Introduction The most expensive tax mistakes for Americans moving to the UK are made before the flight, on the US side of the departure. Upon arrival in the UK, every financial decision is governed by two tax systems at once. Before departure, most actions are still governed only by US rules, making the pre-departure window […]
The most expensive tax mistakes for Americans moving to the UK are made before the flight, on the US side of the departure. Upon arrival in the UK, every financial decision is governed by two tax systems at once. Before departure, most actions are still governed only by US rules, making the pre-departure window the most tax-efficient planning period available.
Specialist accountants for the US and UK, with the hope plan for the US-side departure, understand that the final partial-year return, the 401(k) rollover, the US property sale, the state residency severance, and the S corporation review all carry consequences that cannot be undone after arrival. This guide covers every US-side action that reduces the combined US-UK tax on the move. Visit our advisory service:
https://www.jungletax.co.uk/services/us-uk-tax/
Accountants for the US and the UK who specialize in pre-departure planning understand both the US tax rules governing the departure year — the final partial-year return, state residency severance, 401(k) and IRA decisions — and the UK tax consequences of each US-side action. Many pre-departure decisions have UK consequences that arise in the first UK tax year. A specialist plans the US-side departure alongside the UK-side arrival — ensuring every action is optimized in both systems before the move.
The IRS guidance on US citizens moving abroad is at:
https://www.irs.gov/individuals/international-taxpayers/us-citizens-and-resident-aliens-abroad
Before departure, the investor is a US resident — subject only to US rules. A Roth conversion before arrival is taxable only in the United States. The same conversion upon arrival may also be subject to UK income tax. A PFIC restructuring before arrival attracts only US CGT. Upon arrival, UK CGT at 20 percent also applies. The pre-departure window is always the most efficient planning period — and it closes permanently on the day of arrival in the UK.
The year of departure is a split-year for US tax purposes. From 1 January through the departure date, the individual is a US resident and is taxed on worldwide income. From the departure date to 31 December, they are a non-resident — taxed only on US-source income. Both periods appear on a single Form 1040. The income split, the correct FTC or FEIE application, and any year-end planning actions all require specialist accountants for US and UK knowledge to handle correctly.
California’s Franchise Tax Board and New York’s Department of Taxation both aggressively assert residency for taxpayers who leave without formally severing their connections. A US citizen who moves to the UK without selling their California home, closing California bank accounts, and updating voter registration may receive California tax assessments on UK employment income for years after departure. Formal severance before departure is essential — and must be documented.
Our guide to the UK arrival side of the move is at:
https://www.jungletax.co.uk/jungle-tax-news-updates/us-and-uk-tax-advisors-pre-immigration/
A Roth conversion completed before departure is taxable only in the United States, with no UK income tax interaction. Upon arrival in the UK, the same conversion may also be subject to UK income tax. The pre-departure window is the optimal time for both the Roth conversion and the 401(k) rollover to an IRA. Both actions are governed only by US rules if completed before the UK arrival date.
The departure year US return covers two periods. Every income item is reviewed to determine whether it falls in the US resident period or the non-resident period. Income earned before the departure date is taxed as resident income. US-source income earned after the departure date — rental income from a US property, dividends from a US brokerage account — is taxed as non-resident income.
The adviser also identifies every year-end planning action — loss harvesting, charitable contributions, capital gain realizations — that should be completed in the resident period before departure. Actions completed before departure are governed only by US rules. The same actions deferred to the post-arrival period are subject to UK tax interaction.
A 401(k) left with a former employer after relocation to the UK is subject to that plan’s investment restrictions and withdrawal rules. Managing a US employer plan from the UK — across time zones, with no UK-based support — is significantly more complex than managing a self-directed IRA. Rolling the 401(k) to an IRA before departure is a direct rollover — not a taxable distribution — and provides full investment flexibility and simplified post-arrival administration.
The accountants for the US and UK at Jungle Tax review the state tax treatment of the rollover before execution. Some states treat 401(k) rollovers differently from standard distributions. The rollover should be completed while the taxpayer is still in a state with favorable treatment.
A Roth conversion completed in the US resident period of the departure year is subject only to US federal income tax. Upon arrival in the UK, the same conversion may also attract UK income tax as a receipt from a foreign pension. The pre-departure window eliminates any UK interaction. The accountants for the US and UK at Jungle Tax calculate the optimal conversion amount before departure — the amount that maximizes the benefit at the current US marginal rate without triggering an unnecessarily higher bracket.
A US principal residence sold before departure benefits from the Section 121 exclusion — the first $250,000 of gain is exempt from US federal capital gains tax ($500,000 for married filing jointly). No UK CGT arises because the individual is not yet a UK resident. After arrival in the UK, the same sale of a US property triggers UK CGT at 20 percent on the post-arrival gain. A pre-departure sale eliminates the UK CGT cost entirely.
Severing California residency requires: selling or renting the California home to an unrelated party; closing California bank accounts; updating voter registration to a different state; obtaining a driving license from a different state before departure; and resigning from California-based clubs and professional organizations. New York adds the permanent place of abode test — a taxpayer who keeps a New York apartment after departure may be treated as a New York City resident, subject to NYC income tax on UK employment income.
