What is shadow payroll and when do I need one?

Shadow payroll is a payroll run in the host country solely to report earnings and remit host-country tax on an employee paid from another country's payroll. No net cash is paid to the employee. You need one whenever a host-country withholding obligation exists that treaty relief does not remove, typically where the host entity is the economic employer, bears the cost of the assignee, or the visitor exceeds the treaty day-count limit.

How many days can a US employee work in the UK before PAYE is due?

There is no UK domestic de minimis, so in principle the first workday is taxable. Relief comes from Article 14 of the US-UK treaty, which requires presence of no more than 183 days in a rolling twelve-month period, payment by a non-UK employer, and no UK permanent establishment bearing the cost. All three must hold. Register an Appendix 4 STBV agreement with HMRC to avoid operating PAYE on qualifying visitors.

What is an Appendix 4 STBV agreement with HMRC?

It is an administrative easement that lets a UK entity skip operating PAYE for qualifying short-term business visitors and instead file one annual report by 31 May following the tax year. It must be applied for in advance and is not automatic. It covers income tax only, not National Insurance, and it does not help where the UK entity is the economic employer or bears the assignee's cost.

Does a UK employee working in the US have to pay US tax on short trips?

Often yes. US-source compensation is taxable from the first workday unless Article 14 treaty relief applies, and the relief requires a claim, normally on Form 8233 with a US taxpayer identification number. Crucially, states do not follow the treaty. A visitor fully exempt federally can still owe California or Massachusetts tax immediately, and New York applies a fourteen-day threshold before nonresident withholding starts.

What happens if we fail to operate shadow payroll correctly?

HMRC can determine the unpaid PAYE against the employer, issue a decision for unpaid National Insurance, charge interest from the original due dates and add behaviour-based penalties that rise sharply for deliberate or undisclosed failures. In the US, expect liability for the unwithheld tax, failure-to-deposit and failure-to-file penalties, potential personal trust fund recovery exposure, and separate state assessments. Voluntary disclosure materially improves the outcome.

Shadow Payroll for US-UK Short-Term Business Visitors

Q: What is Appendix 6 modified payroll for tax equalised assignees?

Appendix 6 is an HMRC arrangement for tax-equalised expatriates that solves the circularity created when the employer pays the assignee's tax, making the tax itself taxable. The employer applies in advance, operates PAYE on a reasonable estimate of grossed-up earnings during the year, then submits a reconciliation with actual figures after the tax year ends and settles the balance. The individual still files self assessment.

A shadow payroll is a payroll run purely to report and settle host-country tax on an employee who is paid somewhere else, and it becomes necessary the moment a business visitor's presence creates a host-country withholding obligation that treaty relief does not remove. For executives moving between US and UK group entities, the practical question is narrow: can the visit be covered by the UK Short Term Business Visitor (STBV) arrangement under Appendix 4 of HMRC's PAYE guidance, or must the UK entity operate PAYE on pay it never physically makes? The answer turns on Article 14 of the US-UK income tax treaty, on a careful day count, and above all on who is the economic employer bearing the cost of the individual's work. Get that wrong and the exposure is not the employee's tax bill alone but the employer's, grossed up, with interest and penalties on top.

What is shadow payroll and why does it exist?

An employee remains on the home-country payroll. The host country nonetheless taxes the workdays performed on its territory. Rather than move the employee onto a host contract, the employer sets up a parallel payroll record in the host country that reports the same earnings, calculates host withholding, and remits it, without paying any net cash to the individual. The employee sees one payslip. Two revenue authorities see the compensation.

When does a US business visitor become taxable in the UK?

UK domestic law is blunt. Earnings attributable to duties performed in the United Kingdom are taxable here, regardless of where the employer sits, where the contract was signed, or where payment is made. There is no domestic de minimis for workdays. A single day of substantive work in a London office is, in principle, within charge. Relief comes only from the treaty.

Article 14 of the US-UK income tax treaty deals with income from employment. It exempts remuneration derived by a resident of one state from employment exercised in the other, but only where three conditions are all satisfied. Failing any one of them puts the earnings back into charge in the host state from the first day, not from the day the condition breaks.

The individual is present in the host state for no more than 183 days in any twelve-month period beginning or ending in the tax year concerned.
The remuneration is paid by, or on behalf of, an employer who is not a resident of the host state.
The remuneration is not borne by a permanent establishment that the employer has in the host state.

What does the 183-day test actually count?

