I've managed people who had side projects and I've been the person with a side project reporting to a manager. Both sides taught me things.
The default instinct is to keep it completely hidden. I understand why. But here's what I've actually seen happen when people take both approaches.
When it stays hidden: Works fine until something goes wrong. The project starts taking off and you're mentally distracted. You accidentally send an email from the wrong account. You need to leave early for a customer call that's really a side project call. Or the project gets press and your manager Googles you. All of these have happened to people I know.
When they disclosed proactively: Most managers I know, including myself, do not care if you have a side project as long as: it doesn't compete with the company's business, you're delivering at work, and you're not using company resources. That's genuinely the whole list.
A few things that help if you decide to disclose: Frame it as "I want to be transparent" not "I need permission." Lead with the "it doesn't compete" angle. Be specific about what it is and why there's no conflict. Don't over-explain the revenue potential (upside makes some managers nervous in ways that are irrational but real). One sentence, then let them respond. Don't fill silence with more words.
When to NOT tell your manager and tell HR/legal instead: If your manager is personally controlling and has a history of retaliating for things that aren't actual problems. If your company has a formal outside activities disclosure process (use that, not a manager conversation).
The case for disclosure: it creates a paper trail that it's known and not objected to. That matters if the project ever becomes a company.
The case against: in genuinely dysfunctional work environments, information you give voluntarily can be used against you. Read the room.
For 85% of people in normal tech jobs with a non-competing project: disclose briefly, in writing, and move on. The anxiety of hiding it is not worth it.