Global mobility services exist because moving one employee across one border creates obligations in at least four systems at once — income tax in two countries, social security in two more — and because the employer, not the employee, carries most of the compliance risk. A business visitor who spends ninety days a year in the London office, a secondee from the US parent, a UK hire working "temporarily" from Madrid: each is a payroll question, a treaty question and sometimes a corporate tax question wearing a plane ticket. Here is the UK machinery that keeps cross-border people compliant, and where it breaks.
Short-term business visitors: the exposure nobody budgets
The baseline rule shocks employers: in principle, PAYE obligations can arise from day one of UK workdays — even for a colleague from the group's Frankfurt office attending a fortnight of meetings — with treaty relief affecting the ultimate tax but not, by itself, the strict withholding position. Strict compliance would mean running UK payroll for every visiting employee — which is why HMRC operates the Appendix 4 short-term business visitor arrangement: employers sign up once, and treaty-protected visitors from overseas group companies can be left out of PAYE, with a single annual report due by 31 May.
The reporting scales with days: up to 30 days, essentially nothing; then escalating information bands through 60, 90 and 150 days — names, duties, evidence of overseas tax residence — up to individual applications near the 183-day treaty line. Two conditions do the real work: the UK entity must not ultimately bear the cost of the visitor's remuneration (recharges quietly break this constantly), and the visit must not be part of a longer pattern. For visitors who fail treaty protection — no treaty, or employed by an overseas branch of a UK company — a separate Appendix 8 arrangement allows a once-a-year PAYE settlement for those with 60 or fewer UK workdays in the tax year. The trap in all of this is not the rules but the tracking: without travel data, an employer cannot even complete the report it signed up to file, and diligence exercises routinely find years of unreported visitors priced as a warranty claim.
Inbound assignees: payroll with adjectives
Longer-term arrivals bring their own toolkit. Tax-equalised expats — promised "no worse off than at home" — run through Appendix 6 modified payroll, with grossed-up estimates and a year-end true-up. Employees with duties split between the UK and overseas can have PAYE limited to the UK share through the section 690 process, digitised since 2025: employers now notify online and operate the split on acknowledgment, with a fresh notification needed each tax year — a renewal that calendars miss. Genuinely non-resident employees with no UK duties can hold an NT code. And inbound assignees expected here for 24 months or less can often have travel and accommodation treated as tax-free temporary-workplace expenses — relief that evaporates, prospectively, the moment the expected stay stretches past two years, so extension decisions have a tax cost the business case should show.
The individual-side rules run in parallel: the statutory residence test fixes when UK taxation starts, and the four-year FIG regime (covered in our international tax guide) shapes what new arrivals pay on non-UK income.
Social security: the system people forget
NIC follows its own map, independent of tax. For moves within the EU/EEA and treaty countries, A1 certificates (and equivalents) keep the home country's contributions running — typically up to 24 months for detached workers — and switch off host-country charges; without the certificate, both countries may charge. For the rest of the world, UK NIC continues for the first 52 weeks of an outbound posting under the domestic rules. Social security is routinely the largest single cost line in an assignment package and the least planned: employer contributions abroad can dwarf the UK's, and a certificate applied for late is a certificate arguing with two authorities at once.
Outbound and the new shape of mobility
The classic three-year expat assignment now shares the stage with messier patterns — the employee who wants a month a year from a family home abroad, the remote hire who never relocates, the executive commuting weekly. Each triggers the same checklist in miniature: host-country payroll and tax exposure, social security position, and — the one HR rarely spots — permanent establishment risk for the employer where senior or sales staff work habitually from another country. A written policy on international remote work (who may, where, for how long, doing what) converts an unmanaged exposure into a bounded one, and is now as standard a document as the expenses policy.
Running mobility as a system
Compliant mobility is mostly administration done relentlessly: a travel-tracking source of truth; the Appendix 4/8 arrangements in place and reported by 31 May; s690 notifications renewed each April; A1s applied for before departure, not after; assignment letters that state tax and social security treatment explicitly; and an annual sweep for the accidental cases — the visitor pattern that crossed 60 days, the remote arrangement that hardened into permanence. Acumon runs this machinery for employers through our international payroll and employment tax teams, with expat tax handling the individuals' side of the same moves. If your business has people crossing borders and no one owns the tracking, the exposure is already accruing — quietly, at payroll rates.