Cross-border telework: tax and social security are two different tests

Two questions that sound like one

Asking to work from home two days a week is the most ordinary request there is. Across this particular border it is also one of the easiest ways to change, without meaning to, which country taxes your salary or which country's social security covers you.

That is not obvious, and it is not your fault for missing it: the two systems ask different questions and answer with different numbers, and an employer's homeworking policy usually addresses neither. So we will separate them here — the threshold that applies to each, what document proves which, and the one small habit that keeps both questions answerable all year.

So take the question you actually want answered — how many days can I work from home? — and split it in two, because that is how the rules see it.

The tax question is which state may tax your salary for the days you worked in each place, and whether a France–Switzerland telework agreement preserves the route you were already on.

The social-security question is a different one entirely: which state's system covers you, and which one receives the contributions. Different laws, different calculations, different documents at the end.

This is why "two days from home is allowed" is not an answer. An answer names four things: the percentage, the reference period it is measured over, the tax route, and the social-security certificate that backs it. If your arrangement cannot be written that way, it has not been checked — it has been assumed.

For tax, the working number is 40%

Whether you sit inside the eight-canton frontier regime or on the treaty route outside it, the current France–Switzerland framework can preserve your salary-tax treatment for home telework of up to 40% of working time.

On a five-day week, 40% looks like two days at home. That arithmetic is where people go wrong, because the calculation is annual, not weekly. A stretch of three-day weeks during a quiet month has to be paid back later in the year, and nobody sends you a warning when the running total drifts.

Temporary missions interact with that quota, and the official examples include a separate ten-day tolerance for them. So a week of training in Paris is not simply four more home days.

If you do go over, the consequence depends on which tax route you were on underneath, which is another reason to know your route before you negotiate the percentage.

For social security, the first number is 25%

Now the same week, measured a completely different way. Under the ordinary multi-state rule, "substantial activity" in the state where you live means at least 25% of your working time or your remuneration — and reaching it can shift your social-security affiliation to France.

Note how far apart the two thresholds are. On a five-day week, one day at home is already 20%, comfortably inside the tax framework's 40% and uncomfortably close to the social-security 25%. The tax number tells you nothing about this one.

Below 25%, ordinary procedures can keep you covered in Switzerland. Even then the competent institution is the body that determines which law applies, and the record is worth documenting rather than assuming.

The framework agreement stretches that to 49.9%

France and Switzerland both take part in the cross-border telework framework, and this is what makes a serious home-working pattern possible at all.

An eligible employee of a Swiss employer can do habitual home telework in France below 50% — at most 49.9% — and stay in Swiss social security. The paperwork is your employer's to start: it requests an A1 certificate from its Swiss compensation office, normally through the ALPS platform. The A1 is the document that proves which country covers you.

The framework has real edges, and they are easy to cross without noticing. It does not cover habitual activity in France that is not telework, work in further states, a second employer somewhere in the EU/EFTA area, or self-employment on the side. Any of those puts you outside the framework and into an individual determination.

So the freelance project you picked up last spring is not a detail here. It is potentially the thing that changes which country insures you.

One calendar, two written answers

All of this becomes manageable with a single habit. Keep one calendar and mark five categories: workdays in Switzerland, home telework in France, missions in France, missions elsewhere, and nights spent in Switzerland. Once a month, compare what actually happened with the percentage you planned.

Then get two answers in writing from payroll or HR: the tax treatment they are using for payroll and reporting, and the social-security state supported either by your A1 or by an ordinary determination. Two answers, because there are two tests. Recheck them before you change your percentage, your employer, your canton or the countries you work in.

When the routine does change, update your work-location facts in the tax-residence check and go back for confirmation. An A1 settles the social-security question and only that one; it has never settled anybody's tax.