How to Count Your Telework Days as a Frontalier: the 40% Tax Rule vs the 49.9% Social-Security Rule
Every France–Switzerland frontalier who works from home now lives with two numbers: 40% and 49.9%. They look interchangeable. They are not. The 40% ceiling comes from the Franco-Swiss tax avenant signed on 27 June 2023 and decides which country taxes your salary from 1 January 2026. The 49.9% ceiling comes from the EU/EFTA social-security framework agreement in force since 1 July 2023 and decides which country's social system you contribute to. Two legal instruments, two reference bases, two sets of consequences — and one shared practical problem: you can only show you are under either line if you actually count your days. This guide is the counting how-to: what goes into each count, how mission days are treated, and a simple monthly method that holds up if anyone asks. It is general information drawn from official French, Swiss and EU sources — not personalised tax or legal advice — and for anything specific to your canton or contract it points you to the competent bodies.
