Farmland turns into building land: landfilling, foundation costs, and procedures

Assuming everyone fills up to the same height. Then there would be no slope between the properties.

As I said, I do not expect that, but rather a parallel linear filling:

I assume the existing road N/S is largely level, but the terrain W/E is sloping. The side streets will then probably lead down from the unchanged existing road. [ / ] I assume that the side street slopes downward from the junction with the existing terrain’s inclination and the buyers have to manage to fill up to the (concerning the side street new) street edge.

... so nothing with no slope between the properties.
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But what if the neighboring property, for example, remains free longer? Then you would have to secure it again after all.

Correct, even temporarily drainage onto a neighboring property that has not yet been filled is not allowed. So drainage to the valley-side boundary.

Also drainage at the valley-side boundary.

The small depression should be filled. The house then with the finished floor at least 2 steps above street level. When the construction area is fully built up, in my opinion the depression will no longer exist. If the OP fences it, then no water will run onto the neighboring property.

Updates: We have now visited two local home construction companies and Viebrockhaus. - Massivhaus Wonnegau: Received good reviews on this forum, but we are not pursuing them further because they build KFW55 homes - Weton Massivhaus: We liked the consultation and I also like the construction method (42cm Poroton bricks). We are still waiting on prices; they have probably underestimated additional costs (~80k vs Viebrockhaus ~130k). - Viebrockhaus: Makes a very solid impression. Downsides for us: Possibly a bit more expensive, only 16cm aerated concrete walls and 16cm floor slab which I find unattractive, overall less flexible because they are system houses. - Roth Baumeister-Haus was still on our list, but not pursued so far. The water issue/landfill concerns me less now. The company Weton roughly estimated 10k for landfill with natural gravel. I can live with that amount for now. However, there is a new annoyance: According to the Sparkasse, access is only possible via the dead-end street. On the "main street," parking spaces and trees are planned; access to the plots is not possible. This frustrates our initially favored option to place a garage in the north (Image 1). Alternatively, one could place it in the south (e.g., Image 2). Note that the sizes of the rectangles in the image are only approximate. Since demand is not huge, we will wait for now instead of buying blindly. In particular, there is still no schedule for development on the table.

According to the Sparkasse, access is only via the dead-end street.

That would almost be a disqualifying factor.

Since the demand is not huge,

If that is the case, one could make a preliminary request with access from the main road and see if it really would be such a problem. I would also write a justification for it and make it clear that otherwise it is not interesting for you. Besides, it looks like further south the central plots have to be accessed via the main road anyway.

That would almost be a disqualifying criterion.

Why? It still remains a nice buildable plot.
I consider the inquiry unnecessary, because overall the parking spaces probably also play a role in the development plan.

Alternatively, one could place it in the south.

I see the garage on the east side, boundary construction. Then there remains a nice garden area in the south and west, which can be enclosed with a hedge.

If that is the case, one could submit a preliminary inquiry with access from the main road and see if it really would be such a drama.

There are obviously public parking spaces planned ...

Besides, it looks like further south the middle properties will have to be accessed via the main road anyway.

... which at least for the middle property in the "WA1" block couldn't be done otherwise, but it's a botch: for that, you have to sacrifice a parking space in the plan (which is still possible) and exit under the tree shade (prone to accidents). Somehow this conflicts with taking visibility triangles at junctions into account. That smells like intern planning by the engineering office for development plans.