New single-family house in southern Germany

and that's why no bathtub today because we don't need it today

It's that simple. I build exactly what I and my loved ones need!

Still, it is standard

I need to quickly look up the relevant DIN......where this was defined; it is YOUR own standard, I can understand that. I know what you want to say, don't worry, in the past a basement was standard, attic, party room, etc. but "Times are a changin'," nowadays 70-year-olds build a tiny house or live in senior shared apartments, etc., or travel around the country for 3 years with the motorhome. None of that fit into any standard back then. Fortunately, today there is simply MORE than just the one thing. We ourselves will definitely have a very stylish tub because we want it that way for ourselves but only for that reason. When I often read about "standards," it sounds more like a one-size-fits-all approach; when building, I place special value on my very individual ideas. Such standards in house furnishings/design are foreign to me; my taste and my needs count. Merry Christmas and now I'm going to lie down in ours first..... o_O

I know what you mean, don't worry, basements used to be standard, attics, party rooms, etc.

When was basement standard? 1950??? Certainly not for the past 20 years.

When was a basement standard? 1950??? Certainly not for the last 20 years.

Yes, in the days of "Neuer Heimat," a masonry "Schopfen," for example, was also standard, just like a wash tub in the basement (home slaughter + sausage soup), and this basement was exclusively for homemade juices, own cider, potatoes, etc. That was in the 60s, so my childhood until the 70s. Even in the flatlands by the coast, basements were usually common. True, not so much in the last 20 years, but in my life I have already seen various such transient "standards" come and go. What is sometimes considered a "standard" today will be old hat in 20 years, and no one really knows what it will look like then.

That's also called development or progress. At least there is no washtub in my basement, and for me, my basement is luxury. Nowadays, it hasn't gone out of fashion to build a basement, it's just simply too expensive. I can throw away the junk in the basement, but building a basement afterward is notoriously difficult.

If a missing bathtub can prevent a sale, it can just as well be another "missing" feature or different taste, etc.

Well, there are "nice to haves" and then there are things that simply count as the expected standard for the broad majority in 2020.
Try selling a car without air conditioning...
I never bathe either and neither does my wife (she only uses it to wash her hair), but never once did it occur to us to do without a bathtub.
Families with children, dog owners, wellness enthusiasts, etc. – they all value a bathtub.
If my house is not an option for these groups because of the missing bathtub, the pool of potential buyers significantly thins out.
Of course, you can also be lucky and find the one buyer who thinks just like you and has the necessary cash. But I wouldn’t count on that. No one knows what the market will look like in 20 years and whether times might come when the buyers call the shots again.

At least there is no washtub in my basement

Nope, just old sofas and plenty of "junk"....... fortunately, we're different, and that's allowed

but giving up a bathtub would never have crossed our minds.

It wasn’t my idea, we are self-proclaimed bathtub owners. But would you generally install something in a house today that takes up both money and space, something you yourself never want your entire life, just SOLELY because of a potential buyer after your death, divorce or something else? I ONLY answered that. Of course I have a bathtub myself, because it’s great for us..

then the pool of interested parties narrows significantly

Is that what a young person keeps in mind when planning their house? “Wow” – not me, and I have still sold “everything” well and quickly so far. Nope, I build WITHOUT any thoughts about whether someone might want it like that later or be able to sell it. These considerations are completely foreign to me in my own house and they surprise me among young people who seem so modern. Sorry dear later users, we did it the way we liked o_O and without considering a possible resale. It should be OUR home and not an investment for your generation. Oh dear... now I also learn that I am not a “more sensible” standard... lovely.