Single-family new building: Planning a granny flat for future family expansion?

It should be obvious that a granny flat consists of more than just a kitchen, and unlike normal children's rooms, all rooms must be contiguous, which limits you in the floor plan. But if you think you can get that (almost) "for free," then go ahead. I just refer to the short and concise answer from

Completely idiotic idea. Build the house as it is going to be used. Everything else is pointless in my experience.

So you think two children’s rooms potentially standing empty for years are more sensible?

It should be obvious that an apartment-in-apartment entails more than just a kitchen, and unlike normal children's rooms, all rooms must be contiguous, which limits you in the floor plan. But if you think you can get that (almost) "for free," go ahead. I just refer you to the short and concise answer from

It's not about just "doing it" for me. But I feel that my description of the planning wasn't fully understood and I haven't really received a new argument against it yet. Planning is more difficult, that's true. But as I said, it's about the basement apartment. Therefore, separability and planning aren't more complicated than without the apartment-in-apartment. And kitchen+bathroom as additional cost factors are clear to me. But do the costs really outweigh rental income and additional subsidies?

So you think two children's rooms that may be vacant for years are more reasonable?

But do the costs really outweigh rental income and additional subsidies?

It becomes significantly more expensive. Everything separate – electricity, heating, ventilation, double kitchen. Plus depreciation and possibly taxes and management costs on the income. Completely separate access.
Better a flexible house. Better suggestion: large work and guest room. Plus a "junk room." Later divide the large room into two children's rooms and use the junk room as a combined guest/work room.

It will be significantly more expensive. Everything separate – electricity, heating, ventilation, double kitchen. Plus wear and tear and possibly taxes and management costs on the income. Completely separate access. Better a flexible house. Better suggestion: large work and guest room. Plus a "junk room." Later divide the large room into two children's rooms and use the junk room as a combined guest/work room.

That is already much more concrete, thank you! Do you have a "ballpark figure" for what electricity, heating, and ventilation would cost separately? I can calculate bathroom and kitchen myself, but I have no idea about the connections. I also like your alternative suggestion!

What exactly do you mean by Souterrain? Are we talking about a hillside property with a basement living area?