Is a house for a total volume of 400k still possible at all?

That doesn’t sound too expensive now, all inclusive. A basement was really the last thing we would have given up. Only if no other house is possible. For the price of the basement, we also wouldn’t have gotten an extra 100sqm of space. If I now also calculate the costs of the necessarily larger house you then need, it is, roughly speaking, a zero-sum game.

"Needs" is just so relative. Many believe they need a basement because they are used to it, and in the end it just gets cluttered.

We spend even more on our mini cabin - and are really happy about it :)

That's the most important thing about the house :)

"Need" is pretty relative. Many believe they need a basement because they are used to it, and in the end, it just gets cluttered.

We tried to keep the floor area as small as possible. Therefore, we don't have any excess space above ground and would have had to build bigger. That means more costs for the house and even less garden. A colleague at work was persuaded to build without a basement. Now he regrets it. The double garage is now a substitute basement, and both cars are outside. But everyone is free to do as they please and can. As I said, in an emergency we would have given it up too. But very reluctantly.

Hey, you’re comparing your conditions with those of the OP again. He has a relatively small plot and depends on a basement for his "wishes." We also left out the basement to be able to build bigger on the ground floor. So that the whole thing appears "airier," because I didn’t want to build smaller than 160sqm. With a basement, I would have had to build at least 20sqm smaller. But our plot is almost twice as big. A basement is a fundamental decision.

Now during construction and own services: One room serves as a "storage room" for materials and tools, drink crates, spare tiles, etc. Since our garage is coming later and we don’t have a basement, we are glad that we still have such a room (office/guest room) "left over." I’ve often wished for a basement there or at least that the garage would have been built immediately.

Another disadvantage of a smaller plot with a basement is that not much can be temporarily stored during excavation. Then everything is hauled away just to procure material again later. With a slab foundation, normally not as much has to be hauled away. But that also strongly depends on the soil.

Estimate the costs of a basement realistically once: costs for the basement shell, costs for excavation/deposit ...

Insanity to spend so much money on such a tiny cabin without a basement and garage. For me, that would be the point to say, I’ll leave it and rather rent something proper.

I can only fully agree with KingJulian there. At some point, it stops being fun when you have to give up so much.

But this won’t work with your €1,000,000 for a 130m² cabin in the provinces. Just substitute, pah!
And who is supposed to drive the bubble then if Homo oeconomicus simply no longer does what he is supposed to do?

Do you want a workshop in the basement? Carrying everything up and down?
Bulky parts over a staircase?
In the end, it's more expensive storage space for tools and you end up working in the garden or carport.

I dry most of my laundry on a drying rack upstairs. Luxury with 120 sqm. With that, I would do the laundry on the ground floor and dry as much as possible outside.

120 sqm is already ambitious. 400k too. You have to prioritize. Fewer desires or more budget.

What floor class do you have?
Disposal is also an issue and can range from cheap to unaffordable. Hazard class, landfill fees, travel times.
What about the groundwater?

we have the opportunity to purchase a plot of land relatively cheaply (360sqm for 75k) near Düsseldorf. This is very cheap for the region, as the standard land value is stated as 270€, but the actual value is much higher.

We come from Ratingen and have been looking for a building plot around here this year (and now found one in Mönchengladbach). If you were offered it so out of the blue, you don’t know how crazy the market is.
There are basically NO building plots on the market. If something does come up, it is sold at 50%-80% above the standard land value.
Usually there are also contaminants that have to be demolished first.

Though I do wonder where there is still a 270 Euro standard land value here...
Would you like to say roughly where the plot is located?

Either way, I assume that the plot could be sold on the open market for well over 100k. (Of course depends on slope etc. Quite common in the Bergisches Land.)
So you already made a good "profit" at the time of purchase.
Prices here will not go down anytime soon. Düsseldorf has so far been almost unaffected by the corona crisis. People are doing great. House prices across Germany have risen by more than 6% during the crisis. Sure, in this area even more.
I have no crystal ball, but I assume that all commuter belts around popular cities will gain significantly more value with the increasing share of home office.

Regarding the house price: read the threads about the basement discussions. At least you’ll get new ideas there. We would have liked a basement too. A few sqm more living space above ground and a garden shed are much cheaper than a basement. In my opinion, that’s not a zero-sum calculation.

We are now building 148sqm for about 285,000 euros (plus 65k ancillary construction costs + land costs + outdoor facilities).
So it’s doable. But you have to be very disciplined with the fittings. That sometimes hurts. But that is normal. Even the generations after us didn’t have their dream house standing from day 1. It was improved and upgraded step by step.
But here in the forum you get a distorted picture of the house-building society. Here, only above-average amounts are invested right from the start.
So: simply calculate fittings. For elaborate things, just do the preparation during the construction phase (e.g. photovoltaic). Then it works. Something a bit more comfortable might cost maybe 20k more.

In conclusion: think carefully whether you want to turn down the offer. If you don’t take it, in my opinion you will never build a house. Searching for a plot on your own in 2-3 years will drive you to despair...