Approval for wastewater property connection line

Hello,
for the building permit or the application for connection to the public sewer network, the building authority requires proof of the hydraulic capacity of the shared property connection pipe.
Who calculates or prepares such proof? The architect explains that he is not responsible for this.
Kind regards,
mcox

The architect is funny. Who then?

Which [LP] have you commissioned?

How many joint partners are there then?

So far, two single-family houses are connected. Now, one more single-family house will be added. The supply line is DN 150, approximately 30 meters long, on a private, jointly used driveway to the public street, which has a DN 300 line. It is a combined sewer. Several supply lines to the public street, with 4 terraced houses and 6 prefabricated garages, also have a DN 150 supply line. Sorry, what is LP?

LP is the service phase e.g. 1-9

Are there 8 supply lines to a 300 line?

The proof must be created by a specialist planner for pipeline construction.

Since this is not so easy, the building authority does not want to do it. However, they should have already provided such proof for the construction of the channel. Or is a previously unplanned house being added now?

In 1986, four building plots were created, on two of which a single-family house was built, drained with a DN 150 pipe. Now our house is added.