Construction cost subsidy by builders? Water connection & supply

Let's assume the builder applies for connection to the drinking water supply network. After receiving the offer from the water supplier, the builder realizes that not only the water house connection (basic price + connection length per meter) is charged, but also a construction cost contribution in a considerable amount. Before purchasing the property, the development area was undeveloped. The purchase contract stipulated that the seller bears all public charges in the sense of § 436 of the Building Code, i.e. development costs, beneficiary contributions, and cost reimbursements according to the Building Code. Is the construction cost contribution not already covered in the purchase price of the property or must the seller bear these costs?

To my knowledge, all costs related to the house connections from your boundary to the house are not a public burden. That means you pay these and other costs such as construction contributions if they have not already been paid.

Who acts as the seller here? Municipality or private person?

Private.

Even if that is the case, how do you then come to 10 meters connection length (500 EUR) for the connection and especially to 23 meters construction cost contribution? (5000 EUR).
Water house connection base price 1250 EUR.

...if the house connection room is about 3 meters away from the property boundary.

Minimum dimension or see fee schedule of the provider for the calculation basis (often property boundary length to the street)

Private.

Even if that is the case, how do you then arrive at 10 meters connection length (500 EUR) for the connection and especially 23 meters construction cost subsidy? (5000 EUR).
Basic price for water house connection 1250 EUR.

...if the house connection room is about 3 meters away from the property boundary.

You have to ask your supplier and, if applicable, the municipal utilities. The supplier sets the rate. For us, it is relatively transparent through a kind of fee schedule.

but also a construction cost surcharge of a considerable amount.

A construction cost surcharge is the contribution to the maintenance and expansion of the upstream infrastructure; that means the groundwater wells, etc.

The fees are mostly flat rates or are calculated based on things like plot sizes, etc.

It has nothing to do with the external development.