New single-family house in southern Germany

Where do you want to collect dirty laundry?
Where do you want to soak stains?
Where to store cleaning supplies for the upper floor?
etc. The bathroom on the upper floor is not suitable for this.

Doesn't the arsenic have to be removed anyway, with or without a basement? I would rather not have my house stand on arsenic, or even better, not at all.

In itself, it is harmless* you just must not transport it somewhere else and mix it with uncontaminated soil. Therefore, it won’t become the noise barrier for the new highway but must go to the landfill. Otherwise, as far as I could learn from the soil report, the soil is sufficiently load-bearing to manage with little soil replacement. Assuming 80cm, we would only need 80cbm of excavation, which we can then easily distribute over the rest of the property (calculation: 400sqm distribution area -> an average of 20cm), especially since it slopes slightly to the west anyway. With 2m soil replacement, it would be a completely different matter, which we could no longer distribute, and then the basement would also be set. It was actually with such considerations about soil replacement that I came to ask the municipality for a soil report, and that’s when the arsenic issue came up. *According to everything I found, it is sufficient to wash root vegetables well, then they can be consumed without concern. For strawberries and similar, a straw bed is sometimes recommended to prevent contact with the soil. Otherwise, you can live on arsenic-contaminated soil without problems and don’t have to replace the entire property.

I am currently wondering why nowadays people often do without a basement. Sure, it is (also) a cost issue, but was one practically "forced" due to price increases to build without a basement, or does the majority of home builders simply no longer want a basement? And if the majority no longer wants a basement, was that then partly a reason for the price increases? After all, more money is available to the home builder. Similarly, the correlation between low (construction) interest rates and real estate prices – "cheap money = more money for building/buying a house."

Where do you want to collect dirty laundry?

In the box that has been in the bathroom right next to the washing machine so far. About one load fits in, with a child probably a second box will be needed.

Where do you want to soak stains?

In the sink? Or why do all planners put double sinks in the bathroom?

Where to store cleaning supplies for the upper floor?

Do you mean cleaning supplies like glass cleaner, descaler, and toilet cleaner only, or do you also want to get a separate vacuum cleaner for the upper floor and an extra cleaning bucket? The former fits in a cabinet compartment

..... in the past, basements were standard, attics, party rooms etc., but "Times are a changin", today 70-year-olds build themselves a tiny house or live in senior shared flats etc. or travel around the country with the motorhome for 3 years.

I am still a confessed basement fan, but there are also other opinions, see above, that basements are "out" and a relic of the post-war era for washtub, sausage soup, and grandma’s preserves.

In the washbasin? Or why do all planners put double washbasins in the bathroom?

So that two people can brush their teeth together and don’t get in each other’s way while rinsing. :D