Whereas the labour market for EU citizens has been liberalized, the German employment regulations for other nationals are extremely strict. As a rule, non-EU candidates cannot finance their dissertation by taking up a job and working.
The general prerequisite for obtaining a residence permit is proof of sufficient financial resources, i.e. you need to have organized your financial resources before departing for Germany (e.g. grant or scholarship).
You can look for a temporary job to finance some extras, assuming that you are enrolled as a doctoral candidate.
• Students coming from the EU, EEA countries and Switzerland have the same status as German students. Exceptions are students from Bulgaria, the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Poland, Rumania, Slovakia, and Slovenia, who, under the EU Accession Treaty, only have restricted access to the German labour market until 2011. Before they can take up a job, they always have to obtain approval from the Federal Employment Agency (Bundesagentur für Arbeit).
• Students from countries outside the EU are allowed to work for 90 full or 180 half days per year – up to 4 hours count as half a day. If you plan to work for a longer period of time, you first need the approval of the Federal Employment Agency (www.arbeitsagentur.de) as well as the Foreigners’ Registration Office. To work as a freelancer or to run a self-employed business is not allowed.
• Student assistants and academic assistants are allowed to work for a period longer than 90 full days or 180 half days a year, as long as the job is relevant to their academic work and does not slow down their studies. The approval of the Foreigners’ Registration Office is necessary (you will get a stamp in your passport).
• Foreigners graduating from German universities are allowed to stay in Germany for one more year after graduation to look for a job (approval of the Foreigners’ Registration Office has to be obtained in advance).
• The income limit for all scholarships funded from public sources is € 4,800 per year (according to the scholarship contract).
If you take on a job with more than 20 hours a week, you also have to pay for unemployment insurance!
More information on taxation and social security can be found here.