Wait three years for a relief you will not keep

Figures relate to tax year 2025-26 (UK) · 2025 (US)

CSOP options let you buy shares at a price fixed years ago, and after three years Britain charges nothing at all when you exercise them. Up to £60,000 of them, free of income tax and free of national insurance.

Then your American return arrives and treats the whole thing as salary.

So the relief your employer carefully arranged is worth very little to you, while the colleague sitting beside you keeps every penny of it.

Key takeaways

• CSOP options are tax advantaged in Britain and ordinary options to the IRS.
• Britain charges nothing at exercise after three years, and America charges income tax.
• The spread at exercise is American compensation income, taxed at ordinary rates.
• Britain taxes the later sale as a capital gain instead.
• Different years and different characters mean the credit often fails.
• Selling in the same tax year as exercise usually helps most.
• National insurance on the gain is not creditable against the American charge.
• The planning has to happen before exercise, not afterwards.

Should you sell CSOP options at once?

It usually does, and it is the single most useful lever available. Exercising and selling in the same British tax year puts the capital gains charge and the American income charge into overlapping periods, which gives the credit something to work with. The planning is crude but effective.

Match the British tax year to the American one as closely as you can.

April and December year ends make perfect alignment impossible.

Can you exercise before three years?

The plan rules may allow it, though doing so forfeits the British relief entirely. For an American that decision looks different, because the IRS charges the same tax either way. So the British relief you are giving up is the only thing actually at stake.

Leavers in good standing sometimes exercise early without losing the relief.

Read the plan rules rather than assuming the general position applies.

Do you owe estimated tax?

Very likely, since a large unwithheld gain in one quarter triggers the underpayment rules. Paying an estimate in the quarter of exercise avoids an interest charge that nobody expects. The sums involved on a sizeable exercise are not trivial.

Our guide to estimated tax payments from the UK covers the mechanics.

Quarterly deadlines fall in April, June, September and January.

How are CSOP options valued?

By the market value on the exercise day for a listed company, which is straightforward. A private company needs a valuation, and the figure HMRC accepts may not be the figure the IRS would accept. Where the two diverge you report each country on its own basis and keep the working.

Our guide to cap tables and US citizens covers private company valuations.

Keep the valuation paperwork from the year of exercise permanently.

Does the exchange rate matter?

A lot, because the American charge is a sterling amount converted into dollars. A weak pound shrinks the reported income and a strong pound inflates it, with no economic change at all. Use the rate on the exercise date rather than an average for the year.

Our guide to exchange rates on a US return covers the conversion rules.

Record the spot rate on the day, because reconstructing it later is tedious.

Will payroll withhold on CSOP options?

British payroll will not, because Britain has nothing to collect. American withholding does not apply either where your employer has no American payroll duty for you. So nothing is withheld anywhere and the whole liability lands with you.

Our guide to shadow payroll arrangements covers employers who do operate both.

Set money aside at exercise rather than at filing time.

Do CSOP options count as income for a mortgage?

Lenders rarely count them, because the money is not yet in your hand. Once you sell, the cash counts as savings rather than income. So a big option holding does little for a loan claim until you turn it into money.

Our guide to UK mortgages and currency gains covers the loan side.

Plan the sale around the buy, not the other way round.

Can you give CSOP options away?

Almost never. Plan rules tie the option to you, and a transfer usually kills it outright. You can give the shares away after you exercise, though that is a gift with its own American rules. The option itself stays yours.

Our guide to US gift tax for expats covers gifts of shares.

Check the plan rules before promising anybody anything.

What if you die holding CSOP options?

Most plans let the estate exercise within a short window, often a year. Britain treats that kindly. The American side still charges tax on the spread, and the executor has to deal with it. So the problem passes to whoever sorts out your affairs.

Our guide to a US executor and a UK estate covers the estate work.

Tell your executor the options exist, because they are easy to miss.

Do CSOP options affect your FBAR?

Unexercised options are not reportable accounts, so they stay off the FBAR. Once you own shares held in a foreign brokerage account, that account becomes reportable on the usual thresholds. The transition from option to shareholding therefore brings a new filing.

Our guide to Form 8938 and FATCA covers the asset reporting.

A nominee account holding your shares usually counts.

Are CSOP options worth having?

Yes, on the whole. You still get shares at a price fixed years ago, and that is where most of the value sits. The British relief is a bonus you happen not to keep. Nobody turns down an award because the tax is untidy.

A cheap option is still a cheap option.

Just budget for a tax bill your colleagues will not have.

What are CSOP options?

Share options under a Company Share Option Plan, which is one of Britain's tax-advantaged employee share schemes. According to GOV.UK guidance on employee share schemes, an option held for at least three years before exercise escapes income tax and national insurance entirely. The employee pays capital gains tax only when the shares are eventually sold.

The individual limit stands at £60,000 of market value at grant.

That ceiling doubled in April 2023 from the long-standing £30,000.

How do CSOP options differ from EMI?

Enterprise management incentives suit small, young companies and carry tighter limits on size and trade. A Company Share Option Plan has no such gross asset test, so larger and listed employers use it. The American treatment of both is identical, which is the point that matters here.

Our guide to EMI options for Americans covers the smaller-company scheme.

The British reliefs differ, while the IRS sees ordinary options either way.

Why does the IRS tax CSOP options at all?

Because American law recognises only its own categories of favoured option, and a British plan does not meet them. The rules for incentive stock options require a plan adopted under American statute, so a Company Share Option Plan sits outside them by construction. That leaves the ordinary option rules.

