If you're in the UK and you're really making something in your spare time that might be worth anywhere near as much as your paid income you should really be contracting/self employed. The company will be wondering why you aren't completely focused on their work as generally managers won't understand why people are doing their work as a hobby as well as having a free time job. I do know a couple of employees who added exemptions to their contracts, but if your aspiration is really to be earning from your own projects you need to be considering contracting.