Getting a second job?

I genuinely don't know and have never been in that position before. But if you were looking for a second job, would you need to ask permission from your current job?
I work part time - mornings only. Signed a contract. The rest of my time I take on freelance work, but it's never been my goal to do any freelance work. I like the amount of money I get monthly, to be the same; not dependent on whether I've had enough freelance projects on the side. So I'm thinking of getting another job.
What's been your experience with getting a second job?
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Well, as it turns out, my boss asked me what I'd need to stay full time. So that's the new negotiation in the works. Yay!
Getting a second job?
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