Is it possible to build a new building for 900€ per m²?

How can, for example, Heinz von Heiden be so cheap turnkey? They want to make money too. I also described the equipment in the initial post, which is already very cost-effective, I can hardly imagine that such a house with this equipment costs 3,000€ per m2.

How can, for example, Heinz von Heiden be so cheap with turnkey construction? They want to make money too.

As always, these are "starting from" prices. And turnkey does not mean move-in ready. Furthermore, it does not mean that there are no more tasks left for the builder. And these low prices are only the "starting from" price for the absolutely minimalist houses.
A good example of how you quickly end up at the "starting from" price of €2,200/sqm if you don’t choose the cheap option.
You can live in both. You can also get from A to B in a Smart, but it’s already more comfortable in the C-Class, and the S-Class is great. All from Mercedes. And all are "starting from" prices in the configuration. But almost 100% upgrade. With the base price, no money is actually made.
A client of mine is currently planning social housing for €2,900/sqm...

As always, these are "from" prices. And turnkey does not mean move-in ready. Furthermore, it also does not mean that there are no tasks left for the builder. And these small prices are only for the absolutely minimalist houses at the "from" price.
A nice example – how quickly you get to the "from" price of €2,200/sqm when you don’t choose the cheap variant.
You can live in both. You can also get from A to B in the Smart, but it is more comfortable in the C-Class, and the S-Class is great. All from Mercedes. And all are "from" prices in the configuration. But almost 100% upgrade. No money is actually made with the base price.

The problem is that at the moment I live in an unrenovated attic apartment from 1950 because the prices here are just completely crazy, but through connections I can get cheap developed building land. Therefore, I had the idea to build a new house with cheap equipment and own work, for example if you build a 1.5-storey house you can move in even if only the ground floor is finished. Because compared to the unrenovated attic apartment, even the cheapest new building is a luxury. In the initial post I described the (cheap) equipment of the house from my acquaintance and that he (since he works as a bricklayer) built it in almost 100% own work for €900 per sqm, therefore I had assumed to be able to build a house with such equipment and own work for only €1,700 per sqm (without additional construction costs) except floor coverings and painting.

The problem is that I am currently living in an unrenovated attic apartment from 1950 because the prices here are simply completely crazy, but through connections I can get cheap developed building land. That’s why I had the idea to build a new house with cheap equipment and own labor, e.g. if you build a one-and-a-half-story house you can also move in when only the ground floor is finished. Because compared to the unrenovated attic apartment even the cheapest new build is a luxury. In the first post I described the (cheap) equipment of the house from my acquaintance and that he (he works as a bricklayer) built it with almost 100% own labor for 900€ per m2, therefore I had assumed to be able to build a house with such equipment and own labor only for 1,700€ per m2, just for flooring and painting work. (Without incidental construction costs)

If you have the money lying around for the construction and the time, then good luck. And as I said, you also need masters who put their name on it for you. And remember, the smaller the house, the more expensive it gets per square meter.

Oh yes, you also need "stamping guys," because various trades require master certification - for example, thermal insulation as your own work (ETICS) is forbidden for you. Water pipes are not allowed either. Etc.

The statement by is wrong!

Of course you can insulate your house yourself. If you don’t need a loan and no KfW nonsense, you can do everything yourself.
You can also do the water pipes yourself. You lay everything in copper pipe and the plumber presses everything together for you.
Or you buy a pipe pressing tool for 500 euros and press the fittings together yourself. Nothing more needs to be soldered.
The plumber’s assistants only do that anyway.

You can buy the material either online or in a specialist store.

Same with electricity. Some are even glad when customers lend a hand themselves and their expensive specialists don’t have to knock out cable ducts or sockets. You can do that yourself. Of course, you have to look a bit to find a suitable electrician. But that is possible too.

The statement by @nordanney is incorrect

No. The installation of an ETICS requires either a painter's or a plasterer's company. Both trades are subject to mandatory master certification by law. You may be able to do it, but you are not allowed to.

Or you buy a pipe pressing tool for 500 euros and press the fittings together yourself. Then nothing needs to be soldered anymore.

That is also prohibited. Officially, only specialist companies are allowed to do that. That is why I say you need a "heini" who sticks his neck out for you in various trades. And thereby directly takes on the warranty, since he officially carried out the trades. Could be funny...