Bank does not accept own contribution

If you have already calculated everything so precisely, like the maximum weight of a beam, there is surely a floor plan or some kind of construction specification, right? It would be quite interesting for clarity if you could also upload that here. How do you want to install large windows or even a lift-and-slide door? Mine alone already weighs several hundred kilos

No, if it comes across that way, then I apologize. I don’t think I can do it better or faster. I’m just convinced that you can manage it well enough and learn a lot along the way.

Actually, we had a lot of trouble in recent years with craft firms that simply don’t show up despite appointments, whose invoices almost doubled in 5 years, and so on. There are not only bad ones – by no means – but I was lucky with some topics to find a good one only with the third craftsman, who fixed the work of the predecessors.

What bothers me is the waiting, the calling after, the rescheduling... then I’d rather take the hammer into my own hand

So you want to deliberately cheat the bank? Well, cheers...

if the bank grants you a loan of 400,000,- for the construction of a single-family house, it also expects a house worth 400,000,-
However, with an equity capital of 120,000,-, as a layperson you will never be able to restore the condition in such a way that the bank will value it like that.
Moreover, if you only draw 280,000,-.

That's exactly how it is for us now. If we were to renovate again, with today's knowledge and skills, we probably wouldn't call any craftsmen except for an electrician, and for the latter we have someone in our circle of acquaintances who would do it. We had the above-mentioned problems with almost all trades; in some cases, there was also botched work, all local professional companies that we would never call again. However, I wouldn't build completely on my own, our knowledge and time would not be sufficient for that.

Regarding the project... that's very ambitious. But you don't have any time pressure, no rent. If I were you, I would just have Plan B ready in case you realize halfway through that it won't work out that way and you can reorganize with external companies. You just have to be aware that it might not work out like this. You only have to get sick or something in your personal life can change and it won't work like this anymore. But if you have a backup plan, why not. And you should also have one in case that, despite a ton of DIY, it ends up being much more expensive than planned. Some numbers also seem too low to me.