Are drainage costs justified? Calculation of floor area

We have been finished with the house construction for some time now, and now some more or less (in amount) unexpected invoices are arriving.

Among other things, we have received a notice regarding the construction of a wastewater disposal system. The contribution calculation is based on the floor area (Bavaria). According to Google research, this includes all full floors and the basement only if it meets certain conditions. Also, the development plan can apparently stipulate that other additional areas are included. Our development plan states on the subject:
Grundflächenzahl: 0.4
Geschossflächenzahl: 0.5
Areas of habitable rooms in floors other than full floors including their associated stairwells and including their enclosing walls are to be included in the calculation of the floor area (§ 20 Abs 3 Satz 2 Baunutzungsverordnung).

In the BayBo, I looked up the definition of habitable rooms, and it says that besides a height of 2.40m, they must also be able to be illuminated with a certain amount of daylight. This is only the case in 2 of our basement rooms, if at all.

The contribution determination also states that it is based on §§ 1 to 7 of the BGS EWS. This in turn says that the basement is taken into account with the full area. Has anyone experienced this case before? I find it strange that in an invoice there is suddenly a reference to a law that was never mentioned before in the development plan.

I have translated a notice that says: you have to pay based on your floor area number, how we calculate it is stated in the development plan and the BauNVO. Since it says there that your basement does not count fully, but now we use BGS EWS so that it counts after all???
Is it now correct that the basement is fully included in the contribution determination?

Reference to a law that was never mentioned before in the development plan.

The development plan actually has nothing to do with the assessment. In the development plan, you find the information on what you are allowed to build at most. Nothing more. It also does not state what the power line costs or the drainage.
The specific object is the basis for the assessment or the legal or municipal regulations.

Post the notice. Otherwise, the discussion will lead to nothing.

So I assumed that "Geschossfläche" is a defined term. I would like to respect privacy and therefore am only typing here the text of the notice:

Enforcement of the Municipal Charges Act and the Contribution and Fee Statute for the Drainage Statute (BGS-EWS) of the Municipality XYZ dated xx.xx.2007

Based on §§ 1 to 7 of the BGS EWS, the Municipality XYZ issues the following notice regarding the construction of a wastewater disposal system:

1) Determination of contribution
For the property xyz

2) Calculation of contribution

	Contributory area	Contribution rate
Floor area	343 m2	x Euro
Construction contribution		343 * x Euro

And then a bit of text about where and how I can file an objection. The building permit states 2 floors of 116 sqm each (ground floor/upper floor) and for the basement it even says: no full floor no habitable rooms.

That means the basement was included in the calculation.

Now you need to know which variant was chosen according to § 5. All floors including the basement = Variant 1 Possible floor area due to floor area ratio = Variant 2 That is how it is formulated in the law. There is no restriction for the basement as to whether it is a full floor or not (Variant 1). Since I do not know your plot size and the area calculation is very close to the actual areas, it looks appropriate.

I got hold of this BGS EWS from the municipality and it says:

§ 5
2
Contribution Basis
(1) The contribution is calculated based on the plot size and the floor area of the existing buildings.
(2) In unplanned areas, the contributory plot area
− for commercially used plots and plots for special uses such as schools, kindergartens etc. with at least 4000 sqm area (oversized plots) is limited to 2.5 times the contributory floor area, but at least 4000 sqm,
− for residential plots and other plots with at least 2000 sqm area (oversized plots) is limited to 2.5 times the contributory floor area, but at least 2000 sqm.
(3) The floor area is to be determined according to the external dimensions of the buildings on all floors (building floor plans rounded down to the nearest 10 cm). Cellars are included with the full area. Attics are only included if they are developed. For the calculation of the attic floor area, 66.67% of the area of the floor below is applied. For attics that are only partially developed, only the partially developed floor areas are calculated according to sentence 4. Buildings or independent building parts that according to their type of use do not require connection to the wastewater drainage system or that are not allowed to be connected are not included; this does not apply to buildings or building parts that are actually connected to the wastewater drainage system. Balconies, loggias, and terraces are excluded if and to the extent that they extend beyond the building line.

So apparently variant 1

And as a layperson, one is supposed to know in advance what to expect? Every municipality can choose for itself what it bases its calculation on and when it changes it etc..... and the builder only finds out when the time comes, well bravo..
It's almost as if you get the terms and conditions shown afterwards....

Otherwise, this would surely have been factored into our consideration of cellar yes/no, after all we are talking about almost 4000 euros more. Perhaps therefore also interesting for other builders.

Yes, communication by municipalities is a very special topic...