A few years ago, during my apprenticeship years I wrote a confluence plugin (server), that was later released under the MIT license and published on Github. My employer has since lost interest in the project. For a while I maintained it during my free time and it had a small user base. But when the GDPR was established the project was pulled from the Atlassian Marketplace and I stopped maintaining it. Since then it is basically abandoned.
In the following years I finished my studies, switched teams and became a full time employer with the same company. ~A year ago I reduced my hours, to have more time for private projects and private stuff. Which they weren't cheering about, but also agreed to, without making any issues. There are a number of part time employees at the firm, tough most of them are a lot older than I am.
A few month ago I founded a web agency with two friends, that I work on during my off hours. My employer is aware of that and fine with it. Also by German law I'm allowed to do so as long as it does not effect my main work and my employment contract contains no clauses forbidding for me to do so.
Me and my friends have ported the concept of the original server plugin to a confluence cloud plugin and added a few features. For this we incorporated a small part of the original code (as a maven dependency). We now want to sell this plugin on the market place. If this turns out to be profitable we would also like to overwork the original server plugin, rebrand it and sell it with support on the market place.
Now my question: Should I inform my employer of my plans?
The relationship between myself and my bosses is very good. But I'm afraid of waking a sleeping dragon, by asking for permission. And if they say no I have to either comply with that or openly refuse to do so, potentially even quitting my job.
On the other hand if I just go ahead with it, I doubt anybody will notice or care about it. And I don't think that I'm legally obligated to do so.