Are drainage costs justified? Calculation of floor area

You know this in advance if you have read the fee schedule. You should definitely do that because the construction cost subsidies are usually higher than the (flat-rate) connection costs of the internal development.

This also only serves cost allocation, i.e., even if [Keller] were not counted, the same costs would have been incurred that need to be allocated. Whether you then have 200x50€ or 100x100€ on the invoice ...

This also only serves for cost allocation, i.e. even if basements were not counted, the same costs would have been incurred that need to be allocated. Whether you then have 200x50€ or 100x100€ on the bill ...

That's not entirely correct because now the residents with basements have paid proportionally more than those without.

Oh, so the others are supposed to pay more than you, I see

If you have more space ..

That's how it is. From the area, one can deduce how large the household is, ergo how intensively the infrastructure is used. I find that initially comprehensible. Then there's the aspect of application review or invoice creation. Who should deal with [Grundflächenzahl] etc. to ensure legal certainty? So they simply calculate the external dimensions (so not living space or other debatable figures), e.g. 10x12x3. Anyone without much training can read and calculate that from the plan. Wonderfully simple.

That's how it is. From the area, one can infer how large the household is

Well, the 10*10 bungalow with 2 people and an unfinished attic (storage = basement substitute) uses the infrastructure just like the 10*10 bungalow with a flat roof, but with a basement and 2 people. However, the bungalow with a basement pays double the fees!
Living space would be fairer (although never absolutely fair, that will never exist).