Basically "No, but yes."?

It very much depends on the sick leave though. If it's for a broken leg, no one is going to get offended that OP spent his time coding.e)

@Flater exactly, in this case "coding" != "working".

@CPHPython: Change "coding" in my comment to "working on a personal project" and nothing changes about what I said. The issue isn't so much what OP did during his sick leave, but rather if it obviously contradicts his justification for needing sick leave.

@Flater I was agreeing with you. A construction worker with a broken leg may code all he wants while recovering. Why can't a professional coder do the same?

@Flater when I am on sick leave and doing stuff on my own I can stop and rest anytime I need: I am on my own and can dictate my pace. My line manager will not like if I tell him that I need to take some painkillers and sleep for 4 hours right after clocking in.

@Mindwin: I'm not quite sure what you're responding to, I never argued against that point.

"However, what you do in your own time while you recover is your own choice." - This is wrong. If you do anything that can be thought of as work (and coding your own projects can), your employeer can fire you for not being sick when you are away from work being sick. It will depend on jurisdiction of course.

@Bent I get paid while getting coffee at work. If I decide to make coffee while home sick, I'm basically not sick and should be at work, right? What about thinking? I also get paid to think of ideas. So what happens if I happen to think of a good idea while home sick? I did work, I could be fired! I mean, how pedantic shall we get? Of course you can be legally fired for faking sick leave, but you'd be lucky to find a judge that would consider this to be so.

@Clay07g: That's a good straw man you are trying to set up there but I think anybody reasonable would agree that "I was too sick to do my job coding so instead I did some coding" is the kind of thing that looks bad. People have given reasons elsewhere why you might not be able to make it to work but still be able to code but that doesn't detract from the fact that this looks bad on paper. And sadly on paper is where getting fired starts. Sure, you might be able to take it before an employment tribunal and win but why risk it?

@Chris You don't have to phrase it like that on paper. You can very easily say "I was too sick to reliably produce quality code without risk for an important client on a project that is costing the company thousands of dollars, so instead I updated the CSS on my personal website to make it blue instead of green". You can say that's a strawman if you want, but this is the reality of a programmer. "Coding" covers an infinite range of activities.

@Chris I would disagree with you on that. The problem with your rationale is that the entire job is NOT coding. The entire job would also include transport to and from work, speaking with clients, being available and ready to work for the entirety of a shift. Whereas at home the employee can be comfortable, not have to speak with others, not have to actually travel to work, and can take frequent nap-time breaks of indeterminate time periods. Writing a few lines of code at home and spending eight productive hours in the office are not remotely equivalent.

@UpAllNight: There are no hard rules on this. Some employers would be fine with it. I'm sure there are some out there that wouldn't though. Personally I would assume worst case scenario and not take the risk. Individuals will of course have their own views on how much they are willing to risk it based on factors such as their knowledge of their employer. I would personally stick to my broad advice though that I wouldn't do anything that could look a bit like work if you squinted funny and turned your head to the side a bit if I didn't have to.

Staying away from work sick typically has nothing to do with whether one would be able to carry out typical work duties (and almost always, one would be able to do so in some capacity). It is a often matter of courtesy and public safety, so that other employees are not placed at risk of catching a contagious disease.

@Flater In many IT shops, remote work is supported enough that one could still be fairly productive from home with their broken leg. It would definitely reflect poorly on someone if they forewent this option to work on personal projects.

@jpmc26 But if they work remotely, they're fully working and are not on sick leave, so they have full pay. If they're on sick pay but are expected to work at full capacity, remotely, then there's something wrong.

@Nelson Yes, that's the point. If they are able to work remotely but are not doing so, this reflects poorly on them.

@jpmc26 but it's also what the employer allowed. If I broke a leg, and am living alone, even though I'm able to work remotely, I would not be able to easily buy groceries after work, then there are issues due to logistics that prevent me from working because I am unable to do the other tasks to sustain myself. It's not a hard and fast rule that "If you can be productive, you must work." What if I honestly cannot buy food within a reasonable amount of time? I've broke a hip before... it takes absolutely forever to dress myself. Even going to be bathroom was a chore.

@Chris no one gets paid to write code. People get paid to solve problems for clients. When I am coding at home, I am not delivering any solution to any client problem, therefore what I am doing is not "work".

@jpmc26: Working remotely is not the topic of conversation, as OP would then not be on sick leave, which is the core of the question. WFH is an irrelevant discussion here.

I know this was added a long time ago, but I dare say the last paragraph of the answer isn't... always true... And depending on company culture this may be beneficial to share findings after coming back from work. Something along the lines of "I did some reading into this new methodology XYZ while I was sick and this could be quite useful for us" should be well received. YMMV, however (again, depending on the company culture).

@Flater Nonsense. The core of the discussion is whether doing something similar to work activities is appropriate when you've told your employer you're taking a sick day. The problem is that this (at least intuitively) may give the appearance of impropriety, since perhaps you could work. The question of whether you should even be on sick leave at all is thus obviously relevant.