Preliminary building inquiry, sale of partial plots, procedure

Thank you very much for the answers.
Yesterday I was at the responsible building authority and went through the details with the lady. The tip about the land use plan was worth gold :).
The city wanted to buy the plots years ago, which I was aware of. Proactively, the city apparently already changed the land use plan in this context, which no one had on their radar. However, the sellers no longer wanted to sell because the city apparently behaved a bit inappropriately.
Now the area is already shown as a general residential area in the land use plan (see attachment).
Initially, we imagined dividing it into three plots with a privately created access road (see attachment).

As the next step, we would tackle submitting a preliminary building inquiry and have a soil survey carried out.
In general, I would have the following questions:

[*]Are there certain facts/documents that we might not have considered in our case but definitely belong in the preliminary building inquiry (available are an informal letter, preliminary building inquiry, land use plan, sketch)?
[*]What would be your reassessment of the presented situation and prospects for success?
[*]Should anything be considered since it is three plots at once, also regarding resale?
[*]Does anyone have rough experience of what the filling of topsoil costs per sqm? The area drops about 1 m to 1.5 m to the height of the street.

I would be very grateful for any further tips again.

...
To describe a 4,000 sqm pasture as a "building gap" is pretty much the height of naive hope.

Is the "building gap" part of the inner area?
Are the neighboring developed plots as well?
Is there a similar zoning plan for the neighboring plots?
Are all neighboring plots designated as residential areas?

If all are answered YES, it seems to me quite likely
that the building gap can be classified as a residential area (§34?).

Then the seller will not sell the plots as meadows, but as land with development potential.

How high is the seller's price expectation? Where is ? He will definitely find everything out. But you have to provide precise data. Geoportal has so much more to offer. Is the property located in a residential area?

I would be very grateful for any further tips again.

Then I'll give you one right away: don't be so stingy with the plan excerpts.

Where is @Escroda? He will definitely find everything out.

He was, let's say, "chased away" (short version).

But you have to provide exact data. Geoportal has so much more to offer. Is the property within the inner area?

Looking deeper doesn't hurt, though.

Then the seller will not sell the plots as meadow but as land with development potential.

He will sell them as building land. Because if section 34 applies, it is no longer land with development potential. As a tip: plots must be accessible for development. If I see correctly, the two rear ones have no direct access to the street. Regarding the preliminary inquiry: the fewer questions you ask, the faster and easier the answer. Example: Is the plot with the parcel number fundamentally buildable? You can also formulate several questions or have them build on each other. The deeper you go into detail, the more legally binding information you can later include. From my point of view, the most important thing would definitely be to clarify whether it is building land or land with development potential AND whether the seller is aware of this.