How many square meters are possible for KFW40 turnkey with a budget of 150,000€?

There is certainly more botching by DIY enthusiasts than by craftsmen.

That is your opinion, which you can no more prove than I can prove mine:

My opinion is that a builder does many tasks on his own construction site much more neatly than a contractor under time pressure.
Example: Insulation of the floor under the underfloor heating. We laid the insulation around the pipes so meticulously precisely, a contractor simply cannot afford that. But we also took 2 weeks for it. And nobody needs to tell me that the apprentice/helper of a sanitary company can achieve the same quality in 2 days...

The same applies, by the way, to the insulation between the rafters with mineral wool.

Over both insulations (floor insulation and mineral wool), a foil or vapor barrier is quickly installed and no one can see any sloppy work anymore. I would never pay anyone to do such work in a way that is not done properly.

How do I come to the accusation that I cannot prove? 4 friends/family work in different companies and have told me how such work proceeds when carried out by them and who does it (apprentice etc.).
Certainly not all companies work like this, but nowadays it is certainly no exception!

A good craftsman is recognized by the fact that he works quickly and neatly. Why don't you just do everything yourself... have you ever wondered how a workshop can change 4 tires for 20 €? You have to have a bit of basic trust in the craft; just as I do my work properly, I assume that a craftsman delivers his work according to the state of the art.

..... One must have a little basic trust in the craft; just as I do my work properly, I assume that a craftsman delivers his work according to the state of the art.

I would actually see it 180 degrees the other way: A basic mistrust should always be the starting point....... Anyone who has ever really built something knows what I mean.....

You must NEVER assume that work is done properly. If it is anyway: great. If not, you are glad to have found out in time (inspection!). I have often read here: "Well, it is just the way it is now." Such things must be reduced to an absolute minimum. It is labor-intensive, but sustainable (by that I mean the on-site inspections). Apparently, there is no other way.

But we also took 2 weeks for that. And no one needs to tell me that the trainee/laborer of a plumbing company can achieve the same quality in 2 days...

Again, a bad comparison. Who is "we"? 2 people? Then the plumber alone also has 1 week, not 2 days, if we stay factual here. I am sure that a skilled worker can even manage it in one day. As a student, I once laid insulation for a summer in a screed company. A single-family house is done by a 3-man team by lunchtime. And accurately. As a beginner, you need five times as long because you constantly misjudge the radii and distances!

Over both insulations (floor insulation and mineral wool) a foil or vapor barrier is quickly placed, and no one can see the sloppy work anymore.

If the contractor realizes that he can cheat the client thoroughly, he might sometimes do that. That’s what controls are for.

I experienced it with our carport, the opposite example was my neighbor. Two craftsmen came to me, once they poured concrete footings for 4 hours, they had to remove existing paving and then cut and reassemble it, I think there are a total of 12 footings. On the second appointment, they built the carport from 8:00 to 12:00... the good thing about it is I have a warranty but I don’t think the carport will fall over. My neighbor is building it himself... for now 5 weekends... In the meantime, I mow my lawn, go for walks with my child, and visit the grandmas.

Wow... The topic changes so quickly from the original question to the [Eigenleistungs-Thema].