Construction costs are currently skyrocketing

And who is raising the youth again? Hm… :D
Bullseye…

If your quote is correct, then it says "...can". And not MUST. What is so wrong about this way of thinking that it causes a dispute about the qualification of the politicians? Whoever has the money for this long-term way of thinking may later be at an advantage because he/she has an alternative to possible renting out or even selling. At the latest when the children have left the house, some builders wish for a change in their living situation (e.g. through smaller living space).

But it's not just a question of money, but in practice it is hardly possible to design a floor plan for both. You definitely end up ruining one of the two floor plans quite badly. And then we haven't even solved the separate building services, soundproofing, etc. The idea is tempting in theory, but it simply works poorly in practice. The best you can do is something close with a granny flat that is also accessible through an interior door from the other apartment and the door later disappears behind a cupboard

Anyone who doesn't plan with a granny flat from the start doesn't need to worry about it.... No one will pay for the necessary conversions anymore...

On the other hand, many retirees let their houses fall into disrepair because the pension is not enough to maintain them. And because a house simply cannot go 30, 40, 50 years without renovation... so it’s actually not a bad idea to prepare to rent out part of the house... But unfortunately, it’s not financially without challenges, that’s true...