Development costs for feasibility analysis of building land

First of all, a very impressive analysis, and almost correct as presented, despite my lack of information.

[*]The building land has already been reallocated several times, but there is currently no procedure ongoing. "Similar to what Escroda suspected"
[*]The seller is, as far as I know, not involved.
[*]The area also includes the indicated planned road of about 1000 m2.
[*]The road is marked in the legend as (traffic areas/road traffic areas).

I have looked at the development plan again; it was created in 1980 and no longer corresponds to the current data.
I do not have the current plan but attached a picture from the land-use planning. Yellow marked area totals about 6000 m2.

There was no conflict with the municipality; the seller simply does not depend on the money, already of advanced age, and would only sell due to the good contacts with my father.

So far, I have not considered the municipality's right of first refusal.

However, I do not see why any of the following reasons should apply.

[*]if, within a valid development plan, areas are designated for public purposes or as compensation according to § 1a Abs. 3 (compensation areas for environmental protection),
[*]in reallocation areas (reallocation of land as a reorganization within a development plan),
[*]if a property is located in a redevelopment area or urban development area,
[*]in the course of a statute regulating the securing of implementation measures for urban redevelopment as well as a preservation statute,
[*]if within a land use plan there are undeveloped properties in outer areas marked there as residential building land or residential areas,
[*]if on undeveloped properties in areas according to §§ 30, 33 or 34 mainly residential buildings can be built, and
[*]in areas that are not to be developed for the benefit of preventive flood protection.

But slowly disillusionment is prevailing; I did not imagine it to be easy, but here are probably some hurdles that must be overcome.

Thank you very much, I have already taken a lot from the last 5 posts and am surprised at how much expertise has formed here.

The building land has already been re-parceled several times

I meant a procedure according to §§45ff, Federal Building Code. But you have confirmed my suspicion that this does not apply here.

I looked at the development plan again, it was created in 1980 and no longer corresponds to the current data today.

That may apply to the cadastral basis, but if there is no repeal or re-planning, the development plan should still be legally binding today.

However, I wouldn't know why any of the following reasons should apply as justification.

If my assumption from sentence 1 is correct, I would know two reasons right away:

if within a valid development plan areas are designated for public purposes ...

That applies to the public traffic area.

if on undeveloped plots in areas according to §§ 30, 33 or 34 mainly residential buildings can be built

That applies to the remaining areas: §30 = valid development plan, WA = general residential area

But slowly the disillusionment predominates, I didn't imagine it to be easy, but there are apparently some hurdles that have to be overcome here.

Talk to the decision-makers of the municipality. If there was no dispute and building land is scarce, the solution approach would be §11 urban development contract, Federal Building Code. Enforceable here according to §124 Federal Building Code. But it would be good to have a lawyer among acquaintances.

Well, I wouldn’t want to tie myself to something like that. As already mentioned, the way to go would be a "Städtebaulicher Vertrag". You plan, build, and pay for the road, (very likely) a infiltration solution for rainwater, wastewater, utilities (here only partially). That will roughly cost you at least a million. Out of the 6000m², about 4000m² remain for plots according to the development plan. That means you have to allocate at least €250/m² to the plots, and you haven’t paid for the land yet nor any taxes. In Munich, you can continue calculating, but in the deepest NRW, you don’t have to expend any more energy for that.

First of all, regarding your two image excerpts: both are useless – the second already due to the resolution, but both also show too small a section to capture the overall situation.

A good family friend, already retired, after inquiry on a building plot hinted that he could imagine selling his building plot exclusively to me and not to a developer or the municipality, but only as a total area of approx. 6000 m2.
I am only interested in a plot to build a single-family house with a size of approx. 700 m2 of land.

I assume that the mentioned 6000 sqm refers to the entire image section 1 and that the planned street has not actually been cut out yet, thus will still have to be given to the municipality and thus reducing the stated area. So let’s continue calculating only with 4500 to 4800 sqm, of which you yourself only have 700 sqm "personal requirement" and cannot afford to act as a "dealer" for the rest of the total area – also because you are not a broker and do not have a land development company behind you.
From the depicted planned street I derive the – to be researched for its actuality! – assumption that the municipality already has a concrete idea of the shape of the public traffic area; but first of all, this area has not yet been marked off to them and secondly, they lack the motivation to carry out the development.
From my point of view, you should first make the owner negotiate with you not as an individual buyer, but as chairman of a building association. You should then found this association and, as its chairman, negotiate with the municipality that the association commissions a development contractor (I know comparable situations and can quite certainly tell you that the municipality will not bother to move its own ass for another good ten years but will tell you that you are welcome to take care of it yourself – however, unfortunately the only contractors deemed reliable by the municipality would be Mr. H., Mr. Günter, and Mr. Werner, from whom you would have to choose; and to not immediately slam the door in your face again, you better not voice the idea of patronage out loud). So much for the insight that what you lack for the development business is less experience or money, and much more decisive corrupt energy.
In my opinion, you first need a conversation with the family friend, and then the supporters interested in a plot from the "remaining" area. Potentially these are all those willing to build who are still unserved in the area (and preferably also: who would remain so without you, e.g. because they have too few points for local resident models). Finally, a simple calculation: I said 4500 to 4800 sqm, minus 700 for you, leaves 3800 to 4100. I would consider it naive that they all could or would afford 700 sqm, so I come to ten to twelve supporters. You will have to overbook this number to compensate for interested parties who drop out along the way. Invite them to the founding meeting and then present to the supporters that the association must have you as chairman so that your buddy is ready to sell.
So, my theory is finished, now turn it into your practice.

I have so far assumed as a good-faith citizen that such things work even without knowing the right person, but that is certainly a point not to be underestimated. However, if I broadcast this loudly, the esteemed gentleman surely won't get a moment's sleep. The bank advisor, however, was very enthusiastic about the idea and would provide me with the necessary liquid funds. There also appears to be a substitution of the bank that can carry out the developments. Maybe I'll just have an offer prepared, if only as a comparison. I would also inquire with the local construction contractor, who has already managed a similar project in the community.

The bank advisor was, however, very impressed with the idea and would provide me with the necessary liquid funds.

a) on what contractual basis? b) what exactly did you tell him? c) what exactly did he understand?