[tobilg]

Yes, but my point is that nobody will try to get self-employement status first if he/she doesn't have it, before even knowing whether the project is accepted.

This would have implications on the relationship to the full-time employer, and also from a tax perspective (which is quite a hassle here in Germany, IMHO...).

[Kliment]

You can get self-employment status in Germany for 26€ and about an hour of waiting. It's a single form to fill in, and doesn't have any tax implications until you actually start receiving income from it. Your full-time employer doesn't need to be notified or asked for permission. (your contract may require notification if you actually perform additional work, but getting the status itself is extremely low hassle)