Stock Options US and UK Tax: ISOs, NSOs, EMI and CSOP Compared

Stock options US and UK tax: an introduction

Under stock options US and UK tax rules, the same option can be taxed at different moments in each country. A share option is a right to buy shares in your employer at a fixed price. The US distinguishes incentive stock options from non-qualified options. The UK distinguishes tax-advantaged schemes, such as EMI and CSOP, from all others. Consequently, an option that is tax-favoured in one country is usually an ordinary option in the other. This article sets out when each plan is taxed, and what happens when it crosses the border.

This article supports our guide for US and UK founders and executives. That guide covers equity, companies and executive pay in both tax systems.

Why stock options US and UK tax rules clash

Each country designed its favoured plans for its own employers. Neither recognises the other's schemes. As a result, a US citizen holding EMI options, or a UK resident holding ISOs, gets the favourable treatment in one country only. The mismatch affects when tax falls due, what type of income it is and whether credits line up.

How are stock options taxed in the US?

Non-qualified options are taxed as wages when you exercise. Incentive stock options are not taxed at exercise for regular tax, but may trigger the alternative minimum tax.

Non-qualified stock options

A non-qualified stock option, or NSO, is any option that does not meet the incentive stock option rules. IRS Topic 427 says nothing is taxed at grant where the option has no readily determinable value. On exercise, you include the value of the shares less the price you paid. That spread is compensation, taxed at ordinary rates. A later sale produces a capital gain or loss from that new base.

Incentive stock options

An incentive stock option, or ISO, is an option that meets section 422 of the tax code. Topic 427 says you generally include nothing in income when you receive or exercise one. However, you may owe alternative minimum tax in the year of exercise. Section 422 requires you to hold the shares for two years from grant and one year from exercise. You must also have been an employee from grant until three months before exercise.

The $100,000 limit and the 83(b) election

Section 422(d) treats options as non-qualified to the extent that more than $100,000 of stock, valued at grant, first becomes exercisable in one year. Separately, if you receive restricted shares, for example by exercising early, you can elect under section 83(b) to be taxed at once. The election must be made within 30 days of the transfer. Missing that window cannot be fixed later.

How are stock options taxed in the UK?

Options outside a tax-advantaged scheme are taxed as employment income when you exercise. EMI and CSOP options can be exercised free of income tax and National Insurance if their conditions are met.

Non-tax-advantaged options

HMRC's Employment Related Securities Manual at ERSM110510 sets the gain on exercise as the market value of the shares less the amount paid for them. That gain is employment income. If your employer does not deduct tax through payroll, the GOV.UK guide to tax on employee share schemes says you report it through Self Assessment. Any later growth is subject to Capital Gains Tax on sale.

Enterprise Management Incentives

EMI is a UK scheme for smaller trading companies. The GOV.UK page on Enterprise Management Incentives allows options worth up to £250,000 in a three-year period. From 6 April 2026, the company's assets must be £120 million or less, with fewer than 500 full-time employees. If you pay at least the market value at grant, no income tax or National Insurance is due on exercise. Instead, the growth is taxed as a capital gain when you sell.

Company Share Option Plans

A CSOP suits larger companies. The GOV.UK page on the Company Share Option Plan allows options over up to £60,000 of shares for grants from 6 April 2023. If you exercise between three and ten years after grant, the spread is free of income tax and National Insurance. You may pay Capital Gains Tax when you sell.

Section 431 elections

Shares acquired on exercise are often restricted. A section 431 election treats them as unrestricted, so any later growth is taxed as a gain. HMRC's guidance at ERSM30450 says the election is joint, with your employer, and must be made within 14 days of acquisition. Like the US 83(b) election, the deadline is strict.

What happens when each plan crosses the border?

Each favoured plan loses its advantage in the other country. The comparison below goes plan by plan, so you can see where the timing splits.

An NSO held by a UK resident

This is the simplest case. Both countries tax the spread at exercise as employment income. The UK tax can then be credited against the US tax on the same income, through Form 1116. The main work is splitting the income by workdays if you moved during the vesting period.

An ISO held by a UK resident

The UK does not recognise ISOs. It taxes the spread at exercise as employment income, like any other option. The US, however, taxes nothing at exercise for regular tax, and taxes the eventual sale as a gain if the holding periods are met. Therefore, UK tax arises in one year and US tax in a later year, on income of a different type. Matching the credit across those years is difficult, and practice varies. The cautious approach is to model both outcomes before exercising.

EMI or CSOP options held by a US citizen

For US purposes, EMI and CSOP options are usually non-qualified, unless the plan was also drafted to meet section 422, which is rare. The US therefore taxes the spread at exercise as wages. The UK, meanwhile, charges no income tax at exercise and taxes the growth as a gain on sale. As a result, the US may tax income that the UK has not yet taxed at all.

