Source: https://www.wir-gruenden-in-deutschland.de/en/auslaendische-studierende-doktoranden-aus-nicht-eu-laendern/basic-infos/what-do-i-need/
Timestamp: 2019-04-23 17:58:46+00:00

Document:
Your self-employment must not compromise your degree or post-doctorate. As such, your self- employment must be a part-time occupation; your degree or post-doctorate represents your main occupation. In Germany there are no limitations concerning when a part-time role becomes a full-time role. However, in order to ensure that you do not lose your student status, you must not spend more than 20 hours a week on your self-employment. If you work for more than 20 hours you will lose your student status with your health insurer – in other words, your health insurance will become considerably more expensive.
For the self-employment you need to fulfil the requirements placed on the profession in Germany in which you wish to set up a business. You can find out what these requirements are under "What am I allowed to do?".
You must apply to your local immigration authority (Ausländerbehörde) for a permit to practise self- employment (Erlaubnis zur Ausübung einer Selbständigkeit) in line with § 21 para 6 of the German Residence Act (AufenthG). This permits you to work on a self-employed basis. As a student or postgraduate, you hold a residence permit in line with § 16 para 1 of the German Residence Act. You will continue to hold this legal status. You can find out what § 16 and § 21 of the German Residence Act say here. The immigration authority will check to make sure that your planned part-time self- employment fulfils the other conditions stated.
Your permit granting you the right to self-employment does not affect your legal right to work in an employed role as stated in § 16 para 3 of the German Residence Act. You are still permitted to carry out 120 days of full-time work, or 240 half-days.
"We recommend that you draw up a business plan for your part-time self-employment. Business plans do not just help to give you a better overview of the various steps and requirements required to set up your business, and to help you reflect on these; It will help your application for a permit to practise self-employment in line with § 21 para 3 of the German Residence Act. You can use your business plan to show the immigration authority (Ausländerbehörde) that your self-employment is intended to be on a part-time basis and that your studies will remain the priority."
Inquire at the immigration authority for the reasons for the refusal.
If formal reasons are given, for example missing documents, then hand these in as required.
If the reasons for the refusal concern your business idea and its implementation then revise your business plan and/or seek support – ideally from the business advisory service that has already assisted you. If you do not have a business adviser then find one, for example at your university, or get in touch with one of our advisers.
If the reasons given concern your personal and entrepreneurial skills then ask your business adviser – either at the university, at the local Chamber of Commerce or at the business development organisation – whether there are any seminars or courses available that will allow you to develop the necessary expertise. You can also contact one of our advisers who will do their best to assist you further.
If you receive a positive decision to alter or extend your residence permit, the next step for starting your self- employment in Germany is to register the business or register your self-employment at the local tax office (Finanzamt). The latter applies to freelance professions.
In order to register your trade, go to your local trade office (Gewerbeamt) – the office in the town, district or region in which you want to become self-employed. You can find the contact details for the trade office in your local citizen's support office (Bürgerbüro), or search online at: http://www.bmwi-wegweiser.de/suche/behoerden/. Select "Gewerbeamt" and enter the postcode of the town in which you wish to found your company. The contact information and opening hours for your local trade office will appear on screen. The trade offices' websites often feature forms for registering a business: Some offices expect you to have already completed these forms before coming to the office in person.
If you want to become self-employed as a freelancer, you must register at your local tax office (Finanzamt). There is no form to be completed for the registration; instead, you must write a letter with a short description of the business that you intend to set up. The tax office will send you a "tax registration questionnaire" (Fragebogen zur steuerlichen Erfassung) (Download) and will issue you with a tax number. Aside from registering with the tax office, most freelance professions require the fulfilment of certain qualifications, financial requirements and/or other conditions. You can find out which ones here.

References: § 21
 § 16
 § 16
 § 21
 § 16
 § 21