Social security tax on international assignments.
Context and Challenge
The main challenge with social security taxes on international assignments is that it is often reviewed too late, or not at all. Employers focus first on income tax, payroll and immigration. Social security is then treated as a payroll detail or missed. Common symptoms include contributions paid in the wrong country, missed certificates, double and costly employer contributions, disrupted state pension rights, unexpected social security on equity and employee dissatisfaction.
Who Should Read This
CFOs, payroll teams, HR directors, tax teams, finance directors, reward teams, RemCo members, board members, senior executives, NEDs, founders, start-up and scale-up leadership, internationally mobile employees and professional advisers dealing with social security, A1 certificates, certificates of coverage, totalisation agreements, double contributions or social security on share remuneration.
Core Finding and Summary
Social security can be one of the largest hidden costs of international mobility. The income tax answer does not determine the social security answer. A1 certificates and certificates of coverage must be in-place and in- date when required, even for short trips - there is not usually a minimum number of day threshold. There are not many social security agreements between countries so local rules usually prevail in those circumstances. Employer social security in some countries can reach 45–47% of gross salary but in others it is capped at a low an amount so smart planning is essential. For high earners and equity-heavy businesses, social security planning can materially change assignment cost. Consider if voluntary contributions can be made in the home country, if mandatory contributions stop. How social security is calculated for payroll purposes is often different to how it is calculated for tax purposes.
In practice: Unlike income tax, double social security contributions may not be creditable. Social security contributions can be an expensive tax cost for employers.Planning and compliance is essential.
Technical reference: EU social security coordination regulations, bilateral totalisation agreements and domestic contribution rules each operate independently from income tax treaties.
Decision point
Which country's social security system applies, is an A1 certificate or certificate of coverage exemption available, does the certificate match the actual facts, and the relevant period(s), could double contributions arise, does social security apply to bonus or share remuneration, and are planning opportunities?
The Issue
What triggers the issue?
An employee working in another country may trigger social security contributions in the host country, the home country or both, regardless of the income tax position. Remote workers, digital nomads, business trips, short projects, statutory directors attending board meetings, international commuters and secondees all need to be considered.
Why does it matter?
Incorrect social security can create double contributions, employer cost, employee cost, penalites, disrupted benefit rights, state pension gaps and governance failures.
What decision is needed?
Determine which system applies, obtain certificates when available, align payroll, identify planning opportunites, model costs and communicate impact to employees before the overseas work starts. Consider if voluntary contributions can be made in the home country, if mandatory contributions stop.
Technical Analysis
1.Legal / tax position
Social security on international assignments is the determination of which country's social security system applies when an employee works across borders. Social security is separate from income tax. An employee may be taxable in one country but remain insured in another country's social security system. The income tax treaty does not determine social security. How social security is calculated for payroll purposes is often different to how it is calculated for tax purposes.
The answer depends on the countries involved, the employee's work pattern, the duration of the assignment, the employer structure, the applicable social security agreement and whether a certificate is available. Employers should not assume that social security follows where the employee is paid, where the employment contract sits, where income tax is paid, where payroll is operated or where the employer wants contributions to be paid.
An A1 certificate is evidence that an employee remains subject to one country's social security system for a specific period while working in another country where the relevant rules allow. A certificate of coverage performs a similar role under a bilateral totalisation agreement. These certificates can preserve home-country coverage, avoid host-country contributions, prevent double contributions and maintain benefit continuity. But certificates are not automatic and must match the actual facts. Also unlike tax, there are not many social security tax treaties between countries, so local rules normally prevail and there may be overlap and double contributions and double payroll obligations.
In some countries like the UK, voluntary contributions can be made if mandatory contributions stop.
2.Employer obligations
Where no social security agreement applies, double contributions and double payroll obligations may arise. This can be expensive for employers and difficult to explain to employees. Unlike income tax, social security may not be fully creditable. Double contributions can be a real cost.
There are significant planning opportunities, particularly with higher earners. Social security costs can vary materially between countries. Employer social security in France can reach up to 45–47% of gross salary. One country may have a low employer contribution ceiling. Another may have high or uncapped contributions. That difference can have a major effect on assignment cost, especially for employees with high salary, large bonus, RSUs, options, LTIPs, deferred compensation or pre-IPO equity.
Social security on share remuneration can be a major issue for start-ups, scale-ups and IPO candidates too. Employer social security on share remuneration can run into millions where high-value awards vest, exercise or become taxable in a high-cost country. Reward teams should not design equity incentives without checking international social security exposure. Elections can be made in some countries to determine what contributions employee make on certain long term incentives, on behalf of an employer.
Remote workers, digital nomads, business trips, short projects, statutory directors attending board meetings, international commuters and secondees all need to be considered.
3.Employee / individual consequences
Employees may face disruption to state pension rights, healthcare coverage, unemployment benefits, maternity or parental benefits, disability benefits, family benefits and continuity of contribution record. Some employees will not respond well if the employer disrupts home-country pension or welfare rights because social security was not planned properly. This is particularly sensitive where the assignment was employer-driven. In some countries like the UK, voluntary contributions can be made if mandatory contributions stop so this should considered too.
If you work in another country, you may be required to pay both tax and or social security tax in that country regardless of whether your employer sent you there. Employers work-from-anywhere and other overseas working policies should ideally address social security specifically. The policy should state whether social security review is required before approval, who bears additional contributions, what happens if dual contributions arise, and whether welfare or state pension rights may be affected. If the arrangement is employee-requested, the employer may want the employee to bear certain additional personal costs.
4.Evidence and reporting requirements
Employers should retain assignment letters, employment contracts, work location records, travel calendars, payroll records, A1 certificates, certificates of coverage, certificate applications, social security payment records, equity and bonus records, board appointment letters, NED fee records, tax equalisation policies, employee communications and adviser correspondence.
