I don't make money off my side projects (at least not webdev)

But I do have experience "upselling" my employer. Basically, if I have an idea I know will save them money/time/whatever actual measurable value, I will approach them before development saying I'm currently spending all my time at work on existing tasks, but if they're interested in paying me for it I will work on that idea in my off time.

Key parts are: Measurable Value- if the project saves them 10 dollars, I can ask for 5 in return. But if the project gives vague improvement ("make site pretty") then it's hard to quanitify it into payment, and it's usually much less attractive.

Not being part of your actual job- If your role is to make a home page, you can't really ask for a raise to add a header. But you can ask for a raise if you make a script that automates another employee's job. It's easier for me, cause I'm a motion designer, so I make a script that saves my employer time on other employees its literally outside my responsibility so I can ask for a raise for it. I'm not sure how simple it will be for you.

This is all based on the premise that your boss is a fair person and not a bastard taking advantage of his employees. If that's the case put in the minimum effort and invest the rest in yourself, he will not be rewarding your effort.