Construction costs are currently skyrocketing

So for 140m² standard floor plan + a lot of extras. What do you think? Did I completely miss the mark somewhere with the cost estimates?

Seems very low to me overall. That's just about 2000€/sqm without outdoor facilities. Of course, everything is also set at the very low end.

Yes, of course, but anyone who buys a 99 TEUR offer expansion house will probably not suddenly have marble laid. It is about the absolute minimum.

That's clear, but 10k for earthworks including gravel is really tight. The foundation slab for 15k is too, you'll quickly be at 30k. A neighbor is building with Danwood, he had to use Glatthaar for the foundation slab. No matter the price. Maybe it's similar with Massa Haus. And so on

I based the slab on 120 EUR /m², which is supposedly still the price according to the web. But let's say 150, then rounded up you're at 13,000 EUR.
I had a total of 9k for earthworks and I had to replace 1.2m, which is not exactly easy. There are soils where the 30cm often included in [GU contracts] is enough to remove.

But for my sake, let's add 20 TEUR.
Then we're at 2300 EUR/m².
That's already much cheaper than the average otherwise. Requires a lot of personal effort and it's still much more expensive than some projects here with their 1700 EUR/m² or whatever there was sometimes.
So you can definitely notice the cost increases even in the DIY sector.

Yes, because even if you do a lot yourself - you cannot avoid the significantly higher building material prices. Compared to "having it done," however, it is surely still interesting for some. I would also prefer to do it myself before giving up the dream. We have always done it that way with our renovation so far.

Self-performance at Massa Haus is already feasible. The entire insulation and cladding of walls and ceilings can also be done without much DIY experience. Instructions are easy to understand. It gets more difficult with the installation work. But that can also be outsourced. However, the coordination of the individual trades should not be underestimated when some parts are done in self-performance and other parts by professionals. Furthermore, the time factor should not be underestimated. We renovated back then with three people. Always in the late afternoon until late at night. Six days a week. It took almost 10 months. Electrical and heating installation was outsourced.