Equity compensation — whether Restricted Stock Units (RSUs), Non-Qualified Stock Options (NQSOs), Incentive Stock Options (ISOs), or Employee Stock Purchase Plans (ESPPs) — creates one of the most complex cross-border tax situations for US citizens working in the UK. When RSUs vest or options are exercised, both the US and UK tax systems may want their share — and the rules governing how much each country takes, when it is taxed, and how credits prevent double taxation require careful navigation.
This guide covers the US and UK tax treatment of the most common forms of equity compensation, the apportionment rules that apply when you have worked in both countries during the vesting period, and the planning steps that can significantly reduce your combined tax burden.
Restricted Stock Units (RSUs): US Tax Treatment
RSUs are the most common form of equity compensation for US and UK tech, finance, and corporate employees. For US tax purposes:
- At vesting: The fair market value (FMV) of the shares on the vesting date is ordinary income, reportable on Form W-2 (if from a US employer) or as foreign wages on Form 1040 (if from a UK employer). This income is subject to income tax and, for US employees, FICA (Social Security and Medicare taxes).
- After vesting: Any subsequent change in share value is a capital gain or loss from the date of vesting. The holding period starts on the vesting date.
- Sale of shares: Capital gain or loss = sale price minus FMV at vesting. Long-term rates apply if held more than one year after vesting date.
RSUs: UK Tax Treatment
In the UK, RSUs granted to employees are typically taxed as employment income under the Employment-Related Securities (ERS) rules:
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- At vesting: The FMV of shares on vesting is subject to UK Income Tax (PAYE) and National Insurance Contributions (employee and employer NICs). If the employer has an arrangement with HMRC, the employee may bear the employer NIC charge too.
- After vesting: Subsequent gains on sale of shares are subject to UK capital gains tax (with the £3,000 annual exempt amount).
- Section 431 election: If the RSUs or shares are subject to a risk of forfeiture, a joint election under Income Tax (Earnings and Pensions) Act 2003 Section 431 can cause the full unrestricted value to be taxed at vesting rather than having later value increases treated as employment income.
Cross-Border Apportionment: The Key Issue
When a US expat in the UK receives RSUs that vest over a multi-year period, and they have worked in both the US and UK during that vesting period, both countries typically seek to tax a portion of the vesting income — based on how much of the vesting period was spent in each country.
Both the IRS and HMRC use a source-based apportionment approach:
- Days worked in the UK / Total days in vesting period = UK-source portion (taxable in UK)
- Days worked in the US / Total days in vesting period = US-source portion (taxable in US)
The Foreign Tax Credit (Form 1116) is then used to credit UK income tax paid on the UK-source portion against US tax on the same income. The goal is to ensure no double taxation, but the apportionment calculation must be done correctly to match the credit to the right income source.
Non-Qualified Stock Options (NQSOs)
NQSOs are taxed at exercise (not at grant):
- At exercise: The spread (FMV at exercise minus exercise price) is ordinary income in the US — subject to income tax and, if from a US employer, FICA taxes
- In the UK, NQSO exercise income is employment income subject to UK Income Tax and NICs
- Cross-border apportionment applies based on grant-to-exercise period worked in each country
- After exercise: Gain on subsequent sale (above FMV at exercise) is capital gain in both countries
Incentive Stock Options (ISOs)
ISOs receive preferential US tax treatment but create specific cross-border complications:
- At exercise (US): No ordinary income tax (if the qualifying disposition rules are met) — but the spread may be subject to the Alternative Minimum Tax (AMT)
- At sale: If a qualifying disposition (held more than 2 years from grant date and more than 1 year from exercise date), the entire gain is long-term capital gain
- UK treatment of ISOs: The UK does not recognise the preferential US ISO treatment. HMRC treats ISO exercise income as employment income subject to UK Income Tax and NICs — creating a mismatch where UK tax is due on exercise but no regular US tax is due
- This mismatch can result in significant UK tax with limited US foreign tax credit relief (since the UK tax arises on exercise but the US recognises no income on exercise for regular tax purposes)
Enterprise Management Incentives (EMI) Options
EMI options are a UK tax-advantaged option scheme. For UK purposes, qualifying EMI options are taxed only on sale at CGT rates. For US purposes, EMI options are treated as NQSOs — the exercise spread is ordinary income at exercise. This creates a US/UK timing and character mismatch that requires careful planning.
Reporting Requirements
US reporting for equity compensation as a US expat:
- Form W-2 or foreign wages: RSU/NQSO income at vesting/exercise is reported as wages
- Schedule D / Form 8949: Sale of shares after vesting/exercise is reported here
- Form 1116: Foreign tax credit for UK Income Tax paid on equity income
- Form 3921: Issued by employer for ISO exercises (informational)
- FBAR / Form 8938: If shares are held in a UK brokerage account, the account may trigger FBAR and FATCA reporting requirements
UK reporting on Self Assessment:
- Employment income from RSU vesting or NQSO exercise on SA100/SA102
- Capital gains on share sale on SA108 (Capital Gains Summary)
- ERS annual return filed by employer with HMRC (separate employer obligation)
Frequently Asked Questions
My RSUs vest while I live in the UK but were granted when I was in the US. How is the income split?
Both the IRS and HMRC use a vesting period apportionment. The income is split proportionally based on the number of days you worked in each country during the entire period from grant date to vesting date. The UK portion is taxable by HMRC and the US portion is taxable by the IRS. Foreign tax credits prevent double taxation on the overlapping portion.
Do I pay National Insurance contributions on RSU income?
Yes — in the UK, RSU vesting income is employment income subject to employee Class 1 NICs. Your employer also pays employer Class 1 NICs on the value, which may be passed on to you depending on your employment contract. This NIC charge adds approximately 8% (employee) plus 13.8% (employer) to the UK tax cost at vesting.
Can I use the Foreign Earned Income Exclusion to shelter RSU income?
RSU income at vesting is earned income and potentially eligible for FEIE if it is for services performed in a foreign country and you meet the physical presence or bona fide residence test. However, stock option income at exercise and capital gains on share sales are generally not earned income for FEIE purposes. The interaction with UK income tax foreign tax credits must be modelled before deciding whether to apply FEIE to RSU income.
What if my shares are held in a US brokerage account and I now live in the UK?
The UK generally taxes you on worldwide income and gains as a UK resident. Shares held in a US brokerage account are still subject to UK CGT when sold. Dividends received are UK taxable income. The Foreign Tax Credit (or the US-UK treaty) prevents double taxation on US-source dividends and gains.
Expert RSU and Equity Tax Advice for US Expats
Equity compensation is one of the highest-value and highest-risk areas of US expat tax. A single large RSU vest or ISO exercise can create a six-figure cross-border tax event that, handled incorrectly, results in significant double taxation or missed reliefs. Our advisors specialise in US-UK equity compensation tax and can provide pre-vest planning, vesting-year compliance, and sale-year capital gains optimISAtion.
Contact us today for RSU and stock option tax advice tailored to your cross-border situation.