The situation isn't any different in NL either. Although there is no blanket your-company-owns-your-**s policy (your IPs), the general rule is: if you create something in your spare time that your employer could well have paid you for (i.e. it's in line with your job description), the IP belongs to your employer.

The idea behind this is that your employer pays not just for your time, but also for the skills you acquire during your work. Therefore, the output of those skills has at least partial ownership. There's a huge gray area which contains liabilities and opportunities for both sides, which is why most companies insist on signing off on personal projects. The alternative is fighting it out in front of a judge, should it ever come to that.