General rule for Germany is, that your employer must allow you to have a job besides your main job.

This does probably not really fit to what we understand as "side projects" but if you are making serious money and invest time in it, you´ll have to talk to your employer.

Employers will most likely do not allow it, if it´s very time consuming (you need to be fit your main job) or very related to your main job (say: freelancing graphic design while working for a design agency).

I found that it really depends on the employer and there is no general rule.