Coupling business or not?

Yes, we could include that in the contract. Then the matter would be settled once and for all, right?

Give it a try. But a notarial deed is not a wish concert, a notary is not a means to an end. Report back when the time comes. Please.

The topic of tied transactions is always a gray area.
The tax office will argue one side, you the other.
Therefore, start collecting evidence now that speaks against a developer transaction.
The more indications that speak against a developer transaction, the better for you.
If it is stated in the contract, then that is another argument for you.
Allegedly, the tax office in turn is scouring Immoscout and the like and is gathering evidence on their side.

Back then, I also learned about a construction company from a new development area and got in touch with the relevant contacts through that. At the same time, however, information was also published in the local official gazette that the new building plot was to be developed. Of course, I immediately made a copy of it.

You can also place an advertisement on Immoscout before the contract is signed.

My question now would be whether the tax office has reason to assume a linked transaction and accordingly we would have to pay real estate transfer tax on the house? Purely based on the facts, I would say no,

Don’t ask us this question, but your responsible tax office (preferably both, if the building plot and your current residence are in different jurisdictions), with the facts. They may not advise in a formative / suggestive sense, but they are obliged to provide information (a statement) on the specific case upon request. Unless there is an interim change in the law, this statement is binding for them as well – forum opinions, no matter how knowledgeable, are not.

Therefore start collecting evidence now that argues against a property developer transaction.

We have now been awarded the contract and would work with one of the building companies prioritized by him, as we have already had good experiences with this one (project planning on another plot, for which we did not actually get the award).

You must also include this indication in the assessment by the tax office, because if they do not know all the facts before their answer, their binding effect is lost.

Maybe I misunderstand, but even if the developer had a brokerage contract with the seller: If you buy the plot of land from the seller without any obligation or coercion (contractually) and you have no contract with the developer, you can do whatever you want afterward. It is your property. And if you prefer an architect, the developer may be annoyed, but he can't do anything. If he later submits the best offer and builds the house, how is this (first!) contract connected to a completely different deal?

Yes, we could include that in the contract. Then the matter would be settled once and for all, right?

Does the tax office want to see the notary contract? Or how does the tax office find out about the purchase of the property at all?

What if I am already interested in houses in advance, conclude a purchase contract for a house with the provision that I myself (!) have to obtain a plot of land within period X. Then the contract for the house would have been concluded beforehand but it still would not be a tied transaction?