No election fixes this, and no treaty article rescues it.

The scheme would have to be rewritten entirely, which no employer does for one employee.

What do CSOP options cost in America?

The spread at exercise, meaning the market value of the shares on the day less the price you paid. According to IRS guidance on stock options, that spread is compensation income for a non-statutory option. It goes on your return as employment income, taxed at ordinary rates rather than capital gains rates.

Our guide to UK bonus payments and US tax covers how employment income is treated.

The character is the first half of the problem.

When do CSOP options get taxed in Britain?

Not at grant, and not at exercise if you waited the three years. Britain's charge arrives only when you sell the shares, and then as capital gains tax on the difference between sale proceeds and the exercise price. That is a different tax, in a different year, on a different measure.

Our guide to capital losses for US citizens covers the British gains position.

The timing is the second half of the problem.

Two countries, two moments

Why does the foreign tax credit fail?

Because a credit needs the same income taxed by both countries in broadly the same period. Here America taxes compensation in the exercise year while Britain taxes a capital gain in the sale year, so there is no British tax in the exercise year to credit. The relief exists in theory and vanishes in practice.

Our guide to Form 1116 income baskets covers the basket rules.

Americans in Britain meet this mismatch again and again.

What about national insurance?

Where the three-year condition fails, Britain charges national insurance as well as income tax on the exercise. National insurance is not an income tax for credit purposes, so none of it offsets the American charge. That makes an early exercise a lot worse than it first appears.

Our guide to voluntary national insurance covers the contributions side.

Social security taxes sit outside the credit system by design.

What goes on the British return?

Nothing at exercise where the relief applies, and a capital gains computation when you sell. Your employer reports the scheme to HMRC through an annual return, so the exercise is already known. You simply report the eventual disposal like any other share sale.

Our guide to registering for Self Assessment covers the British return.

A sale above the annual exempt amount brings a filing rule.

What goes on the American return?

The spread as wages in the exercise year, and then a capital gain or loss on the later sale measured from the value you already paid tax on. Getting that basis right is what stops the same growth being taxed twice. People commonly forget the step up and overpay.

Our guide to US-UK share schemes covers the wider position.

Your American basis is the exercise-day value, not the exercise price.

Does the net investment income tax apply?

Not to the exercise, because compensation income sits outside that charge. It can apply to a later gain on the shares once they are an investment in your hands. The two stages therefore attract different surcharges as well as different rates.

Our guide to the net investment income tax covers the surcharge.

No foreign tax credit relieves that particular charge.

What happens to CSOP options if you leave the UK?

Then the gain may be split between countries by reference to where you worked during the vesting period. Britain apportions employment-related securities income on a time basis, and leaving mid-vesting complicates the figures a lot. Keep a record of your working location by month.

Our guide to apportioning UK employment income covers the split.

In our practice this is where the largest errors appear.

What if the shares then fall?

You still owe the American tax on the exercise-day spread, which is the cruellest feature of the whole arrangement. A later fall gives you a capital loss, and capital losses offset capital gains rather than the ordinary income already charged. People have been caught badly by this.

Our guide to wash sale rules for UK investors covers loss planning.

Holding the shares after exercise is a separate investment decision.

A worked example

Take an illustrative case. An American in Manchester exercises after four years, paying £20,000 for shares worth £70,000.

Britain charges nothing, so she assumes the exercise is free.

The IRS treats £50,000 as wages, which at her marginal rate costs several thousand pounds.

Had she sold the same year, the British capital gains tax on the sale would have generated a credit to set against most of it.

Should you hold or sell?

Tax points towards selling, and judgment about the company points wherever it points. The tax argument is strong because selling in the exercise year rescues the credit, while holding strands it. Weigh that against your actual view of the shares rather than treating the tax as decisive.

A partial sale covering the tax is a reasonable middle course.

Never hold shares purely because the paperwork is simpler.

Can your employer help with CSOP options?

It can tell you the valuation, confirm the exercise date and give you the plan rules, all of which you need. It cannot change the American treatment, and HR teams rarely know the problem exists. Ask for the documents and take the tax advice elsewhere.

A written valuation on headed paper is worth asking for.

Do not expect the share plan administrator to understand the American side.

How to plan a CSOP exercise

1. Read the plan rules and confirm your three-year date.
2. Get the exercise-day market value in writing from the company.
3. Model the American charge on the spread before you exercise.
4. Decide whether to sell in the same tax year to rescue the credit.
5. Pay a quarterly estimate in the quarter of exercise.
6. Record the spot exchange rate on the exercise date.
7. Set your American basis at the exercise-day value, not the price paid.
8. Report the eventual sale in both countries on their own measures.

Mistakes and penalties we see with CSOP options

The first is believing the employer's statement that the exercise is tax free, which is true only in Britain.

The second is holding the shares for years and losing the credit permanently.

The third is using the exercise price as the American basis, which taxes the same growth twice.

The fourth is skipping the quarterly estimate and collecting an underpayment charge on top.

How US UK Tax Accountants helps

We model the exercise before it happens, because every useful decision here has to be taken in advance and almost none of it can be fixed afterwards.

Then we run the credit position across both years to see what is actually recoverable. Our US federal returns service covers the filing.

If you hold British share options and an American passport, get in touch before you exercise.

Last reviewed 26 October 2026. This article is general details and not personal tax advice, and share scheme rules vary between plans.