US Tax Preparation for American Expats: Trailing Vesting

An American executive who leaves London does not leave the London tax year behind. Share awards granted during the UK years keep vesting afterwards, and both revenue authorities claim a slice of the same money measured on different periods, in different currencies, in different tax years. Reported correctly, the overlap is relieved. Reported carelessly, it is paid twice.

This guide sets out how trailing share vesting is reported and apportioned in a compliance catch-up. It is written for the situation we see most often in US tax preparation for American expats: an award granted while working in the UK, a departure part-way through the vesting period, several vesting events afterwards, and two or three years on either side that were never filed. Jungle Tax prepares the returns and the supporting apportionment schedules; nothing here is a plan for timing a departure, and no employer or scheme is named.

Why does an award granted in London still belong partly to HMRC after you leave?

The instinct of most departing employees is that residence on the vest date decides everything. It does not. The UK taxes employment-related securities income by reference to where the duties that earned it were performed, across the whole period the award relates to. The statutory machinery for internationally mobile employees, introduced with effect from 6 April 2015, splits the securities income into a chargeable UK portion and a portion that is not chargeable, and it does so on a time basis. HMRC's published guidance for this is in the Employment Related Securities Manual, with the apportionment principle set out at ERSM163120.

The practical consequence is blunt. Three years after leaving, on an award that vests in a year in which you set foot in Britain only for a wedding, a slice of that vest is still UK employment income. It is not a penalty and it is not aggressive. It is the same principle the UK applies to a bonus earned over a period straddling arrival, applied in reverse.

Equally, the US never let go. A US citizen is taxed on worldwide income regardless of residence, so the entire vest is on the US return in the year of vesting. The question on the US side is never whether the income is reported. It is only how much of it is foreign source, because that is what governs the credit for the UK tax.

What is the relevant period, and how does HMRC expect it to be apportioned?

Fixing the period

Everything downstream depends on getting the period right, and the period is not the same for every instrument. For a conditional award such as a restricted stock unit, the relevant period generally runs from the award of the security or right to the point at which the forfeiture or restriction condition lifts, which in ordinary commercial terms is the vest date. For an option, the period generally runs from grant to the satisfaction of the employment-related conditions for exercise. HMRC will depart from the default where the facts show the reward genuinely relates to a different stretch of service; a retention award expressed to reward the following eighteen months is not apportioned over a period that began three years earlier simply because the paperwork was signed then.

In a catch-up this is the first thing we reconstruct, award by award, because a single award with four annual tranches is four relevant periods, not one. Each tranche has its own end date, its own workday fraction, its own vest-date value and, very often, its own tax year on each side.

The workday fraction

The apportionment is by workdays, not calendar days. The fraction HMRC expects is:

• Numerator: the number of days in the relevant period on which employment duties were performed in the UK.
• Denominator: the total number of days in the relevant period on which employment duties were performed anywhere.

Weekends, public holidays and annual leave fall out of both figures. Business travel counts to the territory where the duties were actually performed, which is why a role with heavy travel produces a materially different fraction from a desk-bound one covering the same dates. Where duties are performed in more than one territory on the same day, a sensible and consistent convention is needed and must then be applied to every award in the schedule. The statutory standard is that the apportionment be just and reasonable; in practice that means defensible, consistent and evidenced.

The records that defend it

An apportionment is only as good as the evidence sitting behind it, and in a catch-up the evidence is being assembled years after the fact. The file we build for each client normally contains:

• The award agreement, plan rules and vesting schedule, establishing grant date, tranche dates and conditions.
• The vest-date share price and the number of shares delivered, gross of any share withholding.
• Payslips and payroll reports for each vest, showing what fraction was actually put through PAYE and what was withheld.
• A workday log for the whole relevant period, ideally day by day, reconstructed from diaries, expense claims, travel bookings and passport or immigration records.
• Evidence of residence status on each side, including any split-year position in the year of departure.

Reconstructed logs are accepted, but they carry less weight than contemporaneous ones and they take longer to build. If you are reading this while still holding unvested awards, the single most useful thing you can do is start keeping the log now, for the periods still running.

How does the US source the same award?

The US sources compensation from multi-year arrangements on a time basis over the period to which the compensation is attributable. Conceptually that is the same machinery as the UK's. In application it is not the same answer, for several reasons.

The period the US treats as attributable may start or end on a different date from the UK relevant period, particularly for options, where the US focuses on the period to which the compensation relates and the UK's default runs to the lifting of the condition. Day-counting conventions differ on travel days and part days. And the tax years differ: the US calendar year and the UK year to 5 April mean that a single vest can fall in one US year and two UK reporting periods can be relevant to it, or the reverse.

