I will handle the momentum of the intermittent posts all at once:
I don’t know how it works in the eastern part of Germany, but here in the south the municipality buys the land, develops it, and then sells it. This is meant to prevent speculation. The decision on the size and what will be built is made by the municipal council. "Bauerwartungsland" (land expected for building) has no legal significance. Make a preliminary building inquiry; then you will know how big and how good the chances are.
Here in the north, 80% of the building land is bought by private investors who then negotiate with the municipalities about the development plan (how many single-family houses, multi-family houses, and semi-detached houses should be built, etc.). In the end, there is a compromise that is acceptable for both sides.
The memory of @11ant is admirable. The original poster apparently missed the right time for sensible project development. Now a buyer is supposed to come on board to take the risk.
They say that the land use plan is not suitable for deriving exact boundary lines.
An exact boundary line is only established during the development plan procedure.
Even in the region ("Ländle") not all municipalities have the money to organize social housing developments. And investors only follow the money where they can already smell it – so only in Hintertupfing if, for example, an airport is going to be relocated there soon or something similar. The political general situation calls for limiting land sealing; municipalities themselves therefore only develop building areas under painful demand pressure (or if large abandoned commercial areas threaten to become overgrown). I certainly do not see the original poster as having slept here, but on the contrary dreaming of a building area whose development plan will be drawn up at the earliest in five years. And I have justified doubts that his "investor" even remotely knows what he’s doing. I rather suspect a crackpot without money and connections (the kind whose life path is paved with MoUs and LoIs for gold rush pipe dreams – always broke but just short of a breakthrough). A real experienced investor needs, even with a high sick rate in the town hall, a maximum of four weeks to be – absolutely – fully informed. That the boundary of the area is only fixed during the development plan preparation is correct. But that the thickness of the dashed line would be an inaccuracy is a misinterpretation. The original poster simply cannot read such plans (which is okay for a layperson). But the fact that even the "investor" here has no clue makes me reach for the popcorn. I would simply not extend the lease agreements here – except if they go well beyond 2030. A conversion from allotment gardens to building areas is almost at the very bottom of the priority lists of all professional market participants – and that even in the donor states of the federal financial equalization scheme. Regional development is a slow business and only speeded up by very large money.