You can do this and I know of people who have done this. In virtually all cases, it's never going to come up. But, for companies that are batshit anal about si

You can do this and I know of people who have done this. In virtually all cases, it's never going to come up. But, for companies that are batshit anal about side-projects, they're going to make you sign some onerous contract and have management that are assholes about this kind of thing. What happens in the event that you "hit the lottery" and some side-project becomes a big deal worth a lot of money? Somebody from your company is eventually going to find out about it: maybe from a public interview or from public records. If they put 2 + 2 together about when you built this and figure out that they can file some legal claim to it, you're in a mess of a legal situation that you don't need to be in. Safer to put your cards on the table and make sure there's nothing in an employment agreement that puts something you really care about at risk. The percentage of companies that are very anal about side-projects isn't substantial. They need you more than you need them and it's safer to be honest and look elsewhere for employment.