Wow, okay. I honestly didn't believe that was legally possible - and thanks for pointing out that I'm probably wrong. I checked the wording of my contract, and regarding "my employer has ownership of my work" it explicitly says "im Rahmen dieses Arbeitsverhältnisses" -> "as part of this employment relationship". So I suppose I'm safe (there is a clause that I have to ask for permission to do side jobs; but I know company policy is that permission is always granted and this is about informing them about it).

edit: see geocar's reply.

I asked about this before joining my current employer: I'd like to write a tetris clone and sell it on the AppStore (that was the specific example I used). I promise I won't use company resources. They said please don't. It's a grey area. Worst case you'll have to explain to a judge why your code doesn't infringe on anything the company owns or does. Even if it's something as trivial as a tetris clone.

You are however fairly safe if it's completely different from your work, for example, pottery or writing fiction (assuming your company does neither).

As for "as part of this employment relationship", I'm not a lawyer but I wouldn't assume that it applies only to what you do at the office. It might very well apply to anything while you are employed there. You need to clear the scope of that with HR or a lawyer.

You definitely don't want to base your company on shaky legal grounds. You might lose your company, your product, and your job. It probably won't look good in your Arbeitszeugnis either.

If it doesn't work out Then try again.

If everyone acts in good faith Then nobody should end up in jail.

If too many people end up in jail Then major problems could occur.

Right, and it also depends upon your role (I was in a leadership position). Regardless, the law is intentionally vague (“could be related to your work” leaves room for interpretation), and it’s better to get explicit permission.

I'd say it's always better to ask forgiveness. They probably won't find out anyway, so why waste everyone's time (especially your own.) You'll spend more time writing useless emails to HR / lawyers than working on your side project. The simplest thing is to just do your thing, keep it to yourself. In the very unlikely event someone brings it up, you just play dumb. (Your Tetris clone isn't infringing on the company anyway.)

I'm also from the Netherlands. Can you give examples of cases or precedent because all Dutch lawyers I know think of this law as a joke because it is almost unprovable in court.