US-to-UK Executive Assignments:
Modeling Tax Equalization and Section 690 PAYE
Sending a highly compensated US executive to London on a £250,000 package triggers 45% Additional Rate income tax and 15% Employer National Insurance Contributions. If the US parent guarantees the executive’s net take-home pay through a tax equalization policy, the gross-up arithmetic produces a total payroll cost that can exceed twice the base salary. A Section 690 HMRC direction, properly applied, can limit PAYE to the UK-duties proportion and avoid over-withholding on globally split compensation. Neither the gross-up nor the Section 690 mechanics appear in a standard payroll system. Both require modeled in advance of the assignment start date.
The global mobility director receives the assignment letter and the compensation package. The executive is moving from New York to London. The headline salary is £250,000. The equity component adds another £80,000 in expected annual vesting. The company’s tax equalization policy guarantees the executive will pay no more tax than they would have paid in the US. Nobody has yet modeled what that policy will actually cost the company in UK payroll terms.
That cost is not trivial. At £250,000 of base salary, the UK income tax bill alone is approximately £94,000. Employer National Insurance Contributions at 15% on earnings above £5,000 add another £36,750. The equity component adds further income tax and NIC at the margin. Under the tax equalization policy, the company absorbs the difference between what the executive would have paid in the US and what they owe in the UK. On a £250,000 package in London versus an equivalent package in New York, that differential can reach £50,000 to £70,000 per year. The gross-up arithmetic that converts the net tax equalization obligation into a UK PAYE-compliant payroll figure is not straightforward, and the Section 690 mechanism that allows PAYE to be limited to the UK-duties fraction of the total remuneration adds a further layer of complexity that most payroll teams have never processed.
This article gives global mobility HR directors, expat CPAs, and corporate counsel the analytical framework to model the full cost of a US-to-UK executive assignment before the offer letter is signed.
Practitioner Note
This article is written for global mobility professionals, expatriate tax practitioners, and corporate counsel advising on UK inbound secondment structures. All rates reflect UK tax year 2025-26 parameters unless otherwise noted. Nothing in this article constitutes tax or legal advice. Always engage a qualified UK tax adviser and UK payroll specialist before establishing shadow payroll or applying for a Section 690 direction.
UK Income Tax Bands and NIC Rates for 2025-26
The UK income tax and National Insurance structure for 2025-26 creates a progressive marginal rate environment that is materially more punishing at senior executive compensation levels than the equivalent US federal tax position. Understanding the rate structure is the starting point for any tax equalization modeling.
Income Tax Bands (England and Wales, 2025-26)
Two structural features of the UK income tax system that matter disproportionately for executive compensation modeling are the Personal Allowance taper and the 60% effective marginal rate trap. The Personal Allowance of £12,570 is reduced by £1 for every £2 of income above £100,000. This means that between £100,000 and £125,140, the effective marginal rate on each additional pound of income is 60%: 40% Higher Rate income tax plus 20% effective recovery of the Personal Allowance through the taper. For a US executive whose total UK package takes them above £100,000, this taper band represents the single most expensive portion of the UK tax structure from an equalization cost perspective.
National Insurance Contributions (2025-26)
The April 2025 employer NIC rate increase, announced in the Autumn 2024 Budget, raised the Employer Class 1 NIC rate from 13.8% to 15%, and reduced the Secondary Threshold from £9,100 to £5,000 per year. These two changes combined produce a significantly higher employer NIC bill than in prior years for any employee earning above the Secondary Threshold.