Reading the comparison

In short, ask two questions for each option. First, does each country tax at exercise or only at sale? Second, is the income wages or a gain in each country? Where both answers match, credits usually work. Where they differ, stock options US and UK tax planning starts with the calendar, not the rate.

Selling the shares later

If you keep the shares after exercise, a second difference appears. Each country sets your base cost from its own treatment. For an EMI share, the US base usually includes the spread already taxed as wages, while the UK base generally does not. Moreover, the US measures the gain in dollars and the UK in pounds. Consequently, one sale can show two quite different gains. Under stock options US and UK tax rules, both base costs should be recorded on the day you exercise, not rebuilt years later.

A worked example: an EMI exit for a US citizen

Consider a typical case. Priya is a US citizen working for a London software start-up. She holds 20,000 EMI options with an exercise price of £1, equal to market value at grant. The company is sold, and she exercises and sells on the same day at £6 a share. This is an illustrative example, not a real client.

The UK side

Because the EMI conditions are met, no income tax or National Insurance is due on exercise. Her gain is £100,000, being £120,000 of proceeds less £20,000 paid. The GOV.UK page on Capital Gains Tax rates says higher rate taxpayers pay 24% on gains from 6 April 2026. As a higher rate taxpayer, Priya pays about £24,000, before any reliefs or allowances.

The US side

For US purposes, Priya's EMI options are non-qualified. The £100,000 spread is compensation in the year of exercise, converted to dollars at the rate on that day. Because she sold at once, there is no further US gain. She claims a foreign tax credit for the UK tax on the same gain. However, the UK tax is a capital gains tax and the US income is wages. The IRS has published no guidance on EMI specifically, so the credit position needs care and clear records.

What Priya's case shows

If US tax on the spread exceeds the UK tax, Priya pays the difference to the IRS. She should pay an estimate by the following 15 April, as interest runs from then even with the two-month extension for people abroad. Her EMI advantage is real in the UK, but much smaller once her stock options US and UK tax position is worked out together.

How is option income split when you move between countries?

Both countries split option income by the working days in each country between grant and vesting. The detailed rules differ, so each needs its own calculation.

The UK rule

For internationally mobile employees, HMRC's guidance at ERSM162565 sets the relevant period for an option. It runs from the grant to the earlier of the taxable event or vesting. The UK taxes the share of the gain linked to UK duties in that period.

The US rule

The US sources option income on a similar time basis. Treasury Regulation 1.861-4 generally uses the period from grant until all conditions for exercise are met. The US-source share follows the days you worked in the US. As a US citizen, you are taxed on the whole spread anyway. The split still matters, however, because it decides how much foreign tax credit you can claim.

Keeping a workday record

Keep a diary of where you worked each day from grant to vest. Payroll records rarely show it, and stock options US and UK tax claims rely on it in both countries. Our article on RSU tax in the US and UK explains the same split for restricted stock units. The same diary serves both awards.

What should you check before exercising?

Check the plan type in each country, the deadlines for any elections and the tax in both returns before you commit. Timing an exercise is your decision; good preparation shows you the cost of each choice.

The plan documents

Confirm whether your option is an ISO, an NSO, an EMI option or a CSOP option. Ask whether the plan also meets the other country's rules. Additionally, confirm the market value at grant, as both countries rely on it.

Election deadlines

Note the 30-day window for a US 83(b) election and the 14-day window for a UK section 431 election. Both start on acquisition, not on the date you remember to ask. Missing either cannot usually be put right. In our experience, missed elections are the most expensive stock options US and UK tax mistake, because they cannot be repaired afterwards.

Both returns, modelled together

Ask for the stock options US and UK tax figures to be modelled in both returns, for each exercise date you are considering. Then compare the total tax and the cash you need, and when. Paying US estimated tax on time avoids interest. We model the tax; we do not recommend whether or when to exercise.

How US UK Tax Experts can help

US UK Tax Experts prepares your US and UK returns together, from one file, including every option exercise and share sale. We work out the income in each country, the workday split and the foreign tax credit, and we model the tax for exercise dates you are considering. Our guide for US UK tax for founders covers the company side, and our treaty relief service covers the credits.

Conclusion

Stock options US and UK tax rules rarely agree. NSOs are taxed at exercise in both countries, which makes credits straightforward. ISOs lose their advantage in the UK, and EMI and CSOP options lose theirs in the US. Watch the 30-day 83(b) and 14-day section 431 deadlines, keep a workday diary and model both returns before each exercise.

Contact us

If you hold options and live in either country, email hello@usuktaxexperts.com or use our contact form. Tell us the plan type, grant dates and where you have worked since grant. We reply within one working day.