Land expected for development, who sets the boundary here?

Here in the south, the municipality buys the land, develops it, and then sells it.

That is the standard, but it does not apply in all cases. Former landowners are more often given pre-emption rights and exemptions from compulsory building. And there are also private developments. It's all a matter of networking.

In the North, 80% of the building land is purchased by private investors who then negotiate with the municipalities about the development plan (such as how many single-family houses, multi-family houses, and semi-detached houses are to be built, etc.). In the end, a compromise is reached that is acceptable to both sides.

The 's memory is admirable. The OP apparently missed the time for sensible project development. Now a buyer is supposed to come on board who takes on the risk. In my opinion, the contact person is the municipality.

Which geoportal exactly are you using?
I just checked it in the Sachsen-Anhalt viewer. I do get a map from the land registry there. But no map of the land use plan. Or am I just too stupid? What is it called in the map selection on the left?

Each federal state does as it pleases. Unfortunately, I only know my way around Saxony and Bavaria.

Hello everyone,
I spoke with the responsible building authority yesterday.
They say that the land use plan is not suitable for deriving exact boundary lines.
An exact boundary line will only be determined during the development plan procedure.

That is, of course, pretty annoying for me.

I am now stuck in the following dilemma:
My potential buyer demands that the area be free of tenants at the time of sale.
Without knowing the exact boundary line, I cannot terminate or relocate the tenants of the garden plots and provide compensation.
And the boundary line will only be finalized when the buyer has developed (and planned) the area.

So what to do??
I can hardly “generously” create space and evict gardeners from their plots, even though it might not be necessary.
And I also cannot sell the area with the promise that gardeners who later turn out to be affected will be relocated and compensated. (Formally, I do not even have the right to terminate these tenants and must hope for their cooperation. – If one of them demands several thousand euros in compensation or refuses to cooperate, the whole project will fail.)

How would you proceed??

If I understand correctly, the buyer wants to develop the development plan together with the city or something similar. In this respect, he will probably include exactly the land that he is buying from you. So it will likely be something like "agreeing on a boundary with the buyer and then separating and selling accordingly," while clearly communicating what the city has now told you.

How to best deal with the allotment garden association, no idea what is involved there. So, if all that is super complicated, it sounds to me like "then you just don't sell it." But if it is relatively easy and not problematic at all, then you just sell more. Whether it later becomes part of the development plan or not is, in my opinion, no longer your problem.