New construction on private property allegedly not possible/wanted by the authorities?

An appointment with a specialist lawyer for administrative law / public building law is likely to be more enlightening than groping in the dark.

If the OP already shies away from the costs of a rejection notice, this does not seem to be a solution. And as Yvonne already wrote

On what grounds would one even sue?

If it is only about consultation, the specialist lawyer will only grope in the dark as well, it is just more expensive than obtaining the information from the responsible authorities yourself.

The property is directly on the main road, not in the second row or anything like that.

And you also want to build directly on the main road? A picture is worth a thousand words. Upload a generous section of the cadastral map, then maybe an assessment can be made whether it is an outer area, second row, factual building limits, or something similar, as preparation for the conversation with the building authority.

I am reporting here as a fellow sufferer. An almost identical case. I bought my grandparents' property, which borders two streets. On one street stands my grandparents' house, on the other street I wanted to plan my house.

Q1/2015
Visit to the building authority of the municipality to inform about the building project -> Green light, go ahead, no development plan available, but inner area.

Q2/2015
Submission of building application.

Q3/2015
Rejection of building application -> Reason: Building in the unplanned inner area. If the building gap is too large and no longer appears to be part of a contiguous development, it is necessary to create a development plan for the gap.

Q1/2016
After endless back and forth with the municipality, building authority and co, a resolution to draft a plan was made (basically the starting signal for the development plan).

Q2/2016
Pitiful attempt to push through the building application with a half-finished development plan, with some protection help and "Vitamin B" --> Rejection by the building authority, as necessary planning maturity is not yet available.

Q3/2016
Since then, work has been ongoing on the development plan, in January 2017 then the public display and the involvement of public interest bodies --> Negative statement from the district office and building administration (identical wording from both authorities) --> A noise protection report and an emission protection report must be prepared.

This is not a single case in our district, many acquaintances are fighting on the same fronts.

It is the same for me, the house is supposed to be in first row to the street, looking to the left I see houses in the same line after 100m, looking to the right I also see houses after 100m in the same line.

Ever discussed the matter with the district council member of your constituency? I simply see arbitrariness here. Approval according to [34] would be possible, unless you are grossly understating here and the next houses are much further away. Karsten

Yes, and also with the district administrator of the county, but either people have no more guts or the head of the building authority simply has bigger ones.

Unfortunately can't upload a picture... for whatever reason

Edit. there are 85m free to the right and 130m to the left until the next neighbor has built on the line.

2/2 would be the mentioned area, the grey square on the area is a small shelter for sheep. The meadow is currently used by a well-known shepherd. On the right, the 10 would be my father. On the left at the edge you can see the next house. Allegedly, first a report has to be created by an engineering office. How the demand for building land is. And what building land and possibly old houses for sale already exist... However, I do not quite see why I should buy land in the building area (~ 200 m as the crow flies) for expensive money when there is a sufficiently large plot in family ownership.

A bit scattered up there in the north with you. It’s hard for me to speak of a contiguous built-up district here. But equally, one cannot speak of individual agricultural enterprises either. From that perspective, I find the building authority’s view somewhat petty. However, there are free building plots nearby that have been developed through a development plan. Approving your building project might put the authority in a difficult position as to why a development plan with all formal procedural steps and expert reports had to be prepared there, but you are simply allowed to build. And your building project would also set a precedent for further development.

Ich sehe es aber nicht ganz ein mir für teures Geld ein Grundstück im Baugebiet (~ 200 m Luftlinie) zukaufen, wenn ein ausreichend großes Grundstück im Familienbesitz ist.

Whatever reasons the district may have for the negative stance, you will probably only be able to pursue legal action, which will not advance your building project either in terms of time or finances. Invest the 50k better in a serviced building plot in the development plan area.