Why do the prerequisites to enroll in PhD programs vary between research groups in Germany? I have been trying to get a Ph.D. position in Germany so I follow the advertising sites frequently. A few days ago I saw a position from a group of X Institute. I wrote the professor about the requirement and he replied a bachelor is a prerequisite and he encouraged me to apply. Today again I saw an advertisement for a position from a different group from the same institute and the professor said a master is needed as a prerequisite for becoming a Ph.D. student. My question is why the requirement varies from group to group whereas they come from the same institute? Can a professor or supervisor can play a role to waive master if he wants?
tl;dr: You'd typically need Master's. Read on to find out why.

The "traditional" German higher education system had only one kind of a degree before PhD, the Diplom. It's typically translated as MSc and it's not wrong: basically, it was a 5 years degree. You'd typically need this degree, and having it from a university (and not a Fachhochschule, a polytechnical university) in order to start a PhD.

Then, Bologna happened.

The current scheme in Germany is: 3 years BSc + 2 years MSc. Though the initial thought was to have a US system, where most of the students exit university with a BSc, it did not took off. Most companies and universities themselves saw "just" BSc as inferior and still required a MSc degree. It got a bit better nowadays.

It is possible to get a job with a BSc, even if it would pay less.

It is also possible to start your PhD with a BSc. But it's much, much harder. In many places special "fast-track" programmes exist. Details depend on the location. Typically, they allow a future PhD candidate to study for their MSc, but in a sort-of accelerated manner, for example, the MSc thesis is then allowed to be a topic from the future PhD. Further, typically the non-MSc holders are payed less, E12 level instead of E13.

All this requires, however, further, and in some cases non-insignificant actions from the advisor. So, there might be a certain willingness to employ a BSc-holder. It might be easier, if it's a four-years BSc. The typical "fits all" requirement is, however, a MSc.



A further notice. There are two sides to doing a PhD in Germany. 1) The general advisor-student relationship. It is about research and mentoring. No money is involved in this relationship. (You might want to register as a (PhD-)student with the university, it costs not much.) 2) The employer-employee relationship. The "funded position" is not really connected to the readiness to supervise a student. (Yes, it is often coinciding, but it is not required to be so.)

My point is: you should really ask very directly, does this particular professor really want to supervise you as a prospective PhD student (even with "only" a BSc), or do they do the former and have money for your position.