Tax Equalisation for Americans on UK Assignment: What the Policy Promises, and Where Executives Still Get Caught

Tax equalisation is a promise about your net pay, not about your tax. Under a standard policy you bear a hypothetical home-country tax, deducted from your pay as though you had never left, while your employer bears the actual US and UK tax on the compensation the policy covers. The exposure sits in that last clause: equalisation is a commercial arrangement, and the income where a senior executive's real money lives — investments, property, gains, a spouse's earnings, pre-assignment equity — is usually outside it.

Key takeaways

• Equalisation is contractual, not statutory: no minimum standard, no regulator, and terms that vary by employer.
• Equalisation leaves you neither better nor worse off than staying home; tax protection lets you keep the benefit when the host country proves cheaper.
• Hypothetical tax is a payroll deduction your employer retains, not a remittance to HMRC or the IRS.
• Employer-paid tax is itself taxable pay in both countries, so the calculation must be grossed up — which is why it is circular and your reported income exceeds what you received.
• The year-end true-up routinely lands a year or more late, because the two tax years do not align.
• What the policy excludes — investments, rents, gains, a spouse's earnings, pre-assignment equity — is where the real exposure sits.

What the policy actually promises

HMRC's description is a useful anchor because it is deliberately narrow. Its PAYE guidance describes tax equalisation as an arrangement under which the employee is entitled to specified net cash earnings and non-cash benefits and the employer undertakes to meet the UK income tax arising on them. The Employment Income Manual puts it more plainly: the employer meets on the employee's behalf any additional tax payable above the tax the employee would have paid in his home country.

Two things follow. The promise is framed around specified earnings and benefits — not around you, and not around your household. And nothing obliges an employer to equalise anything: HMRC's manuals describe how these arrangements are administered, not what one must contain.

Equalisation versus tax protection

The two are routinely confused, often by people living under one of them.

Tax equalisation holds you to the home-country outcome in both directions. You bear a hypothetical home tax; the employer bears the actual combined burden. If the assignment proves expensive, the employer absorbs it. If it proves cheap, the employer keeps the saving. You are made whole, never enriched.

Tax protection puts a floor under you but no ceiling over you. You pay the real taxes yourself, and the employer reimburses you only to the extent your actual worldwide tax exceeds what you would have paid at home. If the assignment is cheaper than staying put, you keep the difference.

Protection is more generous and less predictable, which is why large mobility programmes standardise on equalisation. Whether the distinction is worth anything on a given US-to-UK move depends on your income mix and your US state position. A third pattern is often mislabelled as equalisation: the employer pays for tax return preparation and nothing else — common on short postings, and covered in our guide to secondments.

Hypothetical tax: why your payslip stops telling the truth

Hypothetical tax — "hypo tax" — is not a tax. It is a payroll deduction. Your employer estimates what you would have paid at home on your equalised pay, usually stripping out assignment allowances such as housing and relocation, and withholds that from your gross. It reaches neither the IRS nor HMRC: your employer retains it to fund the real bill.

From that moment your payslip stops describing your tax position, for four reasons.

• The hypo figure is a policy output, not a return. It reflects the policy's assumptions about your filing status, family and stock income, which may not match your actual US return.
• Real UK PAYE runs on a parallel, estimated basis. Under the modified PAYE arrangement in EP Appendix 6, the employer prepares a grossed-up best estimate of earnings and benefits at the start of each year and reviews it between December and April. HMRC has also removed the obligation to make payments on account for employees in such a scheme.
• Your UK return reports grossed-up figures. HMRC's tax equalisation helpsheet directs equalised employees to report cash earnings, the cash equivalent of non-cash benefits on a grossed-up basis, and the grossed-up tax itself.
• Your US return ignores all of it. As a US citizen you remain subject to tax on worldwide income from all sources, on the calendar year, whatever you have agreed privately with your employer.

You can therefore hold a payslip, a UK return and a US return showing three different numbers, none of which is your out-of-pocket cost. That cost is the hypo tax plus everything the policy does not cover.

Gross-ups, and why the calculations spiral

When your employer pays tax on your behalf, that payment is itself taxable compensation. HMRC treats equalisation payments made on behalf of employees as part of their earnings, and for National Insurance the tax met by the employer is a profit derived from the employment. The IRS applies the same logic: Publication 15-A states that if you pay your employee's share of social security and Medicare taxes, the amount you pay is wages and must be included in box 1 of Form W-2.

So the employer pays the tax on your pay; that payment is pay; there is tax on that; and so on. The series converges, but two consequences survive. Your reported compensation can be far higher than anything that reached your account, which matters for mortgage and visa applications. And the true cost to your employer of placing you in the UK sits well above the headline tax — worth knowing when you negotiate.

The true-up: what happens after the year end

The settlement compares hypothetical tax withheld during the year against what the policy says you should ultimately have borne, and either you owe your employer or your employer owes you. It is slow for structural reasons.

• The tax years do not align. The UK tax year runs 6 April to 5 April; the US year is the calendar year. Every UK year straddles two US years, so the settlement waits on US returns that may not yet exist.
• The UK liability is not final until the return is filed. Under the Appendix 6 terms, additional tax due is paid following submission of the employee's tax return by 31 January after the end of the tax year, or a repayment claim is submitted.
• Credits cannot be computed until the foreign tax is known, and US extensions lengthen the chain.