| NIC Category | Threshold | Rate | Cap | Assignment Implication |
|---|---|---|---|---|
| Employer Class 1 | Above £5,000 per year (Secondary Threshold) | 15% | No upper cap | Applies to entire salary above £5,000; no limit on employer NIC cost |
| Employee Class 1 (lower) | £12,570 to £50,270 per year | 8% | Upper Earnings Limit | Employee NIC on earnings within basic/higher rate band |
| Employee Class 1 (upper) | Above £50,270 per year | 2% | No upper cap | Additional NIC on earnings above UEL at reduced 2% rate |
| Employee Class 1 (below threshold) | Below £12,570 per year | 0% | N/A | No employee NIC below Primary Threshold |
For a US executive on a £250,000 base salary, the employer NIC bill in 2025-26 is £36,750 (15% of £245,000, being the salary above the £5,000 Secondary Threshold). This is an unavoidable cost to the UK employing entity or host entity regardless of the executive’s personal tax position, and it is entirely separate from the income tax that appears on the executive’s payslip. The employer NIC must be included in the total assignment cost model and in any tax equalization policy calculation that purports to represent the true cost to the company.
Tax Equalization: The Policy and the Arithmetic
Tax equalization is a corporate mobility policy that guarantees an internationally mobile employee will pay no more in total taxes than they would have paid had they remained in their home country. From the employee’s perspective, the policy is a fairness mechanism: it prevents the accident of geography from reducing their net compensation simply because the host country has higher tax rates. From the employer’s perspective, the policy is a cost that must be quantified, approved, and managed across the assignment lifecycle.
The Three Components of Tax Equalization
A tax equalization calculation requires three components to be established for each assignment year. First, the hypothetical tax is calculated: this is the amount of income tax and social security contributions the executive would have paid on their total remuneration package had they remained in the US and earned the same total compensation. Second, the actual host country tax is calculated: this is the amount of UK income tax and NIC actually payable on the executive’s total remuneration in the UK. Third, the equalization adjustment is the difference between the actual UK tax and the hypothetical US tax. Where the UK tax exceeds the hypothetical US tax (which is almost always the case for senior executives), the employer pays the difference.
Why UK Exceeds US for Senior Executives
A US executive earning the equivalent of £250,000 in New York faces US federal income tax at a top marginal rate of 37%, plus New York City and State income taxes of approximately 10.9%, for a combined effective rate in the low-to-mid 30s on the full package. The equivalent UK package attracts 45% Additional Rate income tax on earnings above £125,140, a 60% effective marginal rate on earnings between £100,000 and £125,140 due to the Personal Allowance taper, and employee NIC at 2% on high earnings. The combined effective rate on a £250,000 package in the UK typically exceeds 40%, compared with a US equivalent of approximately 32-35% depending on the executive’s specific circumstances. The equalization gap is real, material, and must be modeled before assignment approval.
The Gross-Up Calculation
The gross-up calculation is the arithmetically demanding step that converts the equalization obligation into a PAYE-compliant payroll figure. When the employer pays the tax equalization top-up on behalf of the executive, that payment is itself a taxable benefit in the hands of the executive. The employer must pay tax on the tax payment, and then tax on that tax payment, in a series that converges on a finite gross figure. In practice, the gross-up is solved iteratively or algebraically using the formula:
Gross-Up Calculation: US Executive on £250,000 UK Package (2025-26)
The example above illustrates the multiplier effect of UK tax equalization. A £250,000 base salary produces a total employer cost of approximately £326,445 once the equalization gross-up and the employer NIC are included. That is a 30.6% premium above the headline salary, driven entirely by the UK tax structure. Before April 2025, when the Employer NIC rate was 13.8% and the Secondary Threshold was £9,100, the same calculation would have produced a materially lower total cost. The rate increases that took effect from April 2025 have added approximately £3,000 to £5,000 per year to the employer NIC bill on a senior executive package of this size.
Section 690 HMRC Directions: Limiting PAYE to UK Duties Only
A Section 690 agreement (named after Section 690 of the Income Tax (Earnings and Pensions) Act 2003, ITEPA 2003) is a formal direction from HMRC that allows a UK employer or host entity to apply PAYE only to the proportion of an internationally mobile employee’s earnings that relates to duties performed in the UK. Without a Section 690 direction, the default PAYE position requires PAYE to be operated on the employee’s full contractual salary, even if they spend only a fraction of their working time in the UK.
