Is a basement in a single-family house useful or rather too expensive?

Also and especially in southern Germany, this is changing. For additional costs of > 60k €, you can easily get 200 sqm more land in rural areas.

Those who have no financial limits when building a house will build a basement. Those who do not have unlimited possibilities should really consider in my opinion whether a basement is more important to them than land area, living space, location, etc...

I find the common argument "resale value" very weak, because the deduction there should in most cases roughly correspond to the additional costs...

You are not entirely wrong there. But I have heard that a foundation slab is not a bargain either. And you need it even without a basement, right? For a sloping plot, you also have to factor in the cost of filling. And on the ground floor, you then need compensation area. With a basement, you might be cheaper and get more out of it.

For me, it remains a matter of habit and type of property.

Slope properties are the big exception, of course.

Sloping plot No basement, but normal living spaces, like many new buildings. In the case of sloping plots, living spaces in the basement are usually suitable. The garden is part of the living spaces. Filling + retaining + floor slab is usually more expensive than basement/cellar

Also, and especially in southern Germany, this is changing. For additional costs of > €60k, you can easily get 200 sqm more land in rural areas.

I am firmly convinced that the marginal utility of more land decreases significantly with the size of the property and after some time even becomes negative. In other words: beyond a certain area, the value decreases for me again because I cannot do anything additionally meaningful with it, but I have more costs and more effort.

I think one rarely reaches this limit. I once read that 750 sqm of land would be ideal for a medium-sized single-family house. Only the fewest reach that.