Real estate transfer tax - developer and construction company same person

The foundation slab is already located on the contractually relevant property. It is the purchaser's responsibility to construct a residential building on it. The construction of a residential building is expressly not a service to be provided by the seller.

we are buying a building plot with an already finished foundation slab from a developer (Person X and Person Y) on 07.08.2020.
Person Y of the developer became self-employed and founded a construction company three months ago.

And the developer is "Person X," who is identical to natural person Y? – and you are buying the plot from the company (without obligation to build with the sole proprietorship of the company's owner), on which a building is already started; at the time of purchase it consists only of the foundation slab, but the complete building plan including its approval is handed over to you?
In fact, the owner of the selling company would then finish building the house for you. Regardless of whether he actually controls the company, I would practically see this as one acting person with a quasi split legal personality and both the initial suspicion and the burden of proof would lie with you that this could be seen as a tied transaction, even if there is no obligation to choose this builder. See a lawyer quickly and inquire whether and how you should possibly ask the tax office in advance for an assessment of how they view the situation. The tax office must not provide advice, but must “lay their cards on the table” upon a concrete question whether the case fulfills the requirements or not.

So Person X and Y are 2 different persons, i.e. the developer is a GbR consisting of 2 persons and one of these persons has founded a GmbH (construction company) and we would like to build with this one. However, I have to obtain the building permit again because we want small changes in the floor plan! This will also only be approved after the land purchase. We will most likely submit the application this week.

The tip about the tax office is great, I will tackle that directly tomorrow.

Looks like cheating. I wouldn't be surprised if someone at the [FA] would see it that way too.

However, I have to obtain the building permit again because we want small changes in the floor plan! This will also only be approved after the purchase of the property.

The timing of the approval is not the crucial point here, and a simple correction may also suffice.

The tip about the tax office is great, I will tackle that directly tomorrow.

No, only after the discussion with the lawyer about if and how.

Thank you for the answers!