Is a penthouse feasible for 350,000€?

A bank will also be found for this, as they also easily finance installments amounting to 50% of the household net income.

Sort of. It depends on the income. 3k net for four people = no. 8k for two = no problem. By the way, the cost of living is coordinated with banking supervision and, due to the high level of NPLs in the banking sector, they are appropriately set, as there are no problems with over-indebtedness.

Thank you for the really uplifting and kind words and the testimonial.

By the way, we would get a loan of 250,000€ more from the bank.
But we told ourselves that 2000€ per month over 25 years is too much, even if we could currently afford it.

Today we have an appointment with a second construction company. We are looking forward to their offer.

Even though I think it’s silly to write a novel now. But I’m happy to list for you everything I did with my dad between 2016 and 2019 plus helpers.

175 sqm
Garage within thermal envelope

Roof frame fixed
Roof covering
Roof insulation
Photovoltaic system
Complete electrical work
Facade insulation
Complete drywall
I got into the excavator and wanted to dig... so off we went --> driveway topped with more than 300 tons of gravel, rough grading, fine grading, and then paved the driveway with 2 helpers (130 sqm)
Almost 90 sqm terrace and access path laid with drain concrete
Garden landscaped and filled
Built concrete block walls
Set L parts
Designed and built garden shed

...and and and.....

You know what, my Excel list doesn’t even start with a 3 at the front… and it didn’t five years ago either.
Forget your plan or whatever you were told. I had many friends who supported me with the project, with tools and materials.

I was in the office every day at 5:30 am and then 4-5 hours on the construction site in the evening. That’s a real grind.

And now you come and want to manage that with 350k. forget it.

come down from your 3,000 EUR/m² already. Only people with a lot of money build like that...

I rather think that you are giving the readers an unrealistic dream world. That is dangerous and not very helpful. The note that the own contribution is underestimated, as you can see, is not enough to bring the OP back down to earth. She only sees what she wants to see.
The 3K is still rather underestimated for the average consumer and downright laughably little on a slope.
And you can also see:

If I had such an uncle, my outdoor facilities would have been finished long ago...

Your final calculation is not even available yet. Or do you want to leave out the outdoor facilities on the slope?

If the bank finances another 250K, then we can relax. It will just be refinanced. More than warn, we cannot do here.

My own experience with a hillside construction in 2018/19 and the assessment of "experienced professionals" showed me that they can be mistaken, I do not assume that they want to be. I no longer remember the exact figures and have no desire to look them up. The architect's initial estimate for the foundation slab including earthworks was ultimately a quarter of the actual costs. Here, the decision to go further into the hillside changed the quantity structure, as we encountered soil class 7, which meant that excavation was no longer possible and chiseling was required (the surveyor who took 2 samples did not recognize this), quickly had to build a crane platform to manage logistics uphill, gave the driveway a slightly different curve... finished were over €200k additional costs compared to the first thrown-out number. All experienced men, all comprehensible. I have to admit that we were not always relaxed during this cost increase process. Overall, however, the construction period was a very nice phase of life for us. If the additional costs had led to financial exhaustion, it certainly would have been different. Take a good look in your wallet – with your cost estimate it would be good to have solid safety reserves.

Today there is an appointment with a second construction company. We are looking forward to the offer.

Our draftsman as well as his son (head of a construction company) assured us that this is a realistic price.

I would be interested – but you don’t answer that – in what exactly the two professionals said. Since you want to do almost everything yourself, I find their statement intriguing.
And regarding your appointment with a second construction company, I am curious why you even need them. Are they supposed to submit an offer for a 150 sqm house but leave out the positions of shell construction, partial roof, electrical, sanitary, heating from the offer, because you are doing that in EL?