The result is that the foreign-source fraction on Form 1116 is computed on the US basis, in the general category, and is not simply the mirror image of the UK chargeable fraction. That income is general-category earned income, so UK tax paid on it relieves US tax on general-category income, not on unrelated passive income such as dividends or interest.

Issue	UK / HMRC treatment	US / IRS treatment
Trigger for taxation	Where duties were performed over the relevant period	Worldwide income of a US citizen, wherever performed
Amount brought into charge	UK workday fraction of the vest value	Full vest value; apportionment affects source only
Default period	Award to lifting of the restriction or forfeiture condition	Period to which the compensation is attributable, on a time basis
Tax year	6 April to 5 April	1 January to 31 December
Currency of measurement	Sterling, at the vest date	US dollars, translated at the vest date
Collection at vest	PAYE operated by the former employer's payroll	Reported on Form 1040; credit claimed on Form 1116
Relief for the other country	Foreign portion outside the UK charge	Foreign tax credit, general category, with carryover
Subsequent sale	Capital gains, sterling base cost	Capital gain or loss in dollars from vest-date basis

Why is the PAYE operated at vest often on a different fraction than the US credit allows?

This is where most catch-ups go wrong, and it is worth being precise about the mechanism.

At vest, a payroll function is applying withholding to a former employee it no longer sees. It has the vest value and it may or may not have a current apportionment. In the absence of a live direction limiting PAYE to a UK fraction, the safe path for the payroll is to operate PAYE on a large fraction, sometimes on the whole award. Directions permitting a reduced fraction are employer applications made on projections, and projections made before departure rarely survive the actual travel pattern that follows. From 6 April 2025 the application process for these directions moved online, which has changed the administration but not the underlying mismatch.

Meanwhile the US credit is capped by the US tax on the income the US treats as foreign source. Three outcomes follow:

• PAYE fraction exceeds the US foreign-source fraction. Part of the UK tax has nothing to credit against in that year. It is not lost, but it sits in carryover rather than reducing the current bill, and it may have been over-withheld in the UK in the first place.
• PAYE fraction is below the correct UK chargeable fraction. There is a UK underpayment to declare through Self Assessment, and the US credit for the year is understated until the UK liability is settled.
• Timing mismatch only. The fractions agree but the UK tax is paid in a different US tax year, which raises the paid-versus-accrued question on the US return and can strand an otherwise perfectly good credit in the wrong year.

The correct starting point in a catch-up is therefore never the payslip. It is the independently computed apportionment. The payslip tells you what was withheld; it does not tell you what was owed. We then reconcile the two and, where the UK tax was over-withheld, deal with it through the UK return rather than by quietly reducing the US credit claim to match.

The year of departure: two calendars, one departure

The departure year is the hinge, and it is the year most often filed incorrectly. On the UK side, split-year treatment may divide the tax year into a UK part and an overseas part, which changes what is within the charge but does not by itself remove the trailing securities income relating to UK workdays. On the US side, a citizen remains a citizen; there is no split year, and the calendar year runs on unbroken.

Practical consequences that recur in every file:

• A vest falling between 1 January and 5 April sits in the same US year as vests that fall after 6 April but in a different UK year. The FX rates, the workday denominators and the credit years all move.
• The UK payroll may cease at departure while vests continue, so the later vests appear on no UK payroll record at all and must be self-reported.
• Where UK tax is paid after the US return for the year of vest has been filed, the credit position for that year may need revisiting, which is one reason the paid-versus-accrued election matters so much to mobile employees.
• Social security is governed separately by the bilateral agreement and does not follow the income tax apportionment; treating the two as one is a frequent source of error.

Currency, basis and what happens when you sell

The vest and the sale are two different events and should never be collapsed into one line.

At vest, the US includes the market value of the delivered shares in compensation income, translated into dollars at the vest date, and that dollar figure becomes your US basis. Sterling never enters the US computation again. At sale, gain or loss is the dollar proceeds less that dollar basis, so movement in the dollar-sterling rate between vest and sale is embedded in the capital result whether or not the share price moved at all. A holding that is flat in sterling can produce a real dollar gain, or a real dollar loss, purely on currency.

Two refinements matter in practice. First, where sale proceeds sit in a sterling account before conversion, the disposal of that currency balance is itself a transaction with its own dollar consequences under the personal transactions rules, and the ordinary-income treatment of currency gains is distinct from the capital treatment of the shares. Second, ordinary share sales are not section 1256 contracts; that regime applies to certain listed derivative positions, so it generally has no application to a plain vest-and-sell. Where a departing executive has genuinely used listed derivative positions around a concentrated holding, that is a separate analysis and must be identified rather than assumed away.

