Building a house at 21 years old .. too young?

in which you are very flexible and also have no problems fetching the shoes from the utility room or elsewhere

You had more than one pair of shoes at that age?

I just find the discussion about whether I would more easily accept some misplanning in my early 20s stupid, honestly. Even in your early 20s, you want/can try to get the optimum out of things.

The optimum in your early 20s is different from the optimum at 25, 35, or 50. I find the word "misplanning" totally misplaced. That is exactly the kind of narrow-mindedness I meant. One tries desperately to find an unknown and changing optimum. Is everything else inevitably bad? No, it is not! Different is not worse and certainly not misplanning. Misplanning would be if, for example, there was no hallway at all.

Believe it or not, but there are people who adapt flexibly to the circumstances and are totally happy with it.

Only the discussion about whether I accept some planning mistakes more easily in my early 20s annoys me, honestly. Even in your early 20s, you want/can try to get the optimum out of it.

You are right. Even at an older age, you have to accept planning mistakes and the consequences of your own decisions, because they happen. I don’t see money that strictly. Amounts are very relative, and some people already have substantial capital and/or large financial and social securities behind them in their early 20s. At 25, I first went bankrupt with high resistance to advice and blind optimism. This is my "minus house" – paid for but never built. A mistake to learn from. Could someone else have done better at 25 than I did back then and turned the idea into a great long-term success? It can work, and then you should do it. Financial mistakes are repairable.

Even at an older age, you have to accept misplanning and the consequences of your own decisions, because they happen.

Exactly. I currently have a wonderful example of how, being over 50 and having just built a house, you realize that you could have done things differently:
we decided against a masonry heater because we are both working and leave the house early and come home late at night. A heating solution that includes a masonry heater was too complicated for us.

Now: Corona!
We both work from home - if we had known that, we might have chosen a different solution for the heating concept.
And that after not even a year in the new house!
That doesn’t mean we are dissatisfied (well – I always wanted a stove, which I don’t have now), but you can see that no matter how well you plan, life goes its own way.

And that happens beyond the age of 50, too!

The optimum in your early 20s is different from the optimum at 25, 35, or 50. I find the word "misplanning" totally misplaced. That is exactly the kind of narrow-mindedness I meant. People try desperately to find an unknown, changing optimum. Is everything else necessarily bad? No, it isn’t! Different is not worse and certainly not misplanning. Misplanning would be, for example, if there were no hallway at all.

Believe it or not, but there are people who adapt flexibly to the circumstances and are totally happy with it.

That’s exactly how I see it too. It is certainly undisputed that there are cardinal errors. But such a cardinal error does not arise from the positioning of the refrigerator or the choice of heating system. I know people who probably did not make what you would call a smart heating choice but are totally satisfied and simply don’t want trouble with it and pay a few euros more... so what? People overpay noticeably for things because searching/checking would cause them stress, others like searching/checking... Both are fine. I keep experiencing that people want to adjust things to a standard or believe things only make sense one way or another. Yes, my refrigerator is in the dining room and I like it there, I see great advantages for a house with radiators as opposed to underfloor heating, I prefer to spend money on a piece of furniture rather than on a garage... and so on. That’s ME and people of the same age find that crazy. Therefore, an age group is not necessarily uniform in perception. "Dear God has a big zoo" and luckily we are all so different. The older person is "cooler" than the youngster and vice versa, the woman can build walls better than the arrogant man, etc., but he sings better. I can only fully agree with and never understand the compulsive categorization or generalization of people because of their age, profession, or something else. I often walk through new housing developments or am inside new buildings and shake my head, while the owner beams with joy. Where are those 101% perfected houses with the over-optimized floor plans? The question is rather, why can’t I be satisfied or why should I like the same as someone else? If the OP is happy with it, he is probably among the top here in terms of quality of life and grins at some comments, because he feels completely comfortable. He thinks about everything else later, when it becomes relevant, and simply adapts it as needed. We will soon build our endlessly over-optimized house and I constantly ask myself what I could not optimize in advance. Thirty years ago I just started building blindly, spent a lot of money on all kinds of things but one half of the house had only a mediocre floor plan, while I always thought the other part of the house was super great. In the end, I always loved living there; I’m not sure if it will be the same in the new house... well, I hope so... and optimize... read... doubt...--optimize again...

Here an extremely large discussion is being spread that I think is not wanted at all...

The OP wanted our help.. improvements/opinions etc. I think it's quite good when you "open their eyes" and say: Hey, look, you want kids, for this reason the tip: do this and that in the nursery or hey: there are these and those things coming up for you, where you will need more space here and there. It is more of a well-meaning advice than saying: No one is allowed to make mistakes.

At that age you don't look into the future, that is the "problem," but as a result the OP might be very annoyed in a few years that they have no coat rack and never have space for one! That can happen, right? We point that out to them! I suspect that people here just want to give tips so that the OP is not unhappy in a few years. That is meant with optimization and thinking about the future. Everyone makes mistakes, that is clear, it is about which mistake can be avoided, that the OP only realizes when they have taken the tips here, which they currently do not see, but might understand if they deal with it!