Side business (Nebengewerbe)
Also: Nebengewerbe, Nebenerwerb, nebenberufliche Selbstständigkeit, part-time self-employment
In short
A side business is a trade that someone runs alongside a main occupation, such as employment or studies. Like any trade, it is registered with the trade office.
Side business (Nebengewerbe) explained
There is no separate law for side businesses. The same rules apply as for any sole proprietorship, including unlimited liability. For VAT, the small business exemption may apply if total turnover did not exceed €25,000 in the previous year and does not exceed €100,000 in the current year. Profit is taxed together with your other income. Whether you need to inform your employer or ask for consent usually follows from your employment contract. Students should clarify the consequences for health insurance, BAföG student finance and child benefit in advance.