For a senior executive whose assignment involves split-location working, with a portion of the year spent in the UK and the remainder in the US or other locations, the Section 690 mechanism produces a significant reduction in the UK PAYE withholding on the portion of compensation attributable to non-UK duties. This is particularly important for assignments where the executive retains dual responsibilities and dual reporting lines, spending perhaps 60% of working time in the UK and 40% on non-UK work for which they are compensated by the US parent.
How HMRC Calculates the UK-Duties Fraction
HMRC applies a time-apportionment approach to determine the UK-duties fraction for Section 690 purposes. The standard methodology uses the number of days of UK duties as a proportion of the total number of working days in the relevant period. HMRC has specific guidance on what constitutes a UK duty day versus a non-UK duty day, and the distinction is important because it affects the fraction that feeds into the PAYE withholding calculation.
A UK duty day is any day on which the employee performs any duties in the UK, regardless of how much time is spent on UK work on that day. A business trip to the UK for a one-hour meeting counts as a UK duty day under HMRC’s interpretation. Days spent entirely outside the UK on non-UK duties, including annual leave and travel days where no UK duties are performed, are typically counted as non-UK days. This interpretation can produce a UK-duties fraction that is higher than the executive’s subjective sense of their workload allocation, particularly for executives who frequently travel to London for brief visits as part of a primarily US-based role.
The Partial Day UK Visit Trap
HMRC’s position that any day with any UK work counts as a UK duty day creates a significant risk for executives on split assignments who make frequent short visits to the UK. An executive who travels to London 40 times per year for half-day or full-day meetings, while spending the remainder of the year in New York, accumulates 40 UK duty days. If their total working year is 230 days, their UK-duties fraction is 17.4%. Without a Section 690 direction, the full salary would be subject to UK PAYE throughout the year. With a correctly calculated Section 690 direction, only 17.4% of the salary is subject to UK PAYE. The difference on a £250,000 total compensation package is approximately £45,000 of UK PAYE withholding that either correctly applies or does not apply, depending entirely on whether the Section 690 has been applied for and granted.
The Application Process for Section 690 Directions
Section 690 directions must be applied for in advance using HMRC’s Section 690 application process. The application requires the employer to provide details of the employee’s assignment structure, the anticipated split of working time between the UK and other locations, the basis for calculating the UK-duties fraction, and confirmation of the employer’s PAYE registration details. HMRC reviews the application and issues a direction specifying the percentage of the employee’s income that should be subject to PAYE. The direction is typically granted for a tax year at a time and must be renewed annually.
The application cannot be made retroactively to cover periods already elapsed. If an employer operates full PAYE on an employee for the first six months of an assignment before applying for a Section 690 direction, the PAYE already withheld for those months cannot be reduced by the direction; the employee would need to claim the overpayment through their self-assessment tax return at year end. This creates a cash flow disadvantage for the executive that many global mobility teams seek to avoid by applying for the Section 690 direction before or immediately at the assignment start date.
UK Shadow Payroll: When It Is Required and How It Works
A shadow payroll is a UK payroll calculation run by a UK entity for an employee who remains on the home country payroll and continues to receive their salary from the home employer, but who performs work in the UK and thereby creates UK tax and NIC obligations. The shadow payroll does not replace the home country payroll; the employee is still paid by the US parent. Instead, it runs in parallel to calculate the UK PAYE and NIC due on the UK-duties portion of the employee’s compensation, remit those amounts to HMRC, and provide the employee with a UK payslip for tax filing purposes.
When Shadow Payroll Is Triggered
The UK shadow payroll obligation is triggered when a non-UK employer has an employee who performs UK duties for long enough to create a UK employment tax liability. The most common scenarios are formal secondment arrangements where the executive remains on the US parent’s payroll while physically working in the UK, and project assignments where a US-based employee travels to the UK for extended periods. The UK trigger for PAYE obligations is generally reached once an employee spends more than 183 days in the UK within a 12-month period (creating UK tax residency), or at an earlier point if the employer has a presence in the UK that can be characterized as the employee’s employer for UK purposes.
