Are drainage costs justified? Calculation of floor area

The rules are the rules. Discussions about justice are completely separate from this. The decision can only be contested if the rules have been broken.

It was not about justice for me but rather that one probably has to read the entire municipal code beforehand in order to know what costs will come up. Terms like floor area are used for the calculation, which are then calculated differently by statute than they are otherwise defined.

But apart from that, I also do not understand why having a basement should result in more wastewater than without...

Even if one has read the legal codes or more precisely the contribution statutes of the municipality here, sometimes it doesn’t help. The fees have now been increased retroactively from 01.01.2020 by almost 40%. At the time of the property purchase (beginning of 2019), the redemption amounts in the notary contract were based on the old amounts. Annoying, but unfortunately there is nothing one can do about it. Sometimes you lose and sometimes the others win

At the end of the day, you have to be glad if you at all got an appealing property and can enjoy it.

Well, the 10*10 bungalow with 2 people and an unfinished attic (attic = substitute for a basement) uses the infrastructure just like the 10*10 bungalow with a flat roof, but with a basement and 2 people. The basement bungalow, however, pays double the fees!
Living space would already be fairer (though not absolutely fair, that will never exist).

You can also live alone in 400 sqm. Sure. But some standard has to be applied, and the one chosen here is a particularly simple one.

And I clearly disagree with the OP, the wastewater statute is a crucial document for budgeting the connection costs for construction. Just as one reads from the power supplier or the telecom what the connection costs are.

The people who receive plots from the municipality save anyway several thousands of euros...

I had a case myself, which also concerned the contribution due for connecting the house to the sewage treatment plant. This was based on the number of full floors. I was counted as having two full floors. I then sent a drawing showing that the upper floor is not a full floor according to the state building code. The response was that this did not matter; one could simply count all areas with a height above 1.40 meters. Fortunately, in my case, it was only a low three-digit amount, of which I could have saved 1/3 if I had been able to "argue away" my upper floor. Therefore, I did not intervene further. But it is interesting how suddenly a "suitable" definition is dug up...