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: fr…
An employer can require you to inform him about side jobs but is legally not allowed to prohibit you from taking a side job unless you work against his interest (eg. negative impact on your performance or competing with your employer).