Which German language level is required to apply for German citizenship?

Foreign citizens who want to apply for a German passport are required to have a relatively good understanding of the German language. If you are holding a B1 certificate then you already hold the proof to have the required language skills.

According to the Common European Framework of Reference for Languages, language skills are divided into the following classes: 

C2 = language skills at native speaker level (competent use of language)
C1 = fluent to business fluent language skills (competent use of language)
B2 = fluent language level (independent use of language)
B1 = good language skills (independent use of language)
A2 = in-depth basic language skills (elementary language use)
A1 = first basic language skills (elementary language use)

To obtain the German citizenship, you will need to have level B1 or higher (B2, C1, C2).

There are a variety of schools and training options in the internet, but what what will count at the end of the day is a passed test with one of the officially recognised language institutes.

The combination of the language certificate and the Einbürgerungstest will then show the authorities that you have high interest in this country and this will then be the best base for getting the German citizenship.