Once you start a side-project, there comes the time when you ask yourself where to host it. You probably started with a simple project page on your website. But inevitably, the more effort you put into the project, the more you feel the urge to host it on its own domain or sub-domain, am I right?
my-cool-project.com or my-cool-project.ukena.de?
Both have their pros and cons. A sub-domain is usually easier to set up and often comes for free with your existing domain. A sub-domain also has the benefit of making your own relationship to your project more explicit: my-cool-new-project.ukena.de is immediately recognizable as one of my projects, while my-cool-new-project.com is not. Is that a pro or a con?
How I would decide
If there is only the slightest chance that you might one day want to sell or handover the project to someone else, then don’t use a sub-domain.
That’s it. If you are 100% certain that you will never part ways with the project, then feel free to use a sub-domain. Otherwise, it’s always better to use a separate domain. Handing over a sub-domain to someone else is not impossible, but it would always show your affiliation to your name. That might be something that the new owner would not want. Of course, they could transfer the content to a new domain, but with respect SEO and Google ranking, that is a non-trivial task and something you would want to avoid.