Many global mobility teams are not aware of the shadow payroll obligation until HMRC correspondence arrives. HMRC’s use of real-time information from airline records, border agency data, and UK bank transactions means that extended UK working patterns are often identified by HMRC before the employer has established a compliant payroll arrangement. The consequences of late shadow payroll establishment include PAYE penalties, interest on late payments, and potential reputational risk with HMRC that can affect future clearance applications including Section 690 directions.
Shadow Payroll Mechanics: The Employer NIC on Top-Up Payments
The shadow payroll calculation for a tax-equalized executive is particularly complex because the UK PAYE and NIC calculation must account for the equalization payment itself. When the employer makes a tax equalization top-up payment, that payment is earnings for UK PAYE and NIC purposes. The shadow payroll must therefore include the top-up in the PAYE calculation, which produces additional employer NIC on the equalization payment, which in turn increases the equalization obligation, creating a recursive calculation that requires iterative resolution.
In practice, UK payroll software handles this iteration automatically once configured correctly. The complexity arises in the setup phase, where the payroll team must identify all components of the executive’s compensation package that constitute earnings for UK PAYE purposes, determine which components are subject to shadow payroll versus home country payroll, and ensure that the Section 690 fraction is applied consistently across all components rather than only to the base salary.
Dual-Contract Structures: Splitting Compensation Between UK and Non-UK Duties
A dual-contract structure is a remuneration arrangement in which an internationally mobile executive holds two separate employment contracts: one with the UK entity for the duties they perform in the UK, and one with the home country entity for the duties they perform outside the UK. The compensation is allocated between the two contracts in proportion to the split of duties, and the two income streams are taxed separately in their respective jurisdictions.
The potential advantage of a dual-contract structure for a UK-resident executive is that the non-UK contract income may be taxable only in the source jurisdiction (the US) rather than also in the UK, depending on the interaction of the US-UK tax treaty and the executive’s UK residency status and domicile position. For non-UK domiciled executives who are eligible for the UK remittance basis of taxation, unremitted foreign-source income may avoid UK income tax entirely.
HMRC’s Scrutiny of Dual-Contract Arrangements
HMRC applies intense scrutiny to dual-contract arrangements and has a well-developed set of anti-avoidance principles targeting structures that it regards as artificial allocations of compensation. The HMRC Employment Income Manual guidance on split contracts confirms that HMRC will seek to re-characterize arrangements where the allocation between UK and non-UK contracts does not reflect a genuine economic distinction between UK and non-UK work. A dual-contract structure where the non-UK contract compensation bears no relationship to the market value of the non-UK duties, or where the executive’s primary economic contribution clearly relates to the UK entity, is vulnerable to challenge on the basis that the non-UK contract is a device for reducing UK tax rather than a genuine second employment.
The conditions for a defensible dual-contract structure require the two contracts to be for genuinely distinct and economically independent sets of duties, the compensation allocation to reflect arm’s-length values for each set of duties, the non-UK contract to be with an entity that actually benefits from the non-UK services, and the structure to have been established with a genuine commercial rationale independent of the tax benefit. Global mobility counsel advising on dual-contract structures should obtain a written opinion on the structure’s defensibility under HMRC’s published guidance before implementation.
The US-UK Tax Treaty: Double Taxation and the Foreign Tax Credit
The Convention Between the United States and the United Kingdom for the Avoidance of Double Taxation, which has been in force in its current form since 2003, governs the allocation of taxing rights over income earned by US citizens and UK residents in each jurisdiction. For the typical US-to-UK executive assignment, the treaty interaction produces the following outcome: the UK has primary taxing rights over employment income for duties performed in the UK, and the US taxes the same income but allows a credit for UK taxes paid against the US federal income tax liability.