Days of presence, not workdays. The treaty test counts physical presence in the host country, and any part of a day generally counts as a day. Weekends, holidays, sick days, and days spent in the country either side of a business trip are included. This is a different measure from the workday count used to apportion earnings, and from the UK Statutory Residence Test day count. Confusing the three is one of the most reliable ways to underreport.

The twelve-month period is a rolling window, not the UK tax year. It can begin or end in the year under review, which means a spring-to-spring travel pattern can breach the limit even when neither tax year does.
Presence for private reasons still counts toward the 183 days, even though private days generate no taxable earnings.
Transit days and partial days ordinarily count as days of presence.
Group companies often track only workdays booked to a project code. That understates the treaty count badly.

Who is the economic employer?

The second treaty condition is where most US-UK arrangements come apart. HMRC follows the economic employer approach reflected in the OECD Commentary. It looks past the legal contract of employment and asks which entity genuinely bears the cost of, and derives the benefit from, the individual's work during the visit. If the UK entity is the real beneficiary, the UK entity is treated as the employer and treaty relief is unavailable.

Cross-charges are the giveaway. Where the US company recharges the visitor's salary cost to the UK subsidiary, whether directly or buried in a management services fee calculated by reference to headcount or time, HMRC will look hard at whether the UK entity is economically employing the individual. A cost-plus recharge that identifies the individual is close to fatal.

Who directs and controls the individual's day-to-day work during the visit?
Who bears the risk and reward of the output produced?
Is the individual integrated into the host entity's organisation, occupying a role in its structure?
Is the cost of the individual charged, recharged, or reflected in the host entity's results?
Does the individual hold a title, office, or reporting line within the host entity?

What is an Appendix 4 STBV agreement?

Appendix 4 is a relaxation agreed with HMRC that removes the practical obligation to operate PAYE for qualifying visitors, replacing it with a single annual report. It does not change the underlying law. It is an administrative easement granted to an employer who applies for it, and it must be applied for; it is not automatic and it cannot be applied retrospectively to years already closed without discussion with HMRC.

The visitor is resident in a country with which the UK has a double taxation treaty containing an employment income article.
The visitor comes to work for a UK company or the UK branch of an overseas company.
The visitor is expected to remain in the UK for fewer than 183 days in any twelve-month period.
The UK entity does not, in substance, bear the cost of the visitor's remuneration, whether directly or by recharge.

When is the Appendix 4 report due?

The annual STBV report is due by 31 May following the end of the UK tax year. It lists visitors by day-count band, with escalating information requirements as the day count rises. Late or incomplete reports put the agreement itself at risk. HMRC can withdraw an Appendix 4 arrangement, and the consequence of withdrawal is that PAYE becomes due on the underlying visits.

What if the visitor comes from a UK branch of an overseas company?

A visitor employed by an overseas branch of a UK company falls outside the ordinary treaty exemption, because the employer is UK resident. HMRC operates a separate special arrangement for these branch visitors, subject to a much tighter workday limit and an annual PAYE scheme settled after the year end. Employers with UK-parented groups that second staff from overseas branches are frequently caught out here, because they assume Appendix 4 covers everyone.

When is UK PAYE shadow payroll actually required?

Whenever treaty relief fails or is not available, the UK entity has a PAYE obligation on the UK-workday portion of the earnings. Because the individual is paid from a US payroll, the only way to discharge that obligation is a UK shadow payroll, usually run through a dedicated expatriate PAYE scheme.

The visitor exceeds 183 days of presence in the relevant rolling twelve-month period.
The UK entity is the economic employer, or the cost is recharged to it.
The remuneration is borne by a UK permanent establishment of the US employer.
The individual is on a formal secondment to the UK entity rather than making discrete business trips.
No Appendix 4 agreement is in place, so the easement simply does not apply.

How does a UK shadow payroll operate in practice?

The UK entity reports the individual's gross earnings through Real Time Information in the ordinary way, applies the appropriate tax code, and remits income tax and, where due, National Insurance to HMRC on the normal monthly cycle. No net pay leaves the UK. The cash is settled internally between the US and UK entities. Equity income, bonuses, and relocation benefits must be swept into the same reporting, and equity is the item most often missed because it vests long after the assignment ends.

Where only part of the earnings relates to UK duties, an employer may apply under section 690 ITEPA for authority to operate PAYE on an estimated proportion of the total. Without that authority, the strict position is that PAYE applies to the whole payment, with the employee reclaiming the excess through self assessment. Securing the section 690 position early is one of the highest-value administrative steps in any inbound arrangement.

Does an STBV agreement cover National Insurance?

