You should check your IP laws in your country. I'm in France but IP principle must be common to many countries. As soon as you create something, you are the au

You should check your IP laws in your country. I'm in France but IP principle must be common to many countries. As soon as you create something, you are the author and you own the IP. Your employer can only claim ownership if the creation was made during an explicit (written) mission to do so, by contract or if the creation was made by using "means" of the company (e.g. PC, data, time,...). The burden of proof is then on your employer, not you. You can retain partial ownership if you can prove that only part of the product were made in the condition yielding ownership to your employer. Remember that it is the employer that has to justify his claim of ownership, assuming you are the author and this is not contested. If such physical proofs would by accident disappear, then your employer could have a hard time to claim any ownership. But keep in mind the perfect "crime" doesn't exist. He might have copy of your stuff, witnesses, mails referring to the creation, etc. The best would be that he never heard of your side project in the first place and that you never used any means (e.g. PC, data, storage, time,...) of the company to create and work on your stuff. Now you know why it was 'Ok' for him that you worked on your side project. Don't waste your time with written things. Get your stuff away of him, if it is still possible. You will be in a much better position to negotiate if this should happen.