Expensive plot + single-family house 155 sqm + cellar KFW40+, financeable?

For financing, you need a bank partner with courage. The equity ratio is below 15%. This creates a risk for the bank that the loan is not sufficiently backed by collateral. Therefore, I agree with @nordaney:

My suggestion: buy the property and pay it down aggressively. From mid-2022, build and finance with the new salary.

On "additional costs":

But does it always have to be brand new furnishings right away?

Bathroom furniture? Terrace? Rooms that don’t currently exist? Lamps? That does add up.

The desire of many to make everything "perfect" right away often stands in the way of realizing dreams. Those who don’t recognize priorities here simply need much more money than necessary. Of course, furniture that moves with you would also work. Of course, a very simple garden lounger works at the beginning. Of course, you don’t need the entire garden facility on day one. Of course, the area where a car parks can just be covered with gravel for now... Suddenly, €100K are quickly spread over the timeline. For that, you can do the things that can’t be postponed properly right away.

how does the "fixed price guarantee" work then? Can you rely on it? I understand that it doesn't look good if the construction supervisor notices that the costs are skyrocketing and tries to save here and there. As a builder, you’re basically cutting off your own nose to spite your face. But what can you actually rely on then?

I would be interested to know how much equity I should have for the above-mentioned planning? etc.

I would be interested to know how much equity I should have for the above-mentioned planning?

T€ 300 or just wait two years until the higher salary is there.

Well then, I'll go ahead and try my luck with the lottery... ;-) Thank you for all the contributions here. I have gained some insights and suggestions.

That often doesn't work for me when it says 30k planned for new furniture but 100% financed....

I understand your opinion and agree with you.
But look at it this way for those affected: this way they still have a buffer. Since furniture comes at the very end and is not financed, the equity can then be used for the more complicated foundation slab or the more expensive earthworks and the forgotten rainwater connection costs ;)