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The Finanzamt are legally required to give you advice.
Yes, this is true, but they are not legally required to help you optimise your tax situation.
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When you say "Umsatz", you're talking about my gross earnings I guess? So the Steuerberater will only charge according to how much I earn?
I don't know all the ins and outs of the details, but in principle, yes, that's what I mean. The only slight problem could be, that it may not be possible to just get the Steuerberater to handle the self-employed bit, which I assume will not actually amount to all that much, if it was basically just a summer job, on the side. Obviously, if you get him to do the whole tax return, then your employed income flows into the equation as well which may not be desirable if this is significantly (say five or ten times) more than your self-employed income. I'd discuss the situation with him and try and persuade him just to do the self-employed bit, after all, this is a separate form on its own which can easily be filled out separately.
On the other hand, if your self-employed earnings only amount to 1000€ - 2000€, then he may not be very interested in taking it on anyway. However, in that case, then you probably can get by by doing the return yourself and, as KäptnKnitterbart says, take advantage of whatever help you can get from the Finanzamt, contrary to the general impression/assumption, they're not all bastards out to screw you, approached in the right way they can be quite friendly and helpful.