How to legally have a 9-5 while working on the side?

Register for self assessment, you'll be able to log extra income. Check your 9-5 employment contract to see if you're obliged to tell them about any side hustles or second jobs. Note, if your side hustle means your tax code changes, you'll need to communicate that code to your employer, so even if they don't know HOW you've earned extra money, they'll know you have done. Generally speaking, the best thing to do is to do it with knowledge and 'blessing', that way you know you're not messing with your 9-5 at all. Some employers are completely fine with it anyway, sometimes they can be a bit weird about the 'competition' but in my view, the stuff I'd do personally is too small for my employer anyway, so you just need to make sure they understand that. Note: some employment contracts expressly forbid it, in which case you have a fight on your hands if they ever find out - it has to be a reasonable condition though, such as protecting IP, avoiding competition etc. They can't stop you working as a florist if you're a mechanic by day, for example, but they could stop you doing mechanic jobs on the side.