Supplementary statute for building land valid only for one buyer

Unfortunately, that's not the case, it's an acquaintance of a municipal council member. It will be developed quite normally.
The question is only, why am I not allowed to do that? The issue is complex regarding what's currently happening.

Maybe the municipality wants it to be developed NOW. You yourself wrote that you want to develop it only in a few years. Maybe the municipality precisely wants to avoid something like land being purchased and then standing empty for years.

But if you believe that things are not being done properly, contact the district authority.

But if you believe that something is not right, contact the district office.

Why should the municipality want construction to take place now on non-building land when there is still sufficient building land available elsewhere? I will contact the district office, by phone inquiry and in writing if necessary, or do they only provide information in writing?

Why should the municipality want construction to take place right now on non-building land when there is still sufficient building land available elsewhere?

You should best ask the municipal council members at their public meetings. You can find out when they take place on the municipality’s website, and there are public notices.

Presumably, the supplementary statute will only be tackled after the sale anyway, and the potential buyer was the driving force behind it? Then it would at least make sense to me that the process might initially be put on hold if someone else ends up buying it?

So I roughly imagine it like this: that I might be looking somewhere for plots of land and find someone who sells this meadow and then inquire with the municipality (and yes, possibly also use contacts who can knock on the door for me) and then discuss with the city whether that’s generally possible (and that eventually leads to the supplementary statute being drawn up there). If I don’t buy/get the land now, it sounds quite normal to me that it will initially fizzle out and it would then depend more on you whether it might still come about (that you would have to be more involved and possibly push for it to happen, hassle the city and authorities, and that it might also be somewhat delayed).

But those are just my assumptions. Is the seller even willing to sell to someone else or is it more or less already decided? Or has the plot been in municipal ownership so far?

The seller basically doesn't care where the money comes from. It is privately owned by a large community of heirs. I would have bought the plot for the children of the neighbors or my daughter. So that this piece of land stays with the neighbors and me. But if we are not allowed to build on it, then it is pointless.

Behind me, 1,500 sqm of meadow land is for sale. This is supposed to be reclassified as building land for the buyer in a supplementary statute. I also showed interest in buying the property, to which I was told that then it would not be building land, the supplementary statute would only be valid for this interested party.

You must have misunderstood. A supplementary statute is necessary to transfer an area in the outer zone into the inner zone at all, and must inevitably precede the establishment of a project-related development plan if its scope is to lie wholly or partly in a previous outer zone. Once this is done, the relevant area is inner zone for everyone, including you. There cannot be a statute valid only for Franzl but not also for Seppl. However, such a procedure (which, due to the size of the drums, can no longer be chamber music) is only undertaken for a symphony of the century. So if there were no chance anyway to turn "Kaltenthal" into "Bad Kaltenthal" or to otherwise build the Zachanassian works next door in Auingen ;-) It is not the statute itself, but the willingness of the municipality to undertake this huge effort that would only be valid for this interested party. The higher-level authorities involved and their control committees (this will, even if only as agenda item 87, be the subject of a state parliament session) would never approve this if it were only about a building plot for your daughter—on the contrary: the highly well-armed sacred cow "outer zone" fills stories in several threads here. It is easier for a camel to pass through the eye of a needle or for a rich man to enter heaven than for a stone marten breeder to use an abandoned farmhouse.