Cross-Border RSU Taxation: Dual-Country Treatment and Credit for Tax Paid

An engineer takes a UK-based job with a US tech employer and receives RSUs that vest over four years. Two years into the grant, she relocates to the US on an L-1 visa. Over the remaining two years, the RSUs continue to vest. Both the UK and the US want to tax the vesting events. Who actually gets what, and how does she avoid paying the same tax twice?

The answer lives in the sourcing rules, the US-UK tax treaty, and the foreign tax credit mechanics of IRC §901. Each vest has to be allocated between countries based on where she worked during the period from grant to vest. The country of source taxes first; the other country taxes the full amount but gives credit for tax paid to the first country, subject to limitations.

Done right, she pays tax only once (or close to once) on each dollar of vest income. Done wrong, she can pay 50%+ combined tax and spend years filing amended returns to recover the excess.

Sourcing rules for RSU income

The core rule: RSU income is sourced based on where services were rendered during the vesting period, not where the employer is located or where the employee lives at vest.

A grant vesting over four years represents four years of service. If the employee spent two years in the UK and two years in the US during that period, half the vest income is UK-source and half is US-source.

For a single-vest tranche (say, year-three vest of $50K), the sourcing depends on the full vesting period from grant to vest, not just the year of the vest. If the employee was in the UK for months 1-24 and the US for months 25-36, the year-three vest is 24/36 UK-source and 12/36 US-source.

Different countries apply different sourcing methods:

US: workdays method (US workdays during vesting period / total workdays).
UK: modified vesting period apportionment under ITEPA 2003, with some simplifications.
India: day count during the vesting period.
Canada: similar day count.

Methodologies mostly converge on similar outcomes, with some edge cases.

Treaty rules: US-UK as an example

The US-UK tax treaty (2001 protocol) allocates primary taxing rights for employment income to the country where services are performed. Article 14 governs dependent personal services and generally cedes primary right to the source country.

For RSU income, the treaty does not have a specific article. It is treated as deferred compensation and is subject to the employment-income sourcing rules with some timing wrinkles.

Practical outcome:

Services in the UK: UK taxes primary.
Services in the US: US taxes primary.
Resident in either country: that country also taxes worldwide income but gives FTC for tax paid to the source country.

The foreign tax credit mechanics

IRC §901 allows US taxpayers to claim a credit against US tax for income taxes paid to a foreign country. The credit is limited to the US tax on the foreign-source portion of income (the §904 limitation).

For RSU income:

Identify the foreign-source portion of the vest using the sourcing rules.
Determine foreign tax paid on that portion.
Calculate US tax on total income (including the full vest amount).
Apply FTC up to the limit: (foreign-source income / total income) × US tax.

If foreign tax paid exceeds the limit, the excess carries back one year and forward ten years under §904(c).

UK PAYE typically withholds at the higher-rate band on large vests, often 45%. US federal withholding on RSU income at a supplemental rate is 22% (or 37% for amounts over $1M). The gap can create cash-flow issues: UK takes its 45% now; US FTC is claimed at year-end return filing.

A numeric example

Engineer with four-year RSU grant for $200,000, vesting 25% per year. Grant date: January 2022. Relocation from UK to US: January 2024.

Year 1 vest (Jan 2023, $50K): entirely UK services. UK-source. UK taxes at 45%. US taxes as resident alien after relocation but not yet; 2023 is UK resident only.

Year 2 vest (Jan 2024, $50K): entirely UK services (earned Jan 2022 through Jan 2024 while in UK). UK-source. But resident in US starting Jan 2024. Both countries tax the full $50K. US gives FTC for UK tax paid on the UK-source portion (100%).

Year 3 vest (Jan 2025, $50K): vesting period Jan 2022 through Jan 2025. 24 months UK + 12 months US. UK-source portion = 2/3 × $50K = $33.3K. US-source portion = $16.7K. UK taxes the $33.3K; US taxes the full $50K as resident; US gives FTC for UK tax on $33.3K.

Year 4 vest (Jan 2026, $50K): vesting period 24 months UK + 24 months US. UK-source = 50% = $25K. US-source = $25K. UK taxes $25K (as it is UK-source income received by non-UK resident). US taxes $50K; gives FTC for UK tax on $25K.

Employment income also triggers social security obligations. UK NICs (National Insurance Contributions) apply on UK-source wages. US Social Security and Medicare (FICA) apply on US-source wages.

Without a totalization agreement, the employee could pay both. The US has totalization agreements with the UK, Canada, Germany, France, and ~25 other countries. These agreements determine which country’s social security system applies during the cross-border work, preventing double contributions.

Typical rule for short-term assignments: if the assignment is under 5 years, the home country’s system continues to apply (so US employer can continue to withhold FICA on a UK assignee whose home country is the US, and the UK assignee is exempt from UK NICs during the assignment).

For longer-term moves (over 5 years) or permanent relocations, the host country’s system applies from the move date.

See the separate article on totalization agreements for the mechanics of obtaining a certificate of coverage.

Common errors

Treating the vest year as the sole sourcing year. Sourcing is done over the full vesting period, not just the year of vest. This error is made frequently by US preparers unfamiliar with international equity.

Claiming FTC on non-creditable taxes. Not all foreign payroll taxes are creditable for US FTC purposes. UK NICs are not creditable (they are a social security tax, not an income tax). UK PAYE withholding for income tax is creditable.

Missing the §904 limitation. Claiming FTC in excess of the limit does not reduce US tax further; the excess carries but cannot offset other-category income.

Forgetting state tax. FTC applies against US federal tax. Most US states do not allow credit for foreign tax paid. A California resident who pays UK tax on RSU income cannot credit against CA tax, producing a higher effective combined rate.

Ignoring the host country’s reporting. UK forms (P60, P11D, self-assessment return) must be filed even after relocation for UK-source compensation still being vested.

Frequently asked

Who is responsible for tracking the days in each country? The employee. Employers sometimes provide day-tracking tools but ultimately the taxpayer substantiates the allocation on their return. A day-counting log is essential.

What if my employer withholds only in one country? Many global employers withhold in the resident country only, leaving the non-resident country’s tax to be paid via estimated tax or at filing. The employee must manage the cash flow.

How does this interact with double trigger RSUs that vest at IPO? Same sourcing analysis based on where work was done during the vesting period. The liquidity-event aspect of double-trigger RSUs does not change the cross-border allocation.

Can I use my employer’s tax-equalization plan to handle this? If the employer provides tax equalization for cross-border assignees (common for senior international assignments), the employer’s tax provider handles the computation. The employee pays a “hypothetical home-country tax” and the employer picks up the rest.

What about state tax on the US-source portion? State of residence at vest generally taxes the full vest. State of prior residence may also have trailing-nexus claims on the portion earned while the employee was in that state. See the California trailing nexus article for specifics.

Next step

If you have cross-border RSUs vesting, build a vesting-period day log today: start date, end date, country of work, days in each. For each vesting tranche, compute the sourcing split. Ask your employer whether they provide a Section 6050B cross-border statement (large global employers often do). Engage a cross-border tax preparer before the first major vest to ensure first-year withholding and FTC planning is correct.