Single-family house with >180 sqm / basement / garage

For 400k€ you won't manage that. At best, you'll end up with 150sqm, if at all.

Somewhat sobering... Does a partial basement save anything?

The costs are shocking... Today you almost have to be a millionaire...

no, you don’t have to.

The basement should be used as a laundry room, wine cellar, heating room, and storage space...

Storage space is expensive. Anyone who can buy wine suitable for aging nowadays (consumption wine is only good for about 2 years) also has the cash.

Building castles in the air, gladly big and even bigger, anyone can do that. But with a bit of realism, knowing that everything has its price, you can also approach building a house.

This includes saying goodbye to old habits. This includes the basement, which is just too expensive for storing some goods. Besides, a living area, especially if kept open, is qualitatively valuable even under 50sqm.
You can wash laundry in a room on the upper floor (where the laundry is generated) and technical equipment can be installed at ground level. And that saves €50,000.

If you then have a big desire for children, the children’s rooms just have to be a bit smaller.
has 5 children and is having an affordable house built. Without a basement. She had previously created a room program and a cost target.

Quite sobering somehow... Does a partial basement make any savings?

nothing to hardly anything. Unless you have a sloping plot.

A house for 4-5 has a living room with kitchen of approximately 37-40. One bedroom 15. Three kids' rooms at 11-12, a utility room around 12, so you can also store something. There are about 100 sqm of good living space. Additionally hallway, easily accessible attic, prefabricated garage, garden shed. If you plan like this, the money is enough too.

A house for 4-5 people has a living room with kitchen of approximately 37-40. One bedroom 15. Three children's rooms each 11-12, a utility room around 12, so you can also store things. About 100 sqm of good living space. Additionally, hallway, easily accessible attic, prefabricated garage, garden shed. If you plan like this, the money will be enough.

And the bathing is done in the rain barrel.

Alright, we'll also provide a bathroom 10 and a guest WC 2.5...

If you can add another 100k, it might just be enough.
Where in BW is the construction supposed to take place?
Depending on the region, there are unfortunately price differences of 50k.
In a cheap area, it might be enough with some own contribution.
The size and location of the plot also play a pretty big role.

I would leave out the basement and the balcony. You hardly ever use a balcony in a single-family house if you have a garden.
At most, maybe in the summer to hang laundry.