5.Practical controls
Employers should implement mandatory social security reviews before cross-border work starts, certificate application processes, renewal tracking, extension review, payroll coding, equity event review, remote-working social security approval, high-earner cost modelling, voluntary contributions and employee communication on benefit rights, and annual audit of certificate coverage against actual travel. The control should be practical and triggered before the cost or risk is created.
Case Scenario
Missing A1 Certificate for EU Assignment
Situation: A UK company sends an employee to France for a 12-month assignment. No A1 certificate is obtained. The employee remains on UK payroll. French social security is not considered.
Issue: Whether the employee should be contributing to French social security and whether UK NIC should continue.
Analysis: Without an A1 certificate evidencing continued UK coverage, France may require local social security contributions. The UK may also continue to require NIC. The result is potential double contributions with no automatic relief.
Outcome: The employer faces double employer social security cost. No certificate exists to evidence the position. Correct the position as soon as possible.
Lesson: Obtain the A1 certificate before the assignment starts. Ensure it matches the actual work pattern and assignment duration. Review renewal before expiry.
Fenton International's Advisory Position
Technical position?
Social security must be reviewed separately from income tax. The income tax treaty does not determine social security. A1 certificates and certificates of coverage must match the actual facts and relevant periods. Remote workers, digital nomads, business trips, short projects, statutory directors attending board meetings, international commuters and secondees all need to be considered.
Professional judgement required
Yes — social security planning involves the interaction of EU coordination rules, bilateral agreements, domestic contribution rules, employer structure, work pattern, equity, bonus and commercial decisions about cost-bearing. Planning opportunities need to be identified to avoid unnecessary costs.
Main risks:
Social security risk
Penalty risk
Cost risk
Payroll risk
Governance risk
Reputation risk
Dissatisfied employee
Evidence risk
Pension / welfare benefit risks
Evidence needed:
Assignment letters
work location records
payroll records
A1 certificates
certificates of coverage
social security payments
equity and bonus records
tax equalisation policies
employee communications
Recommended controls:
Pre-arrangmente social security review
certificate application process
renewal tracking
equity event review
high-earner cost modelling and planning
work-from-anywhere social security approval
annual audit
Professional Judgement & Advisory Application
Professional judgement is required because social security involves the interaction of tax and payroll rules, EU regulations, bilateral social security agreements, domestic social security rules, employer structure, work patterns and commercial decisions. The cost difference between countries can be material, especially for high earners and equity-heavy businesses.
Fenton International's judgement and recommendation: review social security before the employee moves, visits, vests equity or exercises options. Determine which system applies, obtain certificates, identify planning, model employer and employee costs, communicate pension and welfare impact and track renewals.
Frequently Asked Questions
Which country's social security system applies during an international assignment?
It depends on the countries involved, the work pattern, the duration, the employer structure and whether a social security agreement or local rules apply. The income tax position, employment location or payroll location does not determine the social security answer. Each case must be reviewed on its own facts.
What is an A1 certificate?
An A1 certificate is evidence that an employee remains subject to one country's social security system while working in another country where the relevant EU coordination rules allow. It can preserve home-country coverage and avoid host-country contributions. It must match the actual work pattern and assignment duration. Certificates of Coverage do similar things where a nonEU social security agreement exists.
Can social security apply to RSUs, options or LTIPs?
It can, depending on the country and social security rules. Employer social security on share remuneration can be material, especially in countries with high or uncapped contribution rates. For start-ups, scale-ups and IPO candidates, the cost on large vesting or exercise events can run into millions.
Can employees lose state pension or welfare rights because of an international assignment?
Yes. Poor social security planning can disrupt state pension contribution records, healthcare coverage, unemployment protection, maternity or parental benefits and other welfare rights. This is particularly sensitive where the assignment is employer-requested. Employees should be told before the move if their benefit position may change. Voluntary contributions can be made in some countries so should be reviewed.
How Fenton International Can Help?
Fenton International specialises in international social security, cross-border tax, global mobility and international reward and HR advisory. We advise employers, executives, directors and professional advisers on international social security, A1 certificates, certificates of coverage, social security on share remuneration, planning opportunities, voluntary contributions and cost modelling.
High-earner social security cost modelling
Work-from-anywhere social security review
Discuss this issue: Contact Fenton International for a cross-border director tax and compliance review.
Author
Mark Abbs, CEO, Fenton International
Fellow of the Association of Taxation Technicians (FATT)
Enrolled Agent of the IRS (EA)
Global Mobility Specialist – Talent Management (GMS-T)
Accredited Expert Witness (MAE)
32+ years' experience in international tax, cross-border employment tax and global mobility
Advises CFOs, HRDs, and Chairs on cross-border tax governance
Head of Advisory at Global Tax Network
Former Tax Partner, Head of International and Senior Leadership Team at Blick Rothenberg and Senior Tax Adviser in the Big 4.
Key terms: social security, A1 certificate, certificate of coverage, totalisation agreement, double social security contributions, employer social security, employee social security, NIC, social security on equity, RSUs, options, LTIPs, share remuneration, international assignment, work from anywhere, EOR, state pension rights, welfare benefits.
Key terms: social security, A1 certificate, certificate of coverage, totalisation agreement, double social security contributions, employer social security, employee social security, NIC, social security on equity, RSUs, options, LTIPs, share remuneration, international assignment, work from anywhere, EOR, state pension rights, welfare benefits.
Jurisdiction: Multi-jurisdiction | Last reviewed: June 2026 | Next review due: December 2026 | Insight type: Technical Guide
© 2026 Fenton International. All rights reserved.
This article is general information and not legal or tax advice. Professional advice should be taken for specific circumstances.
