Expat payroll is what happens when one employment relationship touches two tax systems at once. An employee working outside their home country can owe income tax where they work, remain in scope of home-country tax and social security, or both, and the employer has withholding and reporting obligations on each side. The machinery that manages this, exclusions, treaties, totalization agreements and shadow payrolls, exists because the default outcome without it is double taxation. This page walks through the US-centred version of that machinery, using the rules as published by the IRS and the Social Security Administration; the specifics of any one assignment turn on the countries and treaties involved, and nothing here is tax advice.
Why expat payroll is different
A domestic payroll answers to one tax authority; an expat payroll answers to at least two, and the two rarely agree on definitions, timing or rates. The employer has to determine where the employee is tax resident, which country's payroll withholding applies, whether social security contributions are due in one country or both, and how equalization or protection policies in the assignment package change the net position. The SSA's overview of the problem is blunt about the stakes: without relief, a worker abroad can face contributions to two systems at once, and where employers gross up taxes on the employee's behalf the costs cascade, because the reimbursement itself is taxable. Structure decided before the assignment starts is far cheaper than structure repaired afterwards.
US income tax: the foreign earned income exclusion
US citizens and resident aliens are taxed on worldwide income wherever they live, which is why the foreign earned income exclusion matters to any US expat payroll. A qualifying individual can exclude foreign earned income up to an annually adjusted ceiling, which the IRS lists as $107,600 for 2020, $108,700 for 2021, $112,000 for 2022 and $120,000 for 2023, and may also claim a foreign housing exclusion or deduction for certain housing costs. Qualifying requires a tax home in a foreign country plus either bona fide residence abroad for an uninterrupted period including an entire tax year, or physical presence abroad for at least 330 full days in any twelve consecutive months. The exclusion reduces regular income tax only; for the self-employed it does not reduce self-employment tax, so Social Security and Medicare liabilities continue on excluded earnings.
Social security: totalization agreements and the detached worker rule
Social security is where double costs bite hardest, because the US covers its citizens working abroad regardless of how long the foreign assignment lasts, while host countries generally cover anyone working on their territory. Totalization agreements, the bilateral treaties the US maintains with a set of partner countries, exist to eliminate dual social security taxation and to fill benefit gaps for careers split between countries. The default rule is territorial: the worker is covered where the work is performed. The key exception is the detached worker rule: an employee temporarily transferred by the same employer stays under home-country coverage, typically where the assignment is expected to last five years or less. A certificate of coverage documents the exemption; employers present it to the host authorities, and it is the proof that only one system's contributions are due.
Shadow payroll: the operational answer
The common operational pattern for an assignment is a shadow payroll: the employee keeps being paid from one country, while a parallel, non-paying payroll runs in the other country purely to calculate and remit the withholding and reporting that country requires. It is exacting work; the shadow side has to track compensation it does not pay, including benefits and equity, convert it correctly and file on the host calendar. Global payroll providers and EORs sell this as a product, and for a company with one or two expats it is usually bought rather than built. Whoever runs it, the assignment letter, the equalization policy and the two countries' rules control the outcome, and the certificate of coverage and exclusion elections need to exist as documents, not intentions.
Questions people ask about expat payroll
What is the foreign earned income exclusion worth?
The ceiling adjusts annually for inflation; the IRS lists $112,000 for 2022 and $120,000 for 2023. Qualifying requires a foreign tax home plus bona fide residence for a full tax year or 330 full days of physical presence abroad in twelve consecutive months.
Does the exclusion remove US Social Security obligations?
No. It reduces regular income tax only. The IRS is explicit that for self-employed individuals the excluded amount does not reduce self-employment tax, and employment-based coverage questions are governed separately, including by totalization agreements.
What stops an expat paying social security twice?
A totalization agreement, where one exists between the two countries. Coverage defaults to the country where the work is performed, with the detached worker exception keeping temporary transfers of five years or less under home-country coverage, evidenced by a certificate of coverage.
What is a shadow payroll?
A non-paying payroll run in one country solely to calculate and remit that country's withholding and reporting on compensation actually paid elsewhere. It is the standard mechanism for keeping both tax systems satisfied during an assignment.