Responsibility Drainage Plan

So for me, the word "entire" before "building application documents" also includes the dependencies in the form of other plans/documents, as long as otherwise no complete building application can be submitted. See

Furthermore, it is usually always better and more efficient if the "experts" coordinate directly, otherwise it becomes a game of broken telephone. However, it sounds as if the general contractor wants to minimize his share of the work...

Due to professional and liability reasons, many BT-GUs employ independent contract architects. The risk (of the GU towards the client) that municipality Y requires more special documents than municipality X will probably be unsatisfactorily regulated in terms of remuneration for the architect.

Moreover, it is generally always better and more efficient if the "experts" coordinate directly; otherwise, it turns into a game of telephone. However, it sounds as if the general contractor would like to minimize his share of the work...

It is indeed the case that the authority is demanding something here that is rather unusual (if I understood correctly), namely proof of the calculation of the necessary dimensioning of an infiltration system.
The architect was (rightly) of the opinion that it would be excessive to have an engineer perform such a calculation (cost probably about 200 euros) – for a completely ordinary single-family house.
This calculation would ideally have been done by the respective manufacturer of the infiltration shaft, but:

Due to professional and liability reasons, many turnkey general contractors employ independent contract architects. The risk (of the general contractor vis-à-vis the client) that municipality Y requires more special documents than municipality X is presumably unsatisfactorily regulated in terms of remuneration for the architect.

That is probably the case.

In the meantime, I have agreed with the city that this calculation is not necessary if we submit a corresponding soil report specifying the infiltration capacity.

Well then, that's fine. Just as another example: In our municipality, the inspection shaft for wastewater (with internal drop) has to be tested for tightness. The general contractor was not aware of this at the time of contract signing either, but ultimately it was commissioned and paid for without complaint.

: The correct sizing of the infiltration system is free of charge with most manufacturers and dealers. This also makes sense, as there are worlds/magnitudes in permeability between karst and clay, and thus in the size of the soakaway/your shaft.

For this, you need the soil survey report including the determination of the infiltration coefficient. With this and the drained area and surface condition, you then get your sizing for free.

The preparation of the entire building application documents"

Now the architect of the general contractor is telling us today that we have to commission a drainage plan because she needs it to complete the building application. How would you see this, who is responsible and who has to pay for the drainage plan?

For us, the drainage plan was included and was also mentioned accordingly.
This was done by the plumber at the request of the general contractor (if I'm not mixing anything up).

Either we are supposed to commission a planner (I have no idea who does that or what it costs)

This calculation is probably done by various suppliers of soakaway pits.
Can you recommend a manufacturer to me?

Infiltration is often a separate matter.
While the drainage plan shows from where the greywater is led out of the house and into the sewer, the infiltration of rainwater from the downpipes is a different matter and definitely has to be arranged by the builder. The earthworks contractor you trust is the contact person for that.

Moreover, from experience, it is always better and more efficient if the "experts" coordinate directly, otherwise it becomes a game of telephone. However, it sounds like the general contractor wants to minimize their share of work...

I think so too. Apparently this will no longer be included here.