Building height of 8.5 m with basement and 2 full floors

It is like this:

[*]8.5 m "building height" from the top edge of the basement floor slab.

That is indeed unbelievable! Unfortunately, once again a case for lawyers. Here is my assessment:
This interpretation will not withstand judicial review. The term "floor slab" alone violates the requirement of definiteness, since further explanations such as "floor slab of the lowest storey" are missing. Even with that, there would still be a lack of definiteness, because the reference point must be determined or determinable in the plan itself, as it is for the reference point of the eaves height. Here, however, the reference point would depend on the architect’s planning.
Also completely unclear is the urban planning purpose of this stipulation. I would be curious about the wording in the justification. The possibility of a height determination aims at design influence on the townscape, which is definitely not achieved with this stipulation, and not at limiting the built-up volume. Other means such as plot ratio/floor area ratio/BMZ are available for that.

For you, the question now is whether you want to pursue legal action if the building application for your dream house is rejected. Are there still no houses in the building area? Do you know other builders there? Have you ever talked to an architect?

We measured these two points.

There is only one point. You determine the midpoint of your street-side boundary (which admittedly is not quite clear with your plot shape). From there you take the shortest way to the street center (I then arrive at point 258.81). The eaves of your house may be a maximum of 6 m above this point (so a maximum of 264.81 above NHN), completely independent of the terrain heights on your property. Although this stipulation is also likely to be challengeable (see judgment OVG NRW, 28.08.2014, Az.: 7 D 8/13.NE).

I know from the Office for Urban Planning itself that especially builders "in the depression," which are all building plots downhill (northeast), had to struggle with this issue.

It is astonishing that all architects complied with this nonsense. I would first take a close look at all existing buildings on site to find out whether everyone really adhered to the regulations or how far they deviated.

just to make sure I understood it correctly

You understood that correctly. I just want to repeat that this regulation is also not legally compliant. But which builder has the time and money to take action against such amateurism.

Surprising that all architects have succumbed to this nonsense. I would take a close look at all existing buildings on site to find out whether everyone has really adhered to the regulations or how far they have deviated.

That will be easy. As soon as one of the almost 20 buildings already constructed has a basement + 2 full stories and no flat roof, the builder has probably clearly exceeded the 8.5 m here. Since I live 300 km away, I'll have someone from the family drive through the construction area over the weekend and assess the situation.

You understood that correctly. I just want to reiterate that this regulation is also not legally compliant. But which builder has the time and money to take action against such amateurism.

Thanks. What exactly is not legally compliant about the specification of 6 m eaves height as well as the reference point to the middle of the street? Or to put it differently: What changes in the development plan would need to happen to legally specify the eaves height? Just so I can also argue accordingly..

I was referring to your shortened quote, from which it is not clear which street must be referred to. In the original, the traffic areas are labeled "A" and "B", so only the somewhat subtle criticism of the not clearly definable center of the property remains. Since the height difference of the reference point will be less than 10cm depending on the method of determining the midpoint for your property, the argument in court would be somewhat weak. However, this does not refer to the nonsensical determination of the building height of 8.5 with the reference point floor slab. In my opinion, this represents a blatant violation of the principle of definiteness, which could lead to the nullity of the [B-Plan].

Since the height difference of the reference point depending on the method of determining the center point for your property will be less than 10 cm, the argument in court would be somewhat weak.

Because I still haven't fully understood exactly how to determine "A" and "B" and why "A" and "B" may not be exceeded by 6 meters when "B" is about a meter below "A" (why do I even need "A" then?), the conclusion from your answers (if I understood correctly) remains that the basement will definitely be completely underground. Because from the center of "B" (if this means the street running east-west) to the center of the property is roughly the 10 cm you mentioned. No idea what the measurement to "A" is relevant for then (probably only for the other properties that lie centrally).

But this does not refer to the nonsensical setting of the building height of 8.5 with the reference point being the floor slab. In my opinion, this constitutes a blatant violation of the principle of legal certainty, which could lead to the invalidity of the development plan.

Where can one find information about how the procedure would be here? Without immediately consulting a lawyer? I am primarily interested in the rough timeline (are we talking weeks, months, or years?) and what costs I would have to bear (thousands of euros would be worth it to me, but would inevitably lead to my house being smaller than 8.5 m anyway because then the house-building budget shrinks ).

Kind goddess, what kind of handyman created this development plan?

The boundary between the areas with 6 m eaves height and 7 m eaves height even runs right through a parcel, since the development plan was obviously created before the land readjustment (even the planned roads merrily swing over the parcel boundaries of the fields, the development area consists according to the plan of nine parcels).

It can definitely be said that the plan consistently lacks definiteness.

The planned roads lie on average about one meter higher than the adjacent properties!

But since you say the area is already 75% built up: then there must already be a plethora of examples of how to deal with a plan under these conditions at all.