Budget calculation single-family house 160 m²

You cannot completely exclude earthworks. Even on a perfect plot of land, there is a bit of digging to do. And if the plot has a slope, the soil is poorly load-bearing, or frost-sensitive, then you need to budget a bit more for earthworks even with a slab foundation. The same goes for the basement. If you do without one, you need a slab foundation instead. Sometimes these are already included in offers, but often the prices for a house are from the top edge of the basement/slab foundation, meaning that comes extra.

And the question whether to have a basement or a slab foundation can really only be answered for a specific plot of land. There are a few points against a slab foundation, for example a sloping site or poorly load-bearing soil. Sure, you can fill up a slope or put three meters of gravel under the slab foundation if the soil is poor, but then your calculation no longer remotely holds true and you have a classic Pyrrhic victory. On the other hand, there are factors that clearly speak against a basement or make it much more expensive: water-bearing layers, contaminated soil (this can also be a problem in idyllic rural areas, e.g. geogenic arsenic). This then drives the costs for the basement significantly higher. In short, without a specific plot of land you can't really plan anything reliable. Without a plot, there should be two rough plans, a preferred one and an alternative plan if a basement or slab foundation is not possible.

In short, without a specific plot of land you can’t really plan anything concrete at first. Without a plot, there should be two rough plans, a preferred one and an alternative plan if a basement or slab foundation is not possible.

I think having a Plan B only for swamp / rock / steep slope is nonsense, since even much less severe obstacles (e.g. Franconian vacuum cleaner knee wall) can ruin a dream house building plan.