I did some research about "work outside your employment" in Germany and found that the main thing is about the amount of hours one can (or is allowed) to work. The idea here was that working more than the "normal" 8 hours a day, would have a negative effect on the actual job, i.e. you're too tired to do your job properly if you work an additional 4 hours at home. In plain English, if you work additional hours daily on a side project, you'll be too tired the next day at work, hurting your productiveness (and upsetting your employer...). I did not find anything relating to weekends and/or to software created in one's spare time.

Are you aware of any specific laws concerning this whole thing?