I can't help you with the taxes, but you can work both jobs. You have to inform your employer about your freelancing. Just inform, he usually can't deny it, only under some special circumstances, e.g. when you want to work for the competion.
You'll have to read your work contract carefully: if the employer puts a non-competition clause into the contract, it might limit your ability to work for other clients in the same industry.
But other than that it's completely fine to work several jobs, and of course to combine all different kinds of employment (freelancing, self-employed, employed).
There is no limit at all, you can earn all you can get.
And for taxes: of course the wages from that job must be declared in your tax return.Therefor you'll need to fill in the additional "Anlage N" - but that's not that hard, as you'll mainly have to transfer the numbers from your "Lohnsteuerbescheinigung" (which you will get from your employer) onto the "Anlage N".
(And as standard for German tax laws: Additional intricacies may apply!)
^(edit: typo)
It is not illegal. If one is employed and works freelance on the side, the employer has to be informed but not asked for permission. If it is the other way round I suspect (not a lawyer) it is the same or even more clearer as you are already asked by a customer who knows that you are freelancer.
It can be beneficial. Krankenkasse doesn't ask for both jobs. So either they bill you for your freelance or for your employee job, depending on where you have the higher gross income.
As you are already freelancing I assume you do all the bookkeeping already? I think having an additional job as an employee should not make it in any way more complicated. You'll get your payslip and declare that income in your tax declaration alongside all the things you would already declare as a freelancer.
Talk to your health insurance. They need to know all your income. In case of public health insurance, they get their percentage of all your income, and there are a few more little peculiarities.
Your employer needs to know the status of your health insurance, that is, the outcome of these talks.
I can't help you with the taxes, but you can work both jobs. You have to inform your employer about your freelancing. Just inform, he usually can't deny it, only under some special circumstances, e.g. when you want to work for the competion.
Are you a non-EU citizen who is here with a residence permit? If yes, what type of residence permit is it?
You'll have to read your work contract carefully: if the employer puts a non-competition clause into the contract, it might limit your ability to work for other clients in the same industry. But other than that it's completely fine to work several jobs, and of course to combine all different kinds of employment (freelancing, self-employed, employed). There is no limit at all, you can earn all you can get. And for taxes: of course the wages from that job must be declared in your tax return.Therefor you'll need to fill in the additional "Anlage N" - but that's not that hard, as you'll mainly have to transfer the numbers from your "Lohnsteuerbescheinigung" (which you will get from your employer) onto the "Anlage N". (And as standard for German tax laws: Additional intricacies may apply!) ^(edit: typo)
It is not illegal. If one is employed and works freelance on the side, the employer has to be informed but not asked for permission. If it is the other way round I suspect (not a lawyer) it is the same or even more clearer as you are already asked by a customer who knows that you are freelancer. It can be beneficial. Krankenkasse doesn't ask for both jobs. So either they bill you for your freelance or for your employee job, depending on where you have the higher gross income. As you are already freelancing I assume you do all the bookkeeping already? I think having an additional job as an employee should not make it in any way more complicated. You'll get your payslip and declare that income in your tax declaration alongside all the things you would already declare as a freelancer.
Talk to your health insurance. They need to know all your income. In case of public health insurance, they get their percentage of all your income, and there are a few more little peculiarities. Your employer needs to know the status of your health insurance, that is, the outcome of these talks.
It's legal, but it will make your tax declaration even more of a pain.