Building with a small budget feasible?

That is individual. You could also sell the house and buy a motorhome and travel around the world with it. The advantage is simply that the money goes into something tangible.

Our original approach regarding the monthly burden was between 750 and 900 euros. That would mean buying a house for around 200 ' . If we found something suitable to rent, we would be at 1000 to 1200 €KM. But that money would then be gone forever, and big consumption wouldn’t be possible either.

Priority is primarily on buying an existing property, but if you don’t find anything there or invest too much in renovation, you come up with funny ideas and ask in a house-building forum about the feasibility of a new build (which at least keeps reserves for maintenance and heating costs low).

14 days ago we looked at a semi-detached house. Good location, large plot with many outbuildings, fully basemented with nicely calculated 85 sqm living space for 140'. We really thought about it for a long time but have now decided against it because so much would have had to be changed for it to fit reasonably. Then we might have ended up at 250 '. But it is still just a semi-detached house....

Then it is still only a semi-detached house....

Don't be mad at me, but I don't know if you can afford these demands. We have already done enough calculations in the thread on how it might just be possible to build a single-family house. The semi-detached house would be the bird in the hand and would not push you as close to the financial limit as the bird on the roof (newly built single-family house).

14 days ago we looked at a semi-detached house. Good location, large plot with many outbuildings, fully basemented with nicely calculated 85 sqm of living space for 140,000. We really thought about it for a long time but have now decided against it because so much would have had to be changed for it to fit reasonably well. Then we might have ended up at 250,000. But it still would have been just a semi-detached house....

What would have needed to be changed there?

And personally, I am now wondering.
"Just" a semi-detached house?
I know many people who earn significantly more than you and live "just" in a terraced house/semi-detached house.
Why is that not good enough for you?

Then it is still only a semi-detached house

What you want is a pie in the sky with kangaroo financing. Big leaps with an empty wallet.

Sorry for my honesty, but my neighbor is the chief physician of a clinic, drives a Japanese SUV, and was only able to build a semi-detached house.

What would have needed to be changed there? And personally, I am surprised now. "Only" a semi-detached house? I know many people who earn significantly more than you and live "only" in a terraced house/a semi-detached house. Why is that not good enough for you?

That does not necessarily have to do with being good enough. Basically, we would have had to move every wall. The upper floor would have needed to be enlarged with dormers at least. An extension on the lower annex would have been possible too. The bathroom is in the basement. On the ground floor only a small toilet, upstairs two tiny rooms with significant sloping ceilings. Windows, electrical system also new. Roof in the foreseeable future as well. The calculation could also be significantly off here with many uncertainties.

What you want is a cloud-cuckoo land with kangaroo financing. Big leaps with an empty bag. Sorry for my honesty, but my neighbor is chief physician of a clinic, drives a Japanese SUV and was only able to build a semi-detached house.

... Even on 85 sqm? Respect.