Actually, you are right, the problem is that Vega82 bears the burden of proof in case of doubt.
We have taken similar precautions so that we could answer the question of whether the construction company was already determined at the time of the land purchase with "No".
Source: Gestaltende Steuerberatung - Issue 06/2004, page 202
If contracts related to property development are also concluded in connection with a land purchase contract, there is regularly a real estate transfer tax burden regarding the total expenditure, which may only be partially offset by a reduced sales tax burden. To document that there is no close factual connection between land purchase and construction contracts, it is advisable to take precautionary evidence measures. This can be done, for example, by keeping separate offer documents regarding land and building or by presenting planning variants after land acquisition with different construction companies, possibly also through contractual wording that there is no commitment regarding the type and scope and choice of further services and companies, provided that no facts are created promptly in this regard.