Single-family house planning with a granny flat - ~230m² - basement - gable roof

Why? There is a door to the residential unit that can certainly be locked.

As far as I know, a separate entrance is not required at all.

The privacy of one of the two apartments is not guaranteed. The additional parking spaces will also be expensive.

A completely new aspect.

Why do you want to plan a house now that you DO NOT need NOW.
Plan a house as it is needed NOW and until the son moves out. Then sell the (possibly already in need of renovation) hut that is no longer needed and buy a cheap new bungalow from me, which is on one level and barrier-free. That way you are really prepared for old age.
I just cannot understand the discussions about "what do I do with the house in old age".

Anyone who needs a big car with children buys one. Those who are just two might buy a Polo, etc.
Why does the German always have to spend their whole life in one house and hope to get the jack of all trades.

Because very few people want to build 2-3 times - building is (often) too stressful and too expensive for that.

We are not in America, where the single-family house (not generally) lasts for generations.

Moreover, not everyone lives in an area where you can easily get rid of the huts again.

Better to build too big than too small :).

Because very few want to build 2-3 times - building is (often) too stressful and too expensive for that.

Own fault, then you have to live with compromises - you don’t have to build, you can also buy. Only a few have that stress.
I don't know any nation that is as inflexible in terms of housing as we are :(

What do you mean by "your own fault"? Even buying can mean stress - not always just seeing the good weather flight, which is rare.

Building always means a compromise - I can only partly understand your opinion there. Maybe you have a comfortable position, but most people don't...

Maybe Germans are inflexible in that regard - but that hasn't really harmed Germans so far.

In the end, I also want to be carried out with the box - no idea why that should be objectionable.