No. Appendix 4 addresses PAYE income tax only. Social security is governed by the US-UK social security agreement, under which a worker sent temporarily from one country to the other can remain in the home system for a limited detached-worker period, evidenced by a certificate of coverage. Without a valid certificate, UK National Insurance can become due even where income tax does not. Certificates are issued on application and should be obtained before travel, not afterwards.

What is Appendix 6 modified payroll for tax-equalised assignees?

Appendix 6 is a modified PAYE arrangement for tax-equalised expatriates. Under tax equalisation the employer bears the assignee's host tax, which means the tax paid is itself a benefit, which is itself taxable, which requires a grossing-up calculation that cannot be performed accurately in real time. Appendix 6 solves the circularity by allowing the employer to operate PAYE on a best estimate during the year and true it up afterwards.

The employer applies to HMRC in advance and, once accepted, reports estimated grossed-up earnings each month through RTI. After the tax year ends, the employer submits a reconciliation replacing the estimates with actual figures and settles the balance. The individual still files a UK self assessment return, and the modified figures feed into it.

Application must be made to HMRC before the arrangement is used; it is not self-elected.
Estimates must be made on a reasonable basis and revisited if circumstances change materially in-year.
The year-end reconciliation and payment deadlines are strict, and HMRC treats persistent inaccuracy as grounds to withdraw the arrangement.
Appendix 6 works alongside, not instead of, the individual's own filing obligations.

Are there equivalent arrangements for National Insurance?

Yes. HMRC operates modified NIC arrangements that mirror the income tax treatment for equalised assignees, allowing contributions to be calculated on estimated earnings and reconciled after the year end. They are applied for separately from the income tax arrangement. Employers frequently secure the income tax easement and forget the contributions equivalent, then discover a mismatch at reconciliation.

How does the US tax a UK assignee coming inbound?

The mirror image applies. A UK employee performing services in the United States has US-source compensation, and the US employer or the US entity acting as employer has a withholding obligation on it. Where the individual is a nonresident alien, withholding applies to the US-workday portion. Where the individual becomes a US tax resident, worldwide income comes into charge and the analysis changes entirely.

Residence for US purposes is determined by the substantial presence test, described in IRS Publication 519. It uses a weighted three-year day count, so a pattern of repeated shorter trips can create residence without any single year looking dangerous. Certain days are excluded, and a closer connection claim or the treaty residence tie-breaker may displace the result, but each of those requires a filing position to be taken and documented.

Can a UK visitor claim treaty relief from US withholding?

Yes, on the same Article 14 conditions, but the relief is not self-executing at the payroll level. A nonresident alien claiming a treaty exemption from withholding on personal services income normally provides Form 8233 to the withholding agent, which the agent files with the IRS. Compensation exempted under a treaty is then reported on Form 1042-S rather than Form W-2. Employers that simply stop withholding without the paperwork have an exposure even where the substantive relief is good.

Form 8233 supports a treaty claim against withholding on personal services income for a nonresident alien.
Form 1042-S reports treaty-exempt compensation; Form W-2 reports taxable wages.
A US taxpayer identification number is generally needed before a treaty claim can be processed.
IRS Publication 901 summarises treaty benefits; the treaty text itself governs.

Does the US have a de minimis rule for short visits?

There is a narrow statutory exception for a nonresident alien performing services in the United States for a foreign employer, where presence does not exceed 90 days in the tax year and the compensation for those services does not exceed a small fixed dollar threshold set out in the Internal Revenue Code. The threshold has not been indexed and is low enough that it rarely helps an executive. In practice the treaty, not the statutory rule, does the work.

What about US social security and Medicare?

Income tax treaties do not relieve FICA. Relief comes only from the US-UK social security agreement, and only where a certificate of coverage confirms the individual remains in the UK system. Without it, the US employer must withhold and pay FICA on covered wages, and the individual accrues UK contributions at the same time. Double contributions are real money and are rarely recoverable once paid.

Why is US state withholding the bigger trap?

States are not parties to the US-UK income tax treaty and most do not follow it. A UK visitor who is fully exempt from federal tax under Article 14 can still owe California, New York, or Massachusetts income tax on the first workday in that state, with a corresponding employer withholding duty. Federal treaty relief and state liability coexist comfortably.

A minority of states offer a day-count threshold before nonresident withholding begins. New York, for example, applies a fourteen-day rule for nonresident employee withholding. Others have no threshold at all. Federal legislation to harmonise state thresholds has been proposed repeatedly and has not been enacted, so the position remains state by state and must be tracked accordingly.

