Affordable building up to 400k, still possible?

How much slope do you have? It's best to measure (e.g. with a water level hose), because estimating by eye is pretty bad for that. It is clear that with that budget you have to build without a basement. As long as the plot doesn’t have too much slope, you can easily add gravel. We did it that way and so far (the base slab is in place) the house comes out of the terrain much less than expected.

For me, the question is whether I shouldn't buy an existing property. The price is already around 400k, and with 8% on top, we're at 432k.

For an existing property, you have real estate transfer tax, broker, and notary fees. At 8%, you’d already be super cheap, with a favorable federal state and an inexpensive broker. The problem is, that’s often not enough. For houses around 400k, you still have to plan for major renovations. Even if you get a house in decent condition, one thing or another will certainly need to be done in the medium term.

maybe you should also ask yourselves how it will continue without building? Here I rather see the "Pandora's box" in rising rents than "closing your eyes and pushing through with the incalculable additional construction costs". Above all: you will probably also have a significantly higher standard of living for the following decades (of course only after the end of construction/construction stress and assuming you don’t sleep badly because of the loan repayment) The loan amount remains the same, so inflation works in your favor. Rent might be the opposite as rents increase more than salaries?

I don’t understand the problem here 0.0 250k loan with 4k net income? You could finance 500k

Yes, the problem is not the loan itself but that I can't imagine how it should work with the 400k.

There remain 340k after buying the land.
If we deduct 40k for additional construction costs, then we still have 300k.

140 sqm x 2400€/sqm = 336k
For 2400€/sqm there is nothing left here, but in your region it might look different.
(A friend was told last week that she should calculate with 3500€/sqm!)
Despite only 2400€/sqm, we are already 36k over budget.

And what about:
Kitchen?
Furniture?
Terrace?
House entrance?
Garden?
Carport?
Driveway?

Furthermore, net salaries cannot be scaled up proportionally.
Example: You need the first 3500 completely for living expenses excluding housing.
Then with 4500 net I can pay 1000€ installment per month and with 5500€ installment already 2000€ per month at the same standard of living.

With the salary of the thread starter, I would never make 500k in debt.