Herkum,
I would like to point out for those reading this thread that it very much depends on what you as the employee has signed regarding intellectual property. I know in this case,
dk indicated he had a verbal agreement but didn't mention what was signed when onboarded with the company. If you sign a legally binding contract, as a number of employees in IT do, not only can the company lay claim to what you write on your free time - they can do so even after you are no longer employed by them. A non-compete and intellectual property contract is usually very specific in terms of time and area so it is best to read carefully before signing.