A US S corporation whose shareholder moves to the UK does not automatically lose its S election — because a US citizen remains a US person, not a non-resident alien. However, the UK may treat S corporation income differently — potentially taxing it as corporate income at UK rates rather than pass-through income at personal rates. The accountants for the US and UK at Jungle Tax review every US business structure before departure and advise on whether to continue, convert, or sell the entity.
The IRS guidance on departing aliens and their US tax obligations is at:
Laura is a US citizen living in Chicago. She is relocating to London in October 2026 for a senior role at a UK financial services firm. She contacted Jungle Tax in March 2026 — seven months before departure. Her US financial position: a Chicago condominium worth $890,000 (purchased for $520,000, with a $280,000 mortgage); a 401(k) worth $310,000; a traditional IRA worth $95,000; a US brokerage account; and a 30 percent interest in an Illinois S corporation.
Jungle Tax identified and completed the following actions before Laura’s October 2026 departure.
The Chicago condominium was sold in August 2026. The taxable gain after the Section 121 exclusion was $120,000 — subject to US long-term capital gains tax only. No UK CGT arose because Laura was not yet a UK resident. Upon arrival, the same post-arrival gain would have attracted UK CGT at 20 percent on top.
The 401(k) was rolled directly to an IRA before departure — a tax-free rollover. Illinois tax treatment was confirmed before execution.
Jungle Tax calculated that Laura could convert $38,000 from a traditional IRA to a Roth IRA during the US residency period, at the 22 percent marginal rate, with no UK income tax interaction. The conversion was completed in September 2026.
Illinois residency was formally severed before departure. The condominium sale eliminated the permanent place of abode, and voter registration was updated before the departure date.
The pre-departure property sale eliminated UK CGT on the post-arrival gain. The Roth conversion was completed at a 22 percent US rate with no UK interaction. The 401(k) rollover simplified post-arrival administration. Illinois residency was formally severed — eliminating ongoing state income tax assessments on UK employment income. The S corporation was reviewed and confirmed to continue, with UK treatment included in the post-arrival annual compliance program.
A US property sale after UK arrival attracts UK CGT at 20 percent on the post-arrival gain — in addition to any US federal CGT above the Section 121 exclusion. A pre-departure sale attracts only US federal CGT. Many Americans defer the sale for logistical reasons — without calculating the tax cost of that delay.
California and New York assert residency aggressively for taxpayers who leave without documenting severance. A US citizen who moves to the UK without selling their California home and updating voter registration may receive state tax assessments on UK employment income for years. The adviser documents every step of the severance and carefully times the departure date.
A Roth conversion after arriving in the UK may trigger UK income tax in addition to US federal tax. The pre-departure window eliminates any UK interaction. Many Americans defer the conversion until after arrival — and pay a higher combined tax as a result.
The UK treatment of S corporation pass-through income for a UK-resident shareholder is not automatic. Some S corporation arrangements require restructuring before the move to avoid unintended UK corporate tax treatment. The accountants for the US and UK at Jungle Tax review every US business structure before the departure date is confirmed.
The IRS guidance on S corporations is at:
https://www.irs.gov/businesses/small-businesses-self-employed/s-corporations
Jungle Tax is a specialist US-UK cross-border tax advisory firm with accountants in the US and the UK, including Enrolled Agents and UK-qualified tax practitioners experienced in US-side pre-departure planning. We conduct a full pre-departure assessment covering all US assets, retirement accounts, business structures, and state residency connections. We model the tax cost of every major decision on both sides — property sale timing, Roth conversion, 401(k) rollover, S corporation review, and state severance. We prepare the US federal departure return covering both the resident and non-resident periods and coordinate the US departure plan with the UK arrival plan.
Read our guide to the UK arrival side of the move:
https://www.jungletax.co.uk/jungle-tax-news-updates/us-and-uk-tax-advisors-pre-immigration/
The US-side pre-departure planning window closes permanently on the day of arrival in the UK. Every action completed before departure is governed only by US rules. Every action deferred until after arrival is governed by both systems — and the combined cost is almost always higher. Specialist accountants for the US and UK plan the US side before the flight is booked.
Three points matter most. First, sell the US principal residence before departure — the Section 121 exclusion applies, and no UK CGT arises. After departure, UK CGT at 20 percent applies to the post-arrival gain. Second, complete the 401(k) rollover and Roth conversion before departure — both are governed only by US rules in the pre-departure resident period. Third, formally sever California and New York residency before departure — without documented severance, state tax assessments on UK employment income can follow for years.
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Yes. The departure year is a split-year return covering both the resident and non-resident periods on a single Form 1040. A specialist handles both periods correctly.
Before departure is almost always more efficient. The Section 121 exclusion applies, and no UK CGT arises. After arrival, UK CGT at 20% applies on the post-arrival gain above the exclusion ceiling.
Yes — and it is generally recommended. The rollover is a direct transfer, not a taxable distribution. It simplifies post-arrival administration and gives full investment flexibility from the UK.
Sell or rent the California home, close California accounts, update voter registration, get a new driving license from another state, and resign from California organizations. Document every step.
A US citizen moving to the UK remains a US person — the S election is not automatically lost. However, the UK may tax S corporation income differently. A specialist reviews the structure before departure.
At least six months before the planned departure date — ideally twelve months. Pre-departure actions cannot be completed after you arrive. The planning window closes permanently on the UK arrival day.
