Property purchase - Risk of a "linked transaction"?

Oh, the clause was included for me. So, if the notarial purchase contract for the specific property does not come about, the construction contract is void. However, this clause does not necessarily help in the assessment of the tax office regarding real estate transfer tax (uniform contract document).

So you actually signed the construction contract even before purchasing the land?
For me, the main point was that they apparently want to force us to take this step, although from my perspective a pure planning contract should suffice and the rest should only be signed after purchasing the land.

I say nothing without my lawyer. (although by now it’s almost statute-barred.)

For us, the land purchase dragged on for a long time because the building plot still had to be developed, various parcels of land had to be merged and then subdivided again into over 50 plots. But the winter lasted deep into April and there were still disputes with a neighboring neighbor about about 1m² on which the neighbor had built a wall during the land consolidation. That dragged on as well until it was agreed that the one square meter was given away as a gift and then the whole thing could continue.

For us it was clear who we would build with and we already had quite a few appointments beforehand and had already planned quite thoroughly (at least that’s what I thought at the time).
The rough plan and the offer were available. At some point, however, they understandably wanted a signature.

In our case, the salesperson of our home builder had some control over the prime plots in the building area because he knew the land seller well. So we were more tied to the independent salesperson than to the home construction company. At that time we could also have switched the land at any time and built with another construction company.
Nevertheless, I was all the more keen to make sure that everything looked proper when assessing the real estate transfer tax. We therefore talked to several construction companies and had offers made. I documented articles in the city’s official gazette that indicated the new building area so that people became aware of the land and so on.

Thank you very much for the input! I just had the opportunity to speak with the property owner again and received the direct statement that there is no obligation to use a specific provider for the house construction on his part. He even had full understanding of my doubts about signing a house contract already now. Therefore, I will somewhat confidently put the pressure back on the sales consultant and give him the choice - either a planning contract or no contract at all. Let's see how he reacts...

So I will somewhat confidently put the pistol back to the sales advisor’s chest and give him the choice – only the planning contract or no contract at all. Let’s see how he reacts...

And – as others have already written here – you should make sure that the planning contract explicitly states that the plans belong to YOU, even if you don’t build with this company. The same applies here: explicitly, don’t let yourself be fobbed off with “we don’t have to put that in writing, it goes without saying”…

On the one hand, we are supposed to sign a planning contract (basic evaluation, preliminary planning, design planning). This is supposed to cost €4000, which will be credited back to us upon conclusion of a house construction contract. As far as I see it, that’s okay, since we want significant changes from the standard design.
BUT, in addition, we are also supposed to sign the house construction contract right away? This is based on the construction and service description of the standard house, supplemented with additional features that we would like to include. However, in terms of layout, window areas etc., it is still far from what we imagine (which is actually supposed to be the purpose of the planning contract?).
There is a right of withdrawal if the financing is “provably not possible.” According to the sales consultant, this applies automatically if we decide against purchasing this particular plot of land (since without the land there is no financing).

Therefore, I will quite confidently put the gun back to the sales consultant’s chest and give him the choice – only the planning contract or no contract at all.

Since you don’t even have the plot of land securely in your hands yet, I think it’s exaggerated that you’re already pushing the rocket launch button so hard. Chill out — impatience has never accelerated anything. For that reason, I strongly advise you against wanting to plan so urgently already. Besides, you should change the provider, or at least the currently focused base model.

since we want significant changes from the standard design.

I read this as a loud warning signal. The more detailed specification

is based on the construction and service description of the standard house, supplemented with additional features we would like to include. However, in terms of layout, window areas etc., it is still far from what we imagine

confirms this qualitatively as well.
It’s an evergreen in the top ten of “things you should NEVER do” — Germany: twelve points — to build a de facto custom design on top of a catalog model!
Because this is a move that backfires massively in multiple respects. Details see the posts by (by the way: why don’t we see a thread here yet with the catalog house you consider suitable for reworking and your completed questionnaire? – details then in that thread).
First of all, you shouldn’t have so much understanding for the attitude of the house building company (“They are imposing an extensive redesign on us, and we don’t want to be stuck with the effort”) and secondly, in my opinion, you should also derive doubts about their reliability from the fact that they even offer you a de facto custom design as an apparent adaptation of a catalog model, which more or less only serves as a value-added floor plan donor. They know at least as well as I do that this will turn into an outrageously expensive nonsense with extra sauce, and certainly it’s not the first time they’ve done this. Any decent owner-managed general contractor would at least take you into a serious conversation in such a case, under the watchful eyes of “pastor’s daughters.”
But you are not the only one who already feels so understood by a “oh, we actually only have to make the windows lower” floor plan that they think this provider must be the right one :-)

Until now, it sounded to me as if it were about a globally undoubtedly ready-to-build plot in a developed building area that just hadn’t yet been developed in terms of its intended parceling because it still lacked the "distributing" cul-de-sac along with the underlying power-gas-water-telephone connections, but the situation now appears fundamentally different to me in : namely, that we have been talking the whole time about the emperor’s nonexistent beard – in the form of a field which only partially extends into a §34 building zone – although at least the future plot of the OP is 40% building land zone :-(
The formulation alone

subject to "nature conservation compensation measures"

indicates to me that there’s still a tough nut to crack here (and to be honest, I don’t see any banker who’d want to hold your hand while waiting for Godot). The aforementioned signal phrase, in my view, means compensation measures in the course of setting up a development plan that expands the existing §34 area by including the neighboring properties which currently appear to me as mere hopeful plots, creating a then fully planned building area. I expect this to involve derivations from existing buildings and their actual building envelopes, as well as at least no shorter a participation process than that of a completely virgin development plan area. So the whole thing lies significantly more in the future than our school wisdom could have dreamed of.
And I, fool that I am, have already taken the whole story seriously. Günni, today I’m having two Doornkaats at once!