Several troubling contract clauses

My counterargument, that we signed the preliminary contract for a house with a fixed price and that this delay is solely due to his company, which has stretched the deadlines so far apart, he did not really accept. [...] In the preliminary contract, there is also no clause stating that the price is limited to a certain period.

I am very confused that you repeatedly talk about a "preliminary contract" (in which prices and binding periods should not yet be included at all), but on the other hand about numerous details of the design and sample selection. A preliminary contract does not yet agree on more than the obligation to enter into a contract with each other. It can also contain resolutive conditions, e.g., "if a soil report reveals a rocky or swampy site, the construction project will be cancelled," but details like stairs or locks do not belong there yet. And prices and their validity only belong in the actual contract, which the preliminary contract primarily stipulates that such a contract is to be concluded at all. Conceptually, a preliminary contract could look something like this:
1. the parties "builders" and "construction company" commit to undertake the project "construction of the Mustermann single-family house approx. XYZ square meters of living space according to the Effizienzhaus 55 standard" together.
2. the party "builders" commissions the party "construction company" to plan the subject of the contract and to obtain a soil report. The implementation of these two assignments will be started immediately.
3. if the soil report results in a presumably uncomplicated foundation of the building, the parties will immediately conclude a construction contract,
otherwise the project is declared finished and the party "builders" pays the party "construction company" for obtaining the report and the planning services rendered up to this point.
Before point 3, according to my understanding of the proper practical procedure, there are no agreements on services and prices of the details from the construction contract. So how could stairs and locks already have been discussed here — and even with fixed prices! — if, on the other hand, it is supposed to be only a "preliminary contract"?

I am very confused that you repeatedly talk about a "precontract" (in which prices and binding periods have no place at all), but on the other hand discuss numerous details of the conception and sample selection. A precontract does not yet agree on more than obliging each other to conclude a contract. It can also contain resolutive conditions, e.g. "if a soil survey shows a rocky or swampy ground, the construction project will be withdrawn," but details such as stairs or locks do not belong there yet. And prices and their validity also belong only in the actual contract, of which the precontract primarily stipulates that such a contract is to be concluded at all. A precontract could conceptually look something like this:
1. the parties "builders" and "construction company" commit to undertaking the project "construction of the Mustermann single-family house approx. XYZ square meters of living space according to the Effizienzhaus 55 standard."
2. the party "builders" commissions the party "construction company" to plan the subject matter of the contract and to obtain a ground survey report. The implementation of these two tasks will be started immediately.
3. if the ground survey report results in a presumably uncomplicated foundation of the building, the parties will promptly conclude a construction contract,
otherwise the project will be declared finished and the party "builders" will remunerate the party "construction company" for obtaining the survey and the planning services provided up to this point.
Before point 3, according to my understanding of the proper practical process, there are no agreements about services and prices of details from the construction contract. So how can stairs and locks already have been discussed here – even with fixed prices! – if it is supposed to be only a "precontract" on the other hand?

The problem could be whether such a precontract is seen by the KfW merely as a "planning contract" or as a contract after which applying for funding is no longer possible?

Hello Tom, I hadn't considered that at all. However, they apparently always do it that way. We are still discussing and negotiating. Nearly 7 months after signing the contract, and not even a building application has been submitted. The construction law lawyer at [Verbraucherschutz] advised us against signing the contract as it is. Significant changes need to be made.