How does tax equalisation interact with US shadow payroll?

Where the employer pays the assignee's US tax, the payment is additional compensation and must be grossed up. Unlike the UK, the US has no general modified payroll easement equivalent to Appendix 6, so the grossing-up must be handled within the ordinary payroll and reconciled through the year-end process and the individual's Form 1040 or 1040-NR. Hypothetical tax deductions taken under an equalisation policy do not reduce taxable wages.

What are the permanent establishment consequences?

Employee mobility and corporate presence are the same set of facts viewed twice. An executive habitually concluding contracts in the host country, or a visitor whose cost is borne by a host branch, generates evidence relevant to permanent establishment under Article 5 of the treaty. STBV reports and shadow payroll records are discoverable by the corporate tax side of the same revenue authority. The employment analysis should never be run in isolation from the corporate one.

What are the most common mistakes employers make?

Assuming a treaty exemption applies automatically without an Appendix 4 agreement or a US withholding claim on file.
Counting workdays instead of days of presence for the 183-day treaty test, and using the tax year instead of a rolling twelve-month window.
Recharging assignee costs to the host entity for transfer pricing reasons, then claiming the host entity is not the economic employer.
Treating federal treaty exemption as state exemption in the United States.
Ignoring equity compensation that vests after the assignment, which is sourced to the workdays over the vesting period, not the vesting date.
Failing to obtain a certificate of coverage before travel, leaving social security due in both countries.
Running a UK shadow payroll on full earnings without a section 690 authority, over-withholding and forcing a reclaim.
Letting travel be booked by business units with no visibility to payroll or tax.

What records should you keep?

A complete travel and day-count record per individual, reconciled to expense claims, corporate card data, calendar entries and immigration records rather than to self-reported estimates.
Copies of the Appendix 4 or Appendix 6 application and HMRC's acceptance, plus each year's submitted report.
Secondment or assignment letters setting out who directs the work, who bears the cost, and the intended duration.
Intercompany agreements and recharge invoices, showing whether individual costs cross the border.
Certificates of coverage under the social security agreement, with their validity dates.
Forms 8233, W-8BEN, 1042-S and W-2 as applicable on the US side, and evidence of any section 690 authority on the UK side.
Equity award records showing grant, vest and exercise dates against the workday location history.
Tax equalisation policy documents, hypothetical tax calculations and settlement statements.

What are the penalties for getting it wrong?

In the UK, HMRC can issue a determination on the employer for PAYE that should have been operated, together with a decision covering unpaid National Insurance. The liability falls on the employer, not the employee, and where the employer meets it without recovery the amount is itself a benefit requiring grossing up. Interest runs from the original due dates. Penalties are behaviour-based, escalating from careless to deliberate and concealed, with materially higher exposure where the failure was not disclosed.

In the United States, an employer that fails to withhold faces liability for the tax, failure-to-deposit and failure-to-file penalties, and interest. Responsible individuals can face a personal trust fund recovery penalty for unpaid withheld amounts. State authorities add their own assessments and penalties, and several are increasingly effective at data-matching nonresident travel. Voluntary disclosure, in both jurisdictions, produces materially better outcomes than waiting for an enquiry.

Where should you look for authoritative guidance?

On the UK side, HMRC's PAYE Manual sets out the Appendix 4 and Appendix 6 arrangements and the special arrangement for branch visitors, and GOV.UK publishes the application routes and the STBV reporting requirements. On the US side, IRS Publication 519 covers taxation of nonresident aliens and the substantial presence test, Publication 515 addresses withholding on nonresident payments, and Publication 901 summarises treaty positions. The US-UK income tax treaty and the US-UK social security agreement are the governing instruments in each case.

How should you build a compliant programme?

Put a pre-travel approval gate in place so that tax sees the trip before it happens, not at year end.
Maintain a single day-count system fed by booking data, not by manual returns from business units.
Agree the recharge policy with transfer pricing colleagues so the employment and corporate positions tell the same story.
Apply for the UK easements in advance, and diarise the 31 May report and the year-end reconciliation.
Review the population annually for individuals approaching thresholds, and escalate before they cross.
Run an annual reconciliation between payroll data, equity records and the day-count system.

This article is general commentary on the US and UK rules applying to short-term business visitors and does not constitute advice. Outcomes depend closely on your facts, on the terms of your intercompany arrangements, and on the individual's residence and travel history. Take professional advice before relying on a treaty position, applying for an HMRC arrangement, or ceasing to withhold in either country.