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

With such a young builder, very likely yes. At twenty-one, you would have to be out of your mind to want to start with the second house right away. You can only slow yourself down by diverting optimization energy. Better the learning house now and to change it and move on when it has fulfilled its purpose, rather than overestimating it and "optimizing" it now with the energy for moving on (which would be foolish anyway with the little experience of your young years). Look at, for example, Yvonne or Karsten or – all on their second house (the latter even with experience abroad) or even as a chronic repeat house builder: gained maturity is a non-physical but still significantly helpful building block, and understanding the property ladder "does no harm." This typically German tic of living in a house "for eternity" with a hereditary monarch perspective until the last breath is incredibly outdated nonsense.

With such a young client, very likely yes. At twenty-one, you would have to be out of your mind to want to start with the second house right away. You can only hold yourself back by diverting optimization energy. Better to build the learning house now and once it has fulfilled its purpose, change it and move on, rather than overestimating it and "optimizing" with the energy for moving on now (which would be foolish anyway with the little experience of youth). Just look at Yvonne or Karsten or – all on their second house (the latter even with foreign experience) or even as a chronic repeat homebuilder: gained maturity is a non-physical but nevertheless significantly helpful building block, and understanding the property ladder "does no harm". This typically German quirk of living in a house "for eternity" with an hereditary monarchy-like perspective until the last breath is incredibly outdated nonsense.

Phew, I consider that a very, very daring statement. You’re never supposed to say never, but I claim that we will live in our house, which is currently still in planning, until old age.

You may be right; many things in life are more transient, and wishes/expectations in areas like family and job have changed insanely. Here today, gone tomorrow. But distinguish: that does not apply to everyone. We have planned our house almost perfectly and invested a lot of time. It will definitely not be perfect, that’s clear, but we have to come to terms with what we have accomplished. When you invest time, you at least manage to build your dream house with which you can definitely live.

Why would I just start building recklessly and say: Oh, I won’t really read up properly, won’t look at what others are doing, don’t want to learn much from others’ mistakes but want to make my own mistakes on a project that really costs me EVERYTHING I HAVE. Honestly, I consider that total nonsense.

Maybe one builds another house sometime? Yes? Okay... Then please tell me what the chance is that I can build in the same place or in a nearby town given the current chances of getting a building plot (which will not change in the future)? Maybe I associate something with the place (family, relatives, friends, clubs, workplace) that you can’t just easily change. Who tells me that I will even have the chance to build again in the future? I’m not building “provisionally.” If I acquire a home that is not built for renting but for living, in a place I would also like to live, then tell me why I shouldn’t put in 100% work here but just say: Oh, come on, we’ll go into debt for 30 years but let’s just see what happens in 8 years.

A house for eternity? Yes!!! Why not? Not for everyone, clearly, but still for many (at least I can speak for the people who live here in the countryside). So much is attached, really individual life situations/relationship situations... so you definitely can’t generalize and say: Just build, see how you like the result.

I don’t know if I think too “Swabian” here, but for me, it’s not something you can do on the side... I could never come to terms with such a quickly planned house. Let’s see what happens, where life takes me... But the money is definitely gone, the loan runs... Maybe I’m just too down-to-earth and security-conscious, which nowadays is no longer the norm.

One should never say never, but I claim that in our house, which is currently still in planning, we will live there until old age.

Yes, and ... do you notice anything when you compare yourself with the OP? - I do, namely: You are now as "old" as the OP will be at the end of the learning phase I predicted. And - which I forgot to add just now: of course, the OP should not expend absolutely no optimization energy, but only an appropriately small amount for a first stage. In the Pareto sense, I do not at all want to encourage him to adopt a don't-care attitude and settle for 80%. But I think he should raise the planning quality to 83% (this is achievable solely through detailed planning of the living-cooking area with concentrated swarm energy and on top of that the described simple correction of the bay window), but beyond that he should not exhaust himself trying to push through the wall with his head to aim for 100% (which in my opinion at such a young age in the end could only lead to the saying, Varus, give me back my legions).

Yes, and ... do you notice anything when you compare yourself to the OP? - I do, namely: you are now as "old" as the OP will be at the end of the learning phase I predicted. And - which I just forgot to add: of course, the OP should not expend absolutely no optimization energy, but only an appropriately small amount for a first stage. In the Pareto sense, I do not want to encourage him to adopt a couldn’t-care-less attitude and settle for 80%. But I think he should raise the planning quality to 83% (this can be done just by detailed planning of the living-kitchen area with concentrated swarm energy and on top of that the described simple correction of the bay window), but beyond that he should not exhaust himself trying to achieve 100% by banging his head against the wall (which in my opinion at such a young age in the end could only lead to the remark that Varus should give him back his legions).

That may be so, but that does not change the points I raised: - Place of residence (can I ever build nearby again at all)? Do I take the risk of building not so "great"? - Everything attached to it family/relationship/job etc. - Degree of debt which, whether 20 or 40, immediately burdens your account. No matter whether you pay off until 65 or 55 None of this changes the fact that this exact home COULD be for the rest of his life. So why shouldn’t I invest exactly this energy? I simply cannot predict where I will end up? I of course assume that this is the dream of a home of one’s own. For me it’s very clear: 100%!!! No matter how old you are, no matter what you plan or still intend. But as I said, maybe I think too "Swabian" or am too perfectionist. But honestly.. we are talking about Pareto and a 3% increase in planning quality etc. To put it in very approachable language: Man alive, invest time, this is your home, your money, you are indebting yourself forever, you want to live in it for a long time, then make the best of it!!

I just googled Pareto.

Basically, it seems fitting to me:
80% of the results are achieved with 20% of the total effort. The remaining 20% of the results require 80% of the total effort and quantitatively the most work.

But that does not mean for me that I should not strive for a 100% solution.

Anyway, nothing is ever 100% "the best".
Inner processes are at least as important as the material ... and the effort for good solutions is not just "hard work" for everyone.

I love working on optimizations.
That is pleasure for me - and "failures" can not only be annoying but also amusing.

... and the fact that you work "Pareto-like" more at 21 than later is nowhere stated anyway.

So for many, 21 would be too young to build. But I have found that there is an incredible range among 21-year-olds, so you can't generalize. And I have also encountered some "younger" builders (online)... by that I mean in their 20s. I therefore find that absolutely legitimate and I think you should definitely use your youthful energy for it. Don't let yourself be held back, but think everything through carefully. However, I would definitely still plan an exit strategy, for example if an income fails. And it certainly always helps to have a father, grandfather, uncle involved who might have a bit more life experience.