On the UK side, the base cost is the sterling amount brought into charge as employment income, and the gain is computed in sterling. There is no reason for the two gains to agree, and a catch-up file that shows identical US and UK gains is almost always wrong.

What if several vesting events sit in years never filed on either side?

This is the common case, not the exception. Departure, a house move, a new payroll, a new country and a dormant UK Self Assessment record produce a quiet three or four years in which two or three tranches vest and nothing is filed anywhere. The sequence we use is deliberate.

1. Build one apportionment schedule first

Before a single form is drafted, we build a master schedule covering every award and tranche: grant date, relevant period, UK workdays, total workdays, fraction, vest date, vest value in both currencies, shares withheld, tax withheld. Both returns then draw from the same schedule. This is the single most important control in the engagement, because it is what makes the US and UK positions explicable to each other and to a later enquiry.

2. Settle the UK figure before finalising the US credit

The US credit depends on foreign tax paid or accrued. Finalising a US return on a UK number that later changes guarantees amendments. Where UK returns are outstanding, they are brought up to date or the liability is otherwise quantified and declared before the US credit claim is fixed.

3. Choose the right US catch-up route

Where the failure to file was genuinely non-willful, the streamlined filing compliance procedures are frequently the appropriate route, and trailing equity income is a very common item within a streamlined package. The procedure requires a certification of non-willful conduct and a filing package prepared to its published specification. Eligibility is fact-specific: it turns on conduct, not on the size of the number. We assess it before anything is submitted, and we do not file into a procedure a client does not qualify for. Our detailed treatment of the mechanics sits with our IRS streamlined filing specialists.

4. Sweep the information returns at the same time

Vesting rarely travels alone. Shares delivered into a non-US brokerage account, sale proceeds landing in a UK current account, an employer share plan account left open after departure, and any residual UK pension or investment accounts all carry their own reporting. FBAR and Form 8938 obligations are tested on the accounts, not on the equity, and a catch-up that fixes the income while leaving the account reporting unfiled has fixed half the problem. Where the account population is large, our US-UK tax accountants map it once and carry the map across every year in the package.

5. Reconcile the UK side properly

On the UK side the position may require Self Assessment returns for the years in which trailing income arose, or a correction where PAYE operated on the wrong fraction. Where UK tax was over-withheld because no direction was in place, the route to recovery is the UK return, not a silent reduction of the US credit. Where it was under-withheld, the shortfall is declared. Either way the schedule from step one is the evidence.

Errors we correct most often

• Treating the vest as wholly foreign source for US purposes because the client was living abroad on the vest date, ignoring US workdays earlier in the period and overstating the credit.
• Treating the vest as wholly UK income because PAYE was operated on all of it, and paying UK tax on a fraction that was never UK-chargeable.
• Using calendar days instead of workdays, which shifts the fraction by several percentage points in either direction and cannot be defended.
• Applying one fraction to every tranche of a multi-year award, when each tranche has its own relevant period and its own answer.
• Using the net shares delivered as the income figure. The gross vest value is the income; shares withheld for tax are a payment of tax, not a reduction of the award.
• Using an average annual FX rate where a spot rate at a specific date is required, and doing so inconsistently between the income and the basis.
• Claiming the credit in the wrong year because the UK tax was paid in a later period and the paid-versus-accrued position was never considered.
• Forgetting the carryover. Excess general-category credits from a heavy vest year are carried back one year and forward up to ten, and are frequently the relief that rescues a later year in which the UK fraction has fallen to nothing.

How this is handled in practice

A trailing-vest catch-up is a reconstruction exercise before it is a filing exercise. The technical rules are not the hard part; the hard part is establishing, from years-old records, a workday history and an award history that stand up on both sides of the Atlantic and tell the same story. Done once, properly, it supports every remaining tranche of every award still outstanding, which for most departing executives means the next three to five years of returns are already half-prepared. For readers whose position extends beyond equity into wider cross-border reporting, our private client work for high-net-worth individuals and our library of technical guides cover the adjacent issues.

If share awards granted during your London years are still vesting and the returns on one or both sides are behind, the position is recoverable and it is routine work for us. To review your awards, your workday history and the years outstanding in confidence, contact our cross-border team for a private consultation. We will tell you what the apportionment actually looks like, which catch-up route fits the facts, and what it will take to close the years cleanly on both sides.