A settlement landing a year or more late is normal. Ask at the outset for the expected timetable and the dispute window: many policies allow very little time to challenge a statement before it is treated as accepted.

What equalisation does not cover

This is the section worth re-reading. A typical policy equalises assignment-related employment income. Outside that boundary you are exposed in two tax systems at once, with no employer cover and usually no adviser. Terms vary, but the following are commonly excluded.

• Personal investment income. UK residence brings your worldwide income into UK charge — HMRC states that residents normally pay UK tax on all their income, whether from the UK or abroad — while US citizenship keeps you in the US net regardless. Pooled investment vehicles are treated very differently in the two systems, and a portfolio built for one rarely behaves well in the other.
• Rental income. The home you kept and let out. Both countries tax the profit, but depreciation, interest relief and allowable expenses do not match, so the taxable figure differs in each.
• Capital gains on personal assets. Whether a disposal of shares, a second property or a business interest falls inside or outside a UK-resident period can change the answer materially, and nobody on the employer's side is advising you on the timing.
• Your spouse's income. Almost never equalised, and almost always forgotten. A spouse who takes UK employment, works remotely for a home-country employer or runs a business creates UK and US filing positions of their own.
• Equity granted before the assignment. Usually the largest single item. Such awards generally have to be apportioned between UK and non-UK workdays across the vesting period, and policies commonly cover only the slice treated as assignment-related — while the entire award remains taxable to you in the US. Our guide to equity compensation sets out the mechanics.

The shape is clear: the employer insures the part of your finances the employer created. It does not insure you.

Whose foreign tax credits are they?

Your US return claims a foreign tax credit for UK tax your employer economically bore. Whose relief is it? Legally, it sits on your return. The IRS sets out four tests for a creditable foreign tax: it must be imposed on you, you must have paid or accrued it, it must be a legal and actual foreign tax liability, and it must be an income tax or a tax in lieu of one. UK tax on your employment income is imposed on you — your employer discharges your liability, not its own — which is why the credit appears in your name. Commercially, the policy claims that benefit straight back: the settlement nets off credits the employer funded.

The contested ground is excess credits. The IRS permits unused foreign tax to be carried back one year and carried forward ten, so an assignment can leave you with a credit pool you might only use years later, after repatriation and against unrelated income. Whether those carryovers are yours or the employer's is a policy question, and a striking number of policies are silent on it. Raise it before you sign, not at settlement; we cover the mechanics in our article on foreign tax credit carryovers.

One related trap is not negotiable: the IRS allows neither a credit nor a deduction for foreign taxes paid on income excluded under the foreign earned income exclusion or the foreign housing exclusion. Where a hypothetical calculation assumes both the exclusion and a full credit, the arithmetic is wrong — and wrong in the employer's favour.

Repatriation, and leaving mid-assignment

Repatriation. Most policies equalise through the assignment plus a trailing period for income relating to assignment services, typically equity vesting after you return. That cover is narrower and shorter than people assume, and your final UK year is likely to be a split year while your US year is a full one, so filings continue after you have left.

Leaving the employer. Resign or be let go mid-assignment and the position can turn sharply against you. Policies commonly provide that settlement balances remain payable after termination, that benefits are clawed back if you leave within a defined period, and that tax preparation support is withdrawn precisely when your returns become most complex. If you are negotiating an exit, the settlement belongs on the table.

What to do about it

1. Obtain the policy document, not the summary. Ask for the hypothetical tax methodology, the definition of covered compensation, the treatment of credits and carryovers, the trailing liability rules and the termination provisions.

2. Model the uncovered side independently. The employer's provider is scoped to the covered income — precisely the scope that excludes your real exposure. A second adviser, engaged by you, is the point.

3. Sequence before you move. Disposals, option exercises and account restructuring almost all have a better and a worse side of the arrival date.

How we work with equalised executives

We are usually engaged as the executive's own adviser, alongside the employer's provider rather than instead of it: reading the policy, testing the hypothetical calculation and the gross-up, and owning what it leaves out — the portfolio, the property, the spouse, the pre-assignment equity. A licensed CPA or Enrolled Agent reviews and signs off every US filing, and the UK side is reviewed by an ACCA-qualified accountant.

Our US-UK expat tax service sets out the engagement. If you are weighing an assignment offer, or already mid-assignment and uneasy about what sits outside the policy, book a confidential consultation.

This article is general information, not tax or legal advice, and does not create a professional relationship. Equalisation and protection policies are commercial documents that vary between employers; your own policy governs your position. US and UK rules change, and treatment depends on your facts. Confirm the current position with a licensed professional before acting.

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Reviewed by a CPA / Enrolled Agent. Last updated: 16 September 2026.

Official sources: HMRC PAYE82002 | HMRC EIM77040 | HMRC NIM02380 | HMRC helpsheet HS212 | HMRC SAM121620 | GOV.UK UK residence | IRS Topic 856 | IRS foreign tax credit | IRS citizens abroad | IRS Publication 15-A