Buying land? - The standard land value in this area is 270 euros

Then leave it. As already said here, it is madness to build a house on someone else's property... it doesn't belong to you, neither the land nor the house. And if you inherit it, you pay inheritance tax on everything, land plus house, plus outdoor facilities...

Just leave it alone, set it aside, don't answer the phone anymore. Better for everyone.

If the uncle now sells to the TE, the municipality can exercise a right of first refusal (which means: entering the deal at the negotiated price!) – particularly annoying if this is a family price. The municipality can resell the property and impose a building obligation, so the uncle will very likely have a house built in front of him within the next three years. Contributing the property to the "Onkel und Georg Grundstücksgesellschaft" would not be a sale, so the municipality cannot interfere there either. The property company can be subject to a building ban by the current sole owner and then by the "only" majority shareholder (for example, the TE buying in with ten percent), and the company (not the uncle!) then grants the TE a shareholder loan in the form of permission to mortgage the property. Immediately and then again every ten years, the uncle gifts portions of his shares to the TE, observing exemption amounts and appeasing co-heirs. The TE is liquid for his construction nearby; the side uncle property remains undeveloped and serves purely as security. Upon the uncle’s death or his move to a nursing home, postponing the construction loses its purpose; all this can be smoothly drafted by a lawyer. This is no rocket science and also no reason for conflicts (internal, familial, or with third parties). Money flows here only from the TE to the "Onkel und Georg Grundstücksgesellschaft," not the other way around. I myself am not allowed to advise the TE, but the lawyer and the tax advisor may. On my part, publicly now over & out ;-)

First, relatives in the direct line are entitled to inherit and also entitled to a compulsory portion (another nasty thing).
If none of them exist, only then can inheritance be passed on elsewhere quite easily.

Siblings have no claim to a compulsory portion, and the uncle's parents no longer exist.

If the uncle sells to the OP now, the municipality can exercise a right of first refusal (that means: enter the deal at the negotiated price!) - especially

with sales up to the third degree of kinship, the municipality has no right of first refusal. and otherwise only if there is a municipal council resolution or a designated redevelopment area. there is no right of first refusal just for speculation (making a profit).

Siblings have no compulsory share claim, and there are no longer any parents of the uncle

But stupid question: Who would then be the heir if there is no will? The closest relatives would be my mother and uncle, unless he excluded them, but then there is also a will.

But stupid question: Who would be the heir if there is no will? The closest relatives would be my mother and uncle, unless he excluded them, but then there would be a will.

First, it goes down the direct line. If there is none, it goes up the direct line (parents). If the direct line upwards is already deceased, it goes sideways from there (siblings). From there, there is no longer a compulsory portion claim. If there is no one there either, it always goes further up and then sideways down again. Until you get an email from the probate court asking if you want to accept a completely over-indebted inheritance from the great-grandcousin of the seventh degree.