Two payrolls, one salary, and nobody gets paid twice

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

You send an employee from London to Chicago for two years. They stay on the British payroll, they keep their pension, and their salary lands in the same account it always did.

America still expects tax on the work done there. A shadow payroll is how an employer reports that pay in the second country without paying the employee twice. Nothing changes for the employee. Quite a lot changes for the finance team.

Key takeaways

• It reports earnings in a second country without paying them again.
• The employee keeps one payslip and one bank transfer.
• It exists so withholding lands where the work physically happened.
• Both directions of assignment can need one.
• Equity awards and bonuses cause most of the complications.
• Set it up before the assignment starts, not after the first filing.

What is a shadow payroll?

A shadow payroll is a second payroll run in the host country that mirrors the pay delivered in the home country. No money leaves through it. Its job is to calculate and remit the host country withholding and to produce the year end reporting that country expects.

The employee sees nothing extra and receives nothing extra.

So it is a compliance mechanism rather than a payment mechanism.

When does an assignment need one?

Once the host country gains the right to tax the employment income and expects withholding at source. According to HMRC guidance, that turns on where the duties are performed and who ultimately bears the employment cost. Day counts and who pays are the two main tests.

A two week trip rarely triggers anything.

A two year posting almost always does.

Why not just move them onto local payroll?

Sometimes that is the right answer. But moving someone onto a local payroll usually breaks their home pension, their social security record and their benefits, and those are often the things an assignee cares about most.

A shadow payroll keeps the home arrangements intact.

It buys continuity at the cost of administration.

A worked example

The figures below are illustrative and use round numbers to show the mechanics.

Helen moves from London to Chicago on a two year assignment, earning £90,000. She stays on the British payroll and keeps paying into her British pension.

Her employer sets up an American shadow payroll reporting the same earnings in dollars. It withholds American federal and state tax and remits it, while the British payroll reduces its own deductions so she is not taxed twice in cash terms.

Helen still receives one payment each month. Two countries now see her pay, and each collects the share it is entitled to.

How do the two payrolls stay in step?

Through a monthly reconciliation. The home payroll sends the gross figures, the host payroll converts them and calculates the host tax, and the home side adjusts its deductions so the net payment stays right. Currency conversion happens on an agreed and documented basis.

Most of the errors we see come from a broken month rather than a broken design.

Agree the process in writing before the first run.

What the arrangement looks like

What happens to social security?

Per IRS guidance on totalisation, a certificate of coverage keeps the employee in their home system for a set period, so contributions continue at home and stop in the host country. That certificate has to be applied for, and it is the single cheapest thing on this list to get right.

Without it, both countries can charge.

Our guide to the certificate of coverage explains how to obtain one.

Do bonuses and share awards complicate it?

They cause most of the difficulty. A bonus earned partly before the assignment and paid during it has to be split between the two countries, and share awards vesting mid-assignment need apportioning across the whole vesting period rather than the payment date.

Payroll software rarely handles that split without help.

Our guide to US-UK share schemes covers the equity side.

Who carries the tax cost?

That depends on the assignment policy rather than the tax rules. Many employers equalise, meaning the employee pays roughly what they would have paid at home and the company absorbs the difference. Others leave the employee to bear the actual host country cost.

Equalisation is kinder to the employee and heavier on the finance team.

Whichever you choose, write it down before the employee boards the plane.

Does it work in both directions?

Yes. An American company sending somebody to London faces the mirror image, with a British arrangement reporting the pay and remitting British deductions. The British side also brings employer National Insurance into the picture, which is a real cost rather than a pass-through.

Budget for that employer cost separately.

It regularly surprises American finance teams.

What about expenses and relocation costs?

Both countries treat them differently, and some items are taxable in one and exempt in the other. Housing, school fees and home leave flights are the usual points of difference on a long assignment.

Each item needs a decision before the first payroll run.

Retrofitting them later means amending returns in two countries.

Does the employee keep their home pension?

Usually, and it is often the main reason for keeping them on the home payroll. A British employee on assignment can generally continue contributing to a British scheme, which protects both the pot and the tax relief attached to it.

The host country may not recognise those contributions.

Americans face a particular version of this. Our guide to employer pension contributions covers it.

How long can an assignment run?

