Is a penthouse feasible for 350,000€?

Now you're getting pretty snippy.

Now you're being quite snippy.

Sorry. Didn't mean it that way - grin

Oh, so according to you, I shouldn’t blindly trust one statement, but if I get another offer, that’s also wrong.

Your approach of obtaining offers is naturally completely correct. When you review the offer, look at how "fixed" the prices are. In order for an offer to be made, it must of course be clearly detailed what exactly is to be built. Otherwise, you will get a price that does not correspond to what you want later or, in the worst case, simply misses things that you then still need.
In civil engineering, you usually have unit prices based on an assumed quantity for an assumed soil class. This means you bear the risk of differing quantities and differing difficulties during excavation. That is common and also fair, provided it is clearly specified. You just have to be able to read it.
It’s nothing complicated and I am sure you will manage well. The figure thrown at you is simply extraordinarily low. It’s not a problem if you can pay more than initially expected. It only becomes a problem if it turns out during construction that additional financing is needed.
I just checked again for you: We paid around €300,000 for earthworks for the driveway, house excavation, insulated concrete foundations to support the slope, natural stone walls, sewer works, and connection. The architect’s initial estimate was under €70,000, and the offers from the involved companies came to just under €200,000 – so I didn’t quite remember that it became twice as expensive as offered. The difference also includes some wishes of ours regarding execution; the bulk of the cost increase was due to soil class 7, which we encountered. Certainly, we got hit "heavily" above average, but many here will have also experienced cost increases for earthworks on hillside construction projects.
Please do not take my skepticism as criticism of your person, but only as a warning. Living on a hillside with a good view is great every day. I wish you success in implementing your construction project well.

Oh, so according to you, I shouldn't blindly trust a statement, but if I get another quote, that's also wrong.
Do you realize that yourselves or what?

What do you actually want? Build it yourself or have it built? If you want to have it built, then you have already received the price statements or a clear "No, that won't work" from the construction industry here. If you want to build it yourself, then you don't need a quote, or the price information from your family craftsmen planned for the construction is sufficient.

It's a pity that you're out of here. But also a pity that, for example, you don't answer the repeated question of how your draftsman and the builder actually calculated for you.

P.S. Firstly, I am somewhat from the construction industry, and secondly, I renovated my own house from the 60s to 95% by myself to a new build. Too bad you don't want to engage in a technical discussion.

It becomes a bit funny when it is even assumed that I am a woman.
But apparently my way of writing does not fit a "solid man"

Nope, "Mala" is clearly a woman's name to me. That's why.

That it is then assumed I am a woman makes it somewhat funny

Mala is a rather rare but still common girl’s name. When I think of Katja, I also don’t think of a man...