Because UK income tax rates at senior executive levels (40% to 60% effective marginal rate) typically exceed US federal rates (37% top marginal rate) on the same income, the foreign tax credit on IRS Form 1116 usually fully offsets the US federal income tax liability on UK-source earnings, leaving no residual US federal tax on the UK assignment income. However, several factors complicate this straightforward analysis for senior executives.
The Limitation on the Foreign Tax Credit
The IRC Section 904 limitation on the foreign tax credit calculates the allowable credit as US tax before the FTC multiplied by foreign source income divided by total income. For an executive whose total income includes US-source income (retained US portfolio dividends, US real estate income, US-based investment returns) as well as UK-source employment income, the limitation may cap the FTC at an amount below the total UK tax paid. The excess credit carries back one year and forward ten years, but in the year of the assignment the effective combined tax burden may exceed the UK rate alone.
Additionally, the employer-paid equalization payment is US-source income for US federal tax purposes (it is paid by the US parent), even though it relates to UK tax. This income sourcing can reduce the UK-source income fraction in the FTC limitation calculation, potentially limiting the credit and creating unexpected residual US tax liability on what appeared to be a fully treaty-protected UK compensation stream.
State Income Tax Exposure During UK Assignments
US state income taxes represent a frequently overlooked component of the executive’s combined tax burden during a UK assignment. Many US states tax their residents on worldwide income and do not recognize the UK tax treaty as eliminating state income tax obligations. A New York-domiciled executive who retains their New York residential nexus during a UK assignment continues to owe New York State and City income taxes on their UK earnings, on top of the UK income tax. The US-UK treaty does not bind US state tax authorities. The executive’s total tax burden on UK assignment income, combining UK income tax, US federal income tax (after FTC, potentially zero but not always), and New York State and City income tax, can produce an effective all-in rate above 50% before the equalization adjustment.
Total Assignment Cost Modeling: The Four-Component Framework
The complete cost of a US-to-UK executive assignment cannot be accurately represented by the base salary alone, or even by the base salary plus the employer NIC. For a tax-equalized assignment, the total employer cost includes four distinct components that must all be modeled before the assignment is approved.
| Cost Component | Example (£250K package) | Nature | Visibility in Standard Payroll |
|---|---|---|---|
| Base salary (gross) | £250,000 | Direct payroll cost | Fully visible |
| Employer NIC (15% above £5K threshold) | ~£36,750 | Statutory employer cost; no upper limit | Visible in payroll |
| Tax equalization gross-up payment | ~£34,518 | Policy cost; iterative gross-up required | Requires separate modeling |
| Employer NIC on equalization payment (15%) | ~£5,178 | NIC on the equalization gross-up itself | Requires shadow payroll modeling |
| Assignment allowances (housing, school fees, repatriation) | £30,000 to £80,000 (variable) | Benefits-in-kind; may trigger P11D and NIC | Requires benefits-in-kind analysis |
| US state tax gross-up (if applicable) | Variable | State income tax differential above hypothetical | Not visible in UK payroll; requires home CPA |
The two components that are most consistently undermodeled are the employer NIC on the equalization gross-up payment and the US state tax component of the hypothetical tax calculation. The employer NIC on the equalization payment is automatically generated once the equalization gross-up is added to the UK payroll, but it increases the total employer NIC bill above what most assignment cost models include. The US state tax component depends on the executive’s pre-assignment state of residency and whether they sever that residency during the UK assignment, which is a fact-specific analysis that the US home CPA must complete independently of the UK shadow payroll team.
Year of Arrival: Split-Year Treatment and the Partial-Year PAYE Position
For a US executive arriving in the UK part-way through a UK tax year (which runs from 6 April to 5 April the following year), the split-year provisions in Schedule 45 of the Finance Act 2013 may allow the individual to be treated as UK resident only from their date of UK arrival. This split-year treatment limits the UK tax liability to income arising from the UK arrival date onwards, rather than from 6 April at the start of the tax year.
