Stay here (and build) or move away (and build/buy there)

What kind of "argument" is that? Please be aware that people like the OP seek confirmation for themselves, rightly do not receive it here (approx. 800k loan with 4.4k income!!!!!) and might therefore be spared from a big mistake/misfortune. When someone like you casually throws out such an untenable sentence without any context, it carries the risk that an OP might latch onto it and ignore the other aspects, which can simply become dangerous.

Your causal chain "I don't want to call your plan impossible, because I find your wish too nice." is, sorry, madness.

That is why I referred to the wisdom of the other forum members ;-) and somewhat relativized the first paragraph again with my last sentence. But you are certainly right as well.

What I notice here in the forum is that building a house/acquiring home ownership is mostly portrayed as if it happens "on the side", easily, casually, alongside fancy cars, demanding hobbies, nice vacations, restaurant visits, etc. Building a house is rarely perceived here as it might have been made possible by "our" parents/grandparents, namely sacrificing something else, which can sometimes be easy or not perceived as a sacrifice because living in your own house with a garden is "dreamlike."

In my opinion, we in the forum cannot estimate how much the "dream of your own house with garden" can motivate someone, how easily other forum members might be able to forego the aforementioned amenities, whether someone perhaps already wants to take on a well-paid second fun job, will inherit in a few years, or can tap additional sources of income, switch from part-time to full-time, etc. Likewise, I am of the opinion that rising rents are not to be neglected, and anyone who buys fruit and vegetables in the store (like me) sometimes thinks about whether a little fruit and vegetables grown in their own garden might not offer savings potential and at the same time greater enjoyment.

For me, for example, the rising rents and the scarce availability of living space are frightening, and I have influence on energy costs/additional costs in my own house, but little to none in a rental property. Also, the thread starter earns too much to receive housing allowance, eligibility certificates, or similar.

I assume that the thread starter and their family will make the right decision for them, and I have pointed out one of the risks.

Nevertheless, you are of course right, and perhaps we will still learn how the thread starter decided.

'Construction ruins' are regularly auctioned off because the builders have overextended themselves. Self-built construction sites / houses also regularly come onto the market because marriages have not withstood this burden. Life is not a pony farm.

Of course, there are people who manage to build under these conditions - but the OP does not seem to be one of them; however, these people save rigorously very early in life for their dream house, help others on their construction sites for years - partly to gain experience themselves, partly to collect 'relationship points' so that they can also expect support for their own work.

I doubt that he will find any bank at all that would consider financing a house for him... his dreams are too far from reality for that.

In his place, I would look for a suitable rental apartment with a garden share; that is far more realistic. Most people successfully raise their offspring in rental apartments.

Regardless of the budget, I have to make a plea for staying.

We are in the opposite situation. We built in Lower Saxony in 2020 and are here 550km away from the family. We also have a child. During Corona, social contacts with friends were severely restricted because most people we know limited their few contacts to their families (to avoid, for example, endangering grandparents). We were very alone here for a long time, and as our son got older, we noticed more and more how lonely we felt here and how much we would like to see the family more often than 1-2 times a year.

Despite the new build and all the associated hurdles, we have now decided to sell, move back to the family (for me after more than 10 years), and build new again there.
I am very relieved that we are doing this and can do this – I can even feel it physically. Despite the stress of selling and moving, I sleep much better at night again. Above all, I can fall asleep better again.

Accordingly, I would definitely not advise you to move away from the family with children.
Or at least to be aware of the consequences.
My mom is over 70 years old. You never know how old people will get, and the thought that if I continue to see her only once a year, I could probably count on two hands how many times we will meet again was unbearable for me.

Edit: Since you can’t completely ignore the finances here: With an 800k loan on 4.4k net income, I certainly wouldn’t advise you to do this.

I think the issue has resolved itself due to the recently increased construction interest rates.
Reason 350k (development, taxes, notary inc.?)
House 450k (standard single-family house with gable roof)
Additional construction costs 50k
further 50k (driveway, garden, carport, fence, kitchen - probably won’t be enough - so 75k)

are pretty realistic 925,000€.
If you leave out the 10k equity buffer for the double burden, the following remains:
925k loan, of which 30k equity, 60k interest-free loan.
835k at approx. 2.3% interest (20 years)

The initial interest alone is 1600€/month.
Under 3000€ rate per month is hardly possible
plus 500€ ancillary and operating costs.
plus the eventual repayment of friends at 60k (when is that still supposed to be saved)

Crazy!
Then is there millet and water to eat or what.

are quite realistic €925,000
If you leave out the €10k equity buffer for the double burden, the following remains:
€925k loan, of which €30k equity, €60k interest-free borrowed.
€835k at about 2.3% interest (20Y)

The initial interest alone is €1600/month.
Less than €3000 installment per month is hardly possible
plus €500 ancillary and operating costs.
plus the eventual repayment to friends of €60k (when is that supposed to be saved up)

With at least 1% repayment (maybe some banks allow it), it would still be just under €2,300 per month.
Possibly the last option remains a repayment suspension loan.
But what is supposed to happen with the remaining debt of €800k in 20-30 years?

With at least 1% repayment (maybe some banks will agree), it would still be just under €2,300 per month.
As a last option, a repayment suspension loan might remain.
But what is supposed to happen with the remaining debt of €800k in 20-30 years?

As my colleague recently said so nicely:
I always wanted to drive a car worth half a million.
I just didn’t know it would be a Ford Focus ;).