Many companies have clauses in your offer/contract for situations like this. They often own what you do in your spare time as well as while you are on the clock

Many companies have clauses in your offer/contract for situations like this. They often own what you do in your spare time as well as while you are on the clock, as the lines are often blurred. I think the first step would be to talk to HR (or if you don't want to do that, take a look at your contract, read it closely, or hire a lawyer) to see what would happen if worst came to worst (if your company took legal action, regardless of if you develop it while working for them, or shortly after quitting).