New construction single - Civil servant permanent service realistic?

But does it make sense to still build at 40-45 at worst? I just want to be done with the financing by retirement, preferably 2-3 years earlier.

Doesn't that contradict itself?

Let’s take the amount of money you can save until retirement. If you save it, then it goes 100% into the account and earns interest *ahem*. If you take the same money and build now, part of it goes as interest to the bank and only the rest adds to your assets in the form of property (and the house value slowly decreases overall).

So if the life plan is not yet set in stone (who can say that with not even 30 years old?), which is purely economically the smarter option?

Maybe instead you start at 40 years old with 60% equity and finish quickly as well.

In your situation, I would continue saving. You are constantly commuting and at risk of transfer.

The first sentence from Nordlys probably hits the mark best. I have, of course, already experienced both transfers and assignments. I had postings where I was home every evening, but also ones where I was a weekend commuter.

The region where I want to settle is 100% certain. I have family, friends, and my child around me here. In addition, it is affordable.
My consideration was simply that, before I throw 400€ a month into the mouth of a landlord here, I might as well build myself. If necessary, you can rent out the home, right?

Besides, it is affordable. My thought was, before I throw 400€ a month to a landlord here

Is that cold or warm? And it’s not just thrown down the throat. After all, you get a service for it.

Net I earn about 3200€ monthly.

How much remains each month?

Does a granny flat possibly make sense, which can be reintegrated into the whole house if needed?

If you have the equity to build that part. At first, a granny flat just barely pays for itself, or is a subsidized business (with zero or little equity).

A small house with a large garage and lift (for the hobby) without other modern frills (highlight would be underfloor heating)

About how big is your idea in sqm?

The 400 are warm, depending on the apartment and size here in the region. The earnings (3200€) are reached in 4 years. 1000€ - 1200€ are realistic, I think. A granny flat was only a consideration. But if I'm not at home during the week, I probably wouldn't feel good about it. Therefore rather unlikely. Size max. 120-130 sqm

26 is usually not the age where you commit so strongly. But I have the feeling the OP knows what he wants.
The 130m² house is a nice size for a single dad with a child and possible family expansion. I would try to save at least 50K before the construction.

If the building area is so fixed, I would look for the dream plot. By the time you find it, you will have saved the 30 to 35 grand equity and buy it. You have time and can find something that does not have a slope location with contaminated soil. Preferably flat and fully developed saves money again. The plot remains as equity for you and nothing works without it anyway.