If it's an add-on to your employers software, yeah. Either you need to let them know up front and get their okay, or keep it secret and release it after you qui

If it's an add-on to your employers software, yeah. Either you need to let them know up front and get their okay, or keep it secret and release it after you quit that day job (and pretend that it was entirely developed after you left the company, and even then you're not safe if you had access to any relevant trade secrets while you were employed). Even when your contract lets you own the IP that you create on your own time, there's usually a really broad exception for anything that somehow interacts with or competes with the employer's business. YMMV, IANAL, this is not legal advice. If you're serious about doing it without your employer's blessing, double-check your employment contract and local law (I don't think the law anywhere in the US would protect you in the stated scenario) and hire an attorney.