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

I think the idea is good if the framework conditions are right.

[A bakery saleswoman also has a regular salary and earns no net 1500€ at 40 hours.

Give me some numbers (salary; how big should your house be and what should it roughly cost?)! There is a reason why currently no one under 25 is building here in the forum. It's not just the reasons that Hilfe mentions.

Besides, not everyone is a Thomas Müller who is already spending half his life with his childhood sweetheart.]

With less than 1500€ I wouldn’t even consider building a house in these years. I’m relatively lucky with my position after training. The salary of me and my girlfriend amounts to 4000€ - 6000€ net. You don’t have to be too exact now.

The plot is about 600 sqm in size. The budget for everything is 300,000€. Everyone in the family is skilled in craftsmanship, so we want to contribute a lot of own work. We have imagined living space of about 130-150 sqm.

With less than €1500, I wouldn't even consider building a house over the years. I'm relatively lucky with my position after training. The salary of me and my girlfriend amounts to between €4000 and €6000 net. You don't have to be that exact now.

The plot is about 600 sqm in size. The budget for everything is €300,000. Everyone in the family is handy, so we want to contribute a lot of own work. We imagined living space of about 130-150 sqm.

That doesn't sound like rose-colored glasses. So €50k more shouldn’t hurt you either!
“From construction” would be better than just “handy,” but it is what it is. You just can’t step on your own family’s toes with government subsidies if something is done wrong or if no one shows up sometimes. One of my five neighbors has been building for 1.5 years and is now getting the screed, feels like 50% own work. But you’re young, have time, and as long as there are no children, you can do it that way.

So the rest (non-financial) just has to be right.

I can understand your desire for a house at a young age! If your relationship feels right and the financing works, I would start building. Especially since you are lucky enough to already own a plot of land and have successfully been together for 5 years. If your girlfriend is just as enthusiastic, then go ahead and fulfill your dream. Breakups, unemployment, and other issues happen at any age, the risk will always be there. Besides, you can set the financing more flexibly now at a young age (e.g., over 25-30 years), allowing you to treat yourselves to something else alongside the house.

however, I have always decided that for me only a prefab house is an option.

I often read this statement here, from people who are twice as "old" as you, and where this attitude is still based on myths. Where does the clarity come from in your case?

I want to build so early because I want to have the house paid off as early as possible.

The first house is built for an enemy, says a folk wisdom. My first impulse was therefore skepticism, thinking that at your age you can hardly have enough life experience to have adjusted your self-assessment accurately enough so that you don't naively plan past your needs. On the other hand: you are never too young to make mistakes – and the younger you make them, the longer you can still apply the insights. So: yes, start now, then you can sell the first place fully paid off again when others are building for the first time. : that sounds good, being 21 again in your building experience situation?

If you want to finance together but marry first and arrange everything contractually. Whoever is listed in the land register owns the house. At your age, I preferred to travel and treat myself to nice things. The girlfriend at the time (7-year relationship) is also history... The question is whether you both are so rooted and can/want to stay there for sure. If the framework conditions are right, there is nothing against building at your age. But I would rather wait one or two more years. Then the crisis will have also reached the construction industry. If you still have a secure job then, you can build more cheaply.