Is a house for a total volume of 400k still possible at all?

If you have a sloping plot filled in to build with a slab, that necessarily means at least one retaining wall. That costs tens of thousands of euros depending on the length and height.

I understand what you mean, but I have to rely on the expertise of the people involved. After all, they were on site and took a look at it. We will now first start the [Bauvoranfrage] with the architect and see where the journey will go.

Sure, but think it through and bombard the experts with questions about costs and execution. These typical ancillary construction costs like earthworks, retaining walls, storage, disposal, clearing, etc. are not initially on many builders' radar. Even if the expert says it is easier to fill in and build with a base slab, that probably means that for him it is less work to support the terrain with L-shaped stones than to build a basement. For me, 29 meters of L-shaped stones with a height of 1 meter came to a total of 15,000 euros. For you (without knowing your property), that will by far not be enough. Therefore, you have to carefully weigh and calculate which model will ultimately pay off for you and which house you can build with which conditions.

I am not a construction professional and have only been dealing intensively with the topic for a few months. But it is no secret.. Building on a slope is more expensive. Since your budget is limited, my recommendation would be, don't do it! Better look for a flat plot of land, which might be a few euros more expensive, and build on it with a slab foundation. There doesn’t necessarily have to be a basement underneath or the plot extensively prepared. Additionally, landscaping the outdoor area is also more complex.

I’m curious what your architect says.

From when does the slope on the property become too much for a slab foundation? Are there any rules of thumb?

I understand what you mean, but I have to rely on the expertise of the people involved.

And unfortunately, this is where the mistake lies. You assume that the contractors want to advise you *correctly* as a whole. This is often simply not true; each of them only sees their own contract and wants to make money. Starting with the GUs who want to sell you a house including a free iPad :p or the architects who are still calculating with €1600/sqm. We all in the construction area have hillside plots. My rear neighbor a) relied on the footing being planned optimally in height relative to the street and b) the GU estimated the usual €10k for the outdoor area. Like you, he simply had no idea and relied on the “professionals.” Now, without reinforcement, DIY store L-beams have to hold his 1.3m fill over 30m for the next 20 years. I am curious. Let’s be honest: at these sums, dreams and later even livelihoods are tied to it, so what reason would the GU have had as a professional to knowingly give my neighbor false additional building costs? But that’s entirely your issue. My impression is that you are currently putting yourself under pressure because of the plot, for whatever reason, especially time-wise.