For PAYE purposes, the arriving employee typically registers with HMRC and begins operating PAYE from their UK start date, with the UK income tax calculation using a code that reflects the partial-year position. The employer’s shadow payroll or UK payroll should reflect the split-year arrival date in the annual equivalent tax calculation to avoid over-withholding PAYE in the first months of the assignment.
The split-year analysis requires confirmation that the executive meets one of the qualifying cases in Schedule 45. The most relevant case for a US executive beginning a UK assignment is Case 4 (starting to have a home in the UK only) or Case 8 (accompanying or joining a partner). The global mobility team should ensure that the executive’s UK tax adviser completes the split-year analysis in the year of arrival rather than treating the full year as a UK-resident year by default, which would produce an over-collection of PAYE in the earlier months of the UK tax year.
Equity Compensation and UK PAYE: RSUs, Stock Options, and the UK Treatment
Equity compensation components of the executive’s total remuneration package require separate analysis for UK PAYE and NIC purposes and are the area where the most significant discrepancies between the modeled assignment cost and the actual UK payroll cost arise in practice.
Restricted Stock Units (RSUs)
RSU vesting events during a UK assignment are employment income for UK PAYE and NIC purposes in the tax year of vesting. The UK employer or host entity is required to operate PAYE on the market value of the RSUs at the vesting date, and both employee and employer NIC apply to the vesting income. For a US executive whose RSU grants were made while employed in the US, HMRC requires a time-apportionment of the RSU income: only the proportion of the RSU value attributable to UK service (from grant to vest) is subject to UK PAYE. The remainder is attributed to non-UK service and is subject only to US tax treatment.
The apportionment calculation requires knowing the grant date, the vesting date, and the total days of service during the vesting period broken down by UK and non-UK days. Where the executive has been on UK assignment for the entire vesting period, 100% of the RSU value at vest is UK-sourced. Where the assignment began partway through the vesting period, the UK fraction is calculated on a time-apportionment basis.
Stock Options
Unapproved share options (the most common type for executive-level grants) are also subject to UK PAYE at the point of exercise, on the spread between the exercise price and the market value at exercise. The same UK-service apportionment analysis applies as for RSUs. HMRC has specific guidance on internationally mobile employees and share plan income that the UK payroll team must apply to each equity event during the assignment period.
Equity Vesting Alert for Tax Equalization
Tax equalization policies typically apply to equity compensation as well as base salary, meaning the employer absorbs the UK income tax and NIC on RSU and option events that exceed the hypothetical US tax on the same equity income. For an executive with a large unvested equity tranche whose RSUs vest during the UK assignment at a time of high share price, a single equity vesting event can generate a UK PAYE liability that exceeds the annual base salary and triggers a very large equalization payment. The equity vesting schedule must be modeled as part of the assignment cost analysis for any executive with significant unvested equity, and the equalization policy wording should be reviewed to confirm exactly how equity income is treated under the policy.
Global Mobility Pre-Assignment Tax Modeling Checklist
The following checklist consolidates the key analytical steps that global mobility HR directors and their tax advisors should complete before an assignment letter is issued for a US-to-UK executive secondment.
Frequently Asked Questions: UK Tax Equalization and Section 690
What is a Section 690 agreement with HMRC?
A Section 690 agreement is an arrangement with HMRC that allows a UK employer or host entity to apply PAYE only to the proportion of an internationally mobile employee’s earnings that relates to UK duties. Without the direction, PAYE must be operated on the full salary regardless of time spent outside the UK. The application must be submitted to HMRC in advance and is typically renewed annually. Full details of the application process are available at the HMRC Section 690 application guidance.
What is a UK shadow payroll and when is it required?
