Is a penthouse feasible for 350,000€?

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

What exactly is the plan? So not the assurance, but the concrete planning of the costs and the implementation through own work? Which construction schedule is this based on?

I believe 2 experienced men who deal with this topic daily and know themselves how it works here in the countryside with a lot of own work and support know what they are talking about :)

Oops, I have been dealing with this topic for almost 30 years. And therefore I ask you for concrete numbers/data/facts. Every seller praises their product and their knowledge as the absolute truth and nothing but the truth, which is also better than the truths of other sellers.

So please get to the point and put the calculation of your experts on the table.

P.S.: The pros who have decades of experience also planned the Elbphilharmonie with €77 million, at the start of construction the plan was already at €241 million, and in the end this fun cost €866 million. Do you want more examples? Also in the small range from daily work?
P.S. 2: Have the assurance given to you in writing including a cost overrun guarantee by the draftsman and head of the construction company...

Yesterday we surveyed the plot and started drawing the plan We now have an external dimension of 12x10 meters and have to reckon with a maximum of €350,000 to €370,000 including ancillary construction costs Our draftsman as well as his son (head of a construction company) assured us that this is a realistic price

And you believe that naively? Nobody here wants to spoil your fun, these are realistic numbers. A 12x10m house costs €350k just like that flat rate, of course you can start like that, but you don't have to. Regards from someone who works at a large construction company.

P.S. 2: Get the assurance in writing including a cost overrun guarantee from the draftsman and the head of the construction company

That doesn't help either. The company goes bankrupt in no time and the money is lost. Personal liability of the boss - that would be an option.

His work is finished after the shell construction.

Since the budget is used up. (;

Since so many people here have a hillside house and can apparently provide experience reports:

How big is your hillside house?
What did it cost?
How much personal effort was invested?
Garage yes or no?

By the way, this is what I imagine as experience reports and not just a: no, that’s not possible

.. it’s just not particularly helpful if the builders give you this information and partly built 2, 4, or even 10 years ago. Especially since everyone built with different specifications.

Therefore, it would be more useful if you create a list of YOUR specifically planned costs and specifications. Then, you will also get information from the local users about possibly forgotten items or items that have been estimated too low or too generously.