you are in the UK — see our guide to US 401(k) and IRA accounts for UK residents. Equally, understand how your traditional 401(k) and IRA will be taxed when you eventually draw them as a UK resident, so your retirement strategy is built around the cross-border treatment rather than blindsided by it.
Clean up investments that become UK problems
Some perfectly normal US investments become awkward once you are in the UK, and vice versa. US mutual funds and ETFs may not have UK 'reporting fund' status, which can subject gains to UK income-tax rates rather than CGT rates for a UK resident. Conversely, once in the UK you must avoid UK funds and Stocks & Shares ISAs that create US PFIC problems. Reviewing your portfolio before you move — and restructuring while it is still simple — heads off years of punitive reporting.
Time your UK residence start date
The UK decides residence using its Statutory Residence Test, which counts days and ties. The exact date you become UK resident — and whether you qualify for split-year treatment in your arrival year — affects how much of your income that first year falls into the UK net. Planning your arrival around the test, and understanding your split-year position, can materially reduce your first-year UK tax. You can read the UK rules on the GOV.UK tax-residence guidance.
Your US home and other US assets
If you own a US home, decide before you move whether to sell or keep it, because the tax outcome differs sharply. Selling a main home can use the US capital-gains exclusion (up to $250,000, or $500,000 for a couple) if you still qualify — a benefit that can erode the longer you are away. Keeping it and renting it out makes you a US landlord and, once UK resident, brings the rental income into UK scope too. Either path is fine, but it should be a decision, not an accident.
Don't forget US state tax on the way out
Leaving the US cleanly for state tax is part of pre-move planning. If you are leaving a 'sticky state' like California or New York, take the steps to genuinely end your domicile before or as you go — see our guide to state tax for Americans abroad. Sorting this on the way out is far easier than unwinding a lingering state residency from London years later.
Estate planning for a cross-border life
Moving to the UK changes your estate-tax picture, especially over time. The US and UK both levy death taxes with different rules, thresholds and treatment of spouses, and a will drafted purely for the US may not work well once you are UK resident. While estate planning is rarely the first thing on a mover's mind, flagging it before the move means your wills, beneficiary designations and asset structures can be set up coherently for both systems rather than retrofitted later.
Common pre-immigration mistakes
- Moving first and asking about tax later — by then the cheapest options have closed.
- Carrying US mutual funds/ETFs into the UK without checking reporting-fund status.
- Buying UK funds or an ISA after arrival and creating a PFIC problem.
- Selling a US home a year too late and losing the main-home exclusion.
- Leaving a sticky US state's ties intact and getting taxed there for years.
The UK's new regime for new arrivals
From 6 April 2025 the UK scrapped the old non-dom 'remittance basis' and replaced it with a new residence-based system. In outline, people who become UK tax resident after a long enough period of non-residence can claim relief on their foreign income and gains for their first few years of UK residence. For an American arriving in the UK, that can mean a valuable window in which foreign (non-UK) income and gains are sheltered from UK tax — but the rules are new, detailed, and depend on your residence history.
The practical takeaway for pre-move planning is that this relief is time-limited and tied to when you arrive, so it should be factored into the timing and structure of your move. It does not change your US obligations at all — you remain fully US-taxable — but it can significantly affect the UK side of your first years, which is exactly why the planning belongs before you land.
Currency gains and your mortgage
A quirk that surprises movers: the US taxes foreign-currency gains, including on the repayment or refinancing of a foreign-currency mortgage. If you take out a pound-denominated mortgage on a UK home and the dollar/pound rate moves, repaying or remortgaging can create a US-taxable 'phantom' currency gain — even though nothing changed in pounds. Knowing this before you buy or borrow lets you plan around it. Similarly, large currency conversions around your move are worth timing and documenting, because the US measures your gains and income in dollars regardless of the currency you actually hold.
A simple pre-move checklist
- Review your investment portfolio for both UK reporting-fund status and US PFIC exposure — restructure while it's simple.
- Decide what to do with your US home before the main-home exclusion erodes.
- Consider realising gains or Roth conversions while still solely in the US system.
- Plan your UK arrival date around the Statutory Residence Test and split-year treatment.
- Cut ties with any 'sticky' US state and plan your final part-year state return.
- Take advice on the UK's new-arrival regime so you use any available relief window.
- Review wills and beneficiary designations for a US/UK estate-tax world.
Prepare for FBAR and FATCA from day one
The moment you start opening UK bank accounts, a pension and perhaps an ISA, you create US reporting obligations that did not exist while all your accounts were American. Once your combined non-US accounts exceed $10,000 you must file an FBAR, and larger holdings can trigger Form 8938 — see our guide to FBAR vs FATCA. Knowing this before you arrive means you can keep good records from your first UK account rather than reconstructing them later, and you can avoid accidentally opening account types (like a Stocks & Shares ISA) that create disproportionate US headaches.
Set your US filing rhythm before you go
Finally, decide how you will stay compliant before the move rather than scrambling each April from abroad. That means understanding the US deadlines for Americans abroad (the automatic June extension, the October backstop, the FBAR date), deciding between the Foreign Tax Credit and the FEIE for your situation, and lining up a US/UK adviser before your first cross-border filing season. Getting the rhythm set in advance turns annual compliance from a source of dread into a routine.
Plan before you pack
Pre-immigration planning is one of the few areas of cross-border tax where a short conversation can save five figures. The themes are consistent: reset what you can before the UK has a claim, avoid creating PFIC and reporting traps, time your arrival, and decide deliberately about your US home and state. A US tax specialist who understands the UK side can map this with you in the weeks before you go. The earlier you start — ideally months before the move — the more of these levers are still available to pull.