A UK shadow payroll is a PAYE calculation run by a UK entity for an employee who remains on the home country payroll but performs work in the UK. The shadow payroll does not pay the employee but calculates the UK PAYE and NIC due, remits those amounts to HMRC, and provides a UK payslip. Shadow payrolls are required when a non-UK employer sends an employee to work in the UK and wishes to maintain the employee on the home country payroll rather than transferring them to a UK payroll. The obligation arises from the date UK employment duties begin, not from a later date when the employer becomes aware of the requirement.
What is tax equalization and how does the gross-up calculation work?
Tax equalization is a policy under which the employer guarantees the executive will pay no more tax than they would have paid in their home country. The gross-up calculation determines the total payroll cost by solving for the gross salary that produces the required net after all UK taxes. Because the equalization payment is itself taxable, the calculation must be performed iteratively or using the formula: Gross-up = Net target divided by (1 minus the applicable marginal rate). The final gross-up figure must also account for employer NIC on the equalization payment, which further increases the total assignment cost.
What are the UK National Insurance Contribution rates for 2025-26?
For 2025-26, Employer Class 1 NICs are charged at 15% on earnings above £5,000 per year (the Secondary Threshold, reduced from £9,100 following the Autumn 2024 Budget). Employee Class 1 NICs are charged at 8% on earnings between £12,570 and £50,270 per year, and at 2% on earnings above £50,270 per year. There is no upper earnings limit on employer NICs; the 15% rate applies to all earnings above £5,000 without cap, making the employer NIC on senior executive packages significantly higher than in prior years.
Can a US citizen avoid UK PAYE on income earned outside the UK during a UK assignment?
A US citizen who is UK tax resident is generally taxable in the UK on their worldwide income. However, a Section 690 HMRC direction can reduce PAYE withholding to the UK-duties proportion only, meaning PAYE is applied only to the fraction of total compensation attributable to UK working days. A dual-contract arrangement may allow part of the package to be characterized as non-UK source income, but HMRC scrutinizes these arrangements carefully. The conditions for a genuine dual-contract structure are demanding and the arrangement must be established with genuine commercial substance independent of the tax benefit.
How does the US-UK tax treaty affect double taxation for US citizens on UK assignments?
The US-UK tax treaty allows US citizens who are UK residents to claim UK income tax as a foreign tax credit on IRS Form 1116 against their US federal income tax liability. Because UK income tax rates at senior executive levels typically exceed US federal rates, the credit usually fully offsets the US federal liability on UK-source employment income. However, the IRC Section 904 limitation may cap the credit where the executive has substantial US-source passive income, and US state income taxes are not covered by the treaty and continue to apply to worldwide income for executives who retain their US state domicile during the assignment.
Key Takeaways for Global Mobility Directors and Expat CPAs
The total cost of a US-to-UK executive assignment under a tax equalization policy is not a linear function of the base salary. It is the output of a multi-variable model that includes the UK income tax at progressive rates up to 45%, the Employer NIC at 15% with no cap from a £5,000 threshold, the iterative gross-up calculation on the equalization payment, the employer NIC on the equalization gross-up itself, and the equity vesting analysis that may produce large one-year spikes in the payroll cost in years with significant RSU vestings.
The three analytical failures that generate the largest surprises in assignment cost management are: first, modeling the assignment cost using the headline salary and a generic tax rate without running the iterative gross-up against the actual 2025-26 rate structure; second, failing to apply for a Section 690 direction before the assignment starts, which causes full PAYE to be withheld on the entire salary for months before the direction is granted; and third, omitting the equity vesting analysis from the equalization cost model, which converts what appeared to be a manageable annual equalization cost into a very large single-year liability when a major tranche vests.
Global mobility directors who build the full four-component assignment cost model into their pre-approval process, coordinate the shadow payroll and Section 690 application before day one of the assignment, and include the equity vesting schedule in the equalization cost projection will consistently produce more accurate assignment budgets and fewer post-assignment cost surprises.