As long as the business needs, but the tax position hardens over time. Longer assignments tend to pull the employee fully into the host system, and a coverage certificate lasts only a set number of years before it has to be revisited.

Five years is a common natural review point.

Beyond that, a local hire often makes more sense than a continuing assignment.

What records should the employer keep?

Assignment letters, the policy on who bears the tax, the monthly reconciliations, and the currency basis used. Keep the coverage certificate and the host registration paperwork in the same place.

Auditors ask for the reconciliation first.

A clean monthly file makes a year end enquiry a short conversation.

How to set one up

1. Confirm the assignment length and where the work will physically happen.
2. Decide whether the employee stays on the home payroll.
3. Register the employer with the host tax authority and obtain the references.
4. Apply for a certificate of coverage for social security.
5. Agree the currency conversion basis and the monthly reconciliation process.
6. Set the assignment policy on who bears the host tax cost.
7. Run a parallel test month before the first live cycle.

What does it cost to run?

More than a normal payroll and less than getting it wrong. Expect a setup fee, a monthly cost per assignee, and personal tax return preparation in both countries at year end. Small programmes often cost more per head than large ones.

Build the figure into the assignment budget rather than the payroll budget.

Ask providers for a per assignee monthly figure and a separate setup quote. Bundled pricing hides which part scales.

Finance teams that skip this line item find it anyway, three months late.

Can you run it in house?

Rarely, and not for a first assignment. Host country registration, local filing formats and the year end forms all need local knowledge, and an in-house team learning on one assignee usually costs more in time than an external provider charges.

Larger programmes sometimes bring it in house later.

Start with help and decide again at ten assignees.

What if the assignment ends early?

Close it properly rather than simply stopping. The host registration needs a final return and a formal deregistration, and a certificate of coverage covering a longer period should be cancelled so records stay accurate on both sides.

An abandoned registration keeps generating filing notices.

Those notices become penalties if nobody answers them.

Does the employee still file personally?

Almost always, in both countries. Withholding is a payment on account rather than a final settlement, so a personal return still reconciles the position and claims the relief that prevents double taxation.

Most assignment policies include return preparation for this reason.

Our guide to US secondment tax covers the employee view.

Does it change the employment contract?

Not usually, though most employers issue an assignment letter alongside it. That letter sets out the duration, the location, the policy on tax, and what happens if the assignment ends early.

Employment law in the host country can still apply to the working arrangements.

Take legal advice on that separately. Tax advice does not cover it.

What if the employee has family with them?

Their position follows separately. A spouse who starts working in the host country has their own filing obligation, and children can pick up obligations of their own where citizenship is involved.

Employers often forget the family entirely.

A short briefing at the start prevents a scramble the following spring.

Can one person be on two payrolls permanently?

It happens, though it is rarely the intention. Someone splitting their time evenly between two countries can end up with genuine obligations in both, and the arrangement stops being a temporary assignment.

At that point the structure deserves a proper review.

Permanent splits usually need a different answer from an assignment.

What triggers a host country enquiry?

Late registration is the most common trigger, followed by year end figures that do not reconcile to the amounts remitted during the year. Authorities compare the two, and a gap invites a letter.

Equity income appearing suddenly in the final month is another.

Spread the reporting across the year and the figures look like what they are.

A shadow payroll that reconciles monthly rarely attracts attention at all.

Mistakes and penalties we see with shadow payroll

The first is starting late. Registering after three months of pay means three months of late withholding, with interest.

The second is ignoring the equity. A share award vesting mid-assignment is the most commonly missed item of all.

The third is skipping the certificate of coverage. Two social security bills on one salary is an avoidable expense.

The fourth is leaving the host registration open after the employee comes home.

How US UK Tax Accountants helps

We work out whether an assignment genuinely needs a second payroll, handle the host registration and the coverage certificate, and set the monthly reconciliation up so it runs without drama. Then we prepare the personal returns at both ends.

Where an assignment started without any of this, we sort the catch-up. Our UK self assessment service covers the British filings.

If you are moving staff across the Atlantic, get in touch before the first payday. Setting it up early costs a fraction of fixing it late.

Last reviewed 28 September 2026. This article is general information and not personal tax advice. Every assignment turns on its own facts, so take advice on yours before starting.