Three home-office days a week sounds like a matter of taste. For someone living in Alsace and employed in Germany, three days out of five is 60 percent – and from 50 percent, social security moves to France. That isn't a grey area, it's arithmetic. Here it is, worked through in full.
As of 08/2026. The percentages refer to social security, not to tax. Sources at the end.
The two thresholds in play
In social security there are exactly two numbers for cross-border commuters, and they do different jobs.
The first is the 25% threshold in Article 13 of Regulation (EC) 883/2004. If you regularly work in two states and carry out a *substantial part* of your activity in your country of residence, that country's law applies to you. Substantial means 25 percent. Below that, nothing happens – you stay in the German system, with no application and no form.
The second is the 49.9% limit from the multilateral framework agreement on cross-border telework, in force since 1 July 2023 and signed by both France and Germany. It is a derogation: on a joint application by employer and employee under Article 16 of the regulation, you may telework up to 49.9 percent from your country of residence and still remain in German social security. The application isn't automatic, but it has become routine – the outcome is evidenced by an A1 certificate.
The worked example: an ordinary 5-day week
Take someone living in Strasbourg and employed by a company in Offenburg. Full time, five working days. Every day happens either in Germany – at the office or at another workspace on the German side – or at home in Alsace.
| Weekly pattern | Share in France | Competent system | What to do |
|---|---|---|---|
| 5 days Germany, 0 home-office days | 0% | Germany | Nothing. |
| 4 days Germany, 1 home-office day | 20% | Germany | Nothing – below the 25% threshold. |
| 3 days Germany, 2 home-office days | 40% | Germany, but only with the framework agreement | Joint application under Art. 16, obtain the A1 certificate. |
| 2 days Germany, 3 home-office days | 60% | France | The framework agreement no longer helps. The employer must register in France. |
Source: Regulation (EC) 883/2004, Art. 13 and 16, framework agreement on cross-border telework since 1 July 2023. As of 08/2026.
The interesting row is the third. Two home-office days feel moderate, but they already sit in application territory. A great many people work in exactly this pattern without ever having filed anything.
And now the swap
This is where the calculation gets practical. A workday that isn't tied to a place doesn't have to happen at home. It can just as easily happen at a desk on the German side of the Rhine – and then it counts as a day in Germany.
| Starting point | Change | New share | Result |
|---|---|---|---|
| 3 home-office days (60%) | 1 day → coworking in Germany | 40% | Back under 50%: Germany stays competent, with an application and an A1. |
| 3 home-office days (60%) | 2 days → coworking in Germany | 20% | Under 25%: no application and no A1 certificate needed. |
| 2 home-office days (40%) | 1 day → coworking in Germany | 20% | The application drops away entirely. |
Own calculation on the basis of the legal sources cited above. As of 08/2026.
The gap between row one and row two is the actual point. One swapped day rescues the competence but still demands paperwork. Two swapped days make the paperwork unnecessary. Given the choice, most people find the second version considerably more relaxing – and it costs nothing beyond deciding where to open the laptop.
Why a coworking day in Schutterwald is a working day in Germany
The attribution is unspectacular: what counts is the place where the work is actually carried out. If you sit at a desk in Schutterwald and work, you are working in Germany – regardless of where the employer is based or where the laptop is registered.
The tax picture fits: Schutterwald is about 13 km from the border and therefore inside the 30 km border zone. So a coworking day here also consumes nothing from the 45-day limit that protects frontier-worker status. From Strasbourg it's roughly 20 minutes.
For employees without frontier-worker status – someone from Strasbourg working in Stuttgart, say – a second effect comes in: days worked in France lead to a salary split and therefore filing obligations in both countries. Days in Germany do not.
For employers: the paragraph to forward
The most common obstacle isn't the rule, it's the conversation with HR. So here is a paragraph you can copy and send on.
For the employer the interest is tangible: if competence moves to France, they have to register there and pay French contributions. That's doable, but it's a project – and rarely proportionate for a single employee.
Where the model reaches its limits
For the calculation to stay honest, the caveats belong with it.
- The assessment is forward-looking. What counts is the expected distribution over the coming twelve months, not any single week. One week with four home-office days tips nothing; a permanent pattern does.
- Part-time shifts everything. On a 4-day week, one home-office day is already 25% and therefore exactly on the threshold; on three working days it's 33%.
- The framework agreement covers telework only. Client meetings, field work or site visits in France don't fall under it – they count, and the Art. 16 application can't rescue them.
- It applies to two states only. If you also work regularly in a third country, you fall outside its scope.
- Tax and social security are two calculations. The 45-day rule and the 25% threshold measure different things. A pattern can be spotless on tax and critical on social security.
- Nothing gets decided in a blog post. This calculation is a starting point for the conversation with your fund, your employer and your adviser – not a substitute for it.
Frequently asked questions
How many home-office days can I do as a cross-border commuter?
On a 5-day week, one day (20%) is unproblematic. Two days is 40% and therefore above the 25% threshold: Germany stays competent, but only with an Art. 16 application and an A1 certificate. From three days (60%) France becomes competent, and the framework agreement no longer helps.
Does a coworking day in Germany count as a working day in Germany?
Yes. What counts is the place where the work is actually carried out, not where the employer is based. A working day at a desk in Schutterwald is a working day in Germany – for social security and for tax alike.
What is the A1 certificate?
It documents which social security law applies to you. With an Art. 16 application under the telework framework agreement, it is the evidence that German law continues to apply despite more than 25% telework. It is applied for in Germany through the responsible body, usually initiated via your Krankenkasse.
Is it calculated weekly or yearly?
Forward-looking over the coming twelve months. A single unusual week doesn't change the attribution; the expected pattern decides. Which is exactly why it's worth choosing the pattern deliberately rather than letting one form by itself.
Does this apply to tax as well?
No, these are two separate calculations. For tax, the 45-day rule is what counts for frontier workers, and home office in Alsace consumes none of it because your home is itself inside the border zone. The 25% and 49.9% figures apply to social security only.
Takeaway
For a cross-border commuter, "home office or coworking?" isn't a question of style but a calculation with two thresholds. If you spend at most one of five working days at home, you stay in the German system with no application at all. Everything beyond that is possible but costs paperwork. The home-office days calculator helps you keep your own share in view.
Official Sources
- EUR-Lex: Regulation (EC) No 883/2004 (Art. 13, Art. 16) →Substantial part of the activity, the 25% threshold, exception agreements. Accessed 08/2026.
- Framework agreement on cross-border telework →In force since 1 July 2023, Belgium as depositary, with the list of signatory states. Accessed 08/2026.
- DVKA: information for cross-border commuters →Competences and the A1 procedure from the German side.
- Frontaliers Grand Est →Free Upper Rhine advice service – for employers too.
Try one of your remote days on the German side of the Rhine? The trial day at Innergarden is free – 20 minutes from Strasbourg.
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