I have taken the list from the construction contract of my house provider. Unfortunately, I did not notice that the calculation of the floor area ratio AND the plot ratio is missing in the list... (as already mentioned, I am doing all this for the first time....)
The following results came from my phone calls today: The land registry office had no idea and immediately referred me to the building authority. In a conversation with a city building councilor, I was informed that it is common practice to submit an official site plan, even if it is not strictly necessary. Because whether it is necessary or not would only be checked upon the submission of a non-official site plan. And if the caseworker then thinks they actually need an official site plan, the processing time of the building application is delayed due to the additional request. And naturally, nobody wants that... Therefore, in practice, it is handled in such a way that an official site plan is generally submitted, even if the regulation does not require it in most cases.
Even though I myself am a caseworker in an authority and write official decisions, building law is unfamiliar to me. Therefore, I cannot assess with absolute certainty whether we definitely do not need an official site plan. Of course, we do not want to risk a delay in the processing of the building application. Therefore, my current view on our further procedure is: commission an official site plan from the publicly appointed surveyor (costs €2100 according to the fee schedule) and commission the rest from the non-publicly appointed surveyor.