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

I am asking a general question in the hope that you either completely dismantle the idea or maybe it can be worked with. It is more of a quick idea than a concrete plan:

We (a married couple wishing to have children, but without children yet) would like to build a single-family house on a relatively large plot (1000 sqm). Since we plan to have two children in the future, we would like to plan for two children's rooms accordingly. A study and possibly a guest room would also be nice to have. A friend who is an architect suggested including a granny flat in the single-family house.

Therefore, we came up with the idea to possibly plan the house on the 1st floor with a master bedroom, bathroom, study, and guest room, and to plan a granny flat in the basement with two rooms, a small kitchen, and a bathroom, and then rent it out. If there is a need for two children's rooms at some point, the granny flat could be used as a study and guest room, and the freed-up rooms on the 1st floor could be used as children's rooms. In the more distant future, the granny flat could possibly be used again as a separate apartment for teenage/adult children or even as a barrier-free retirement residence for ourselves.
This way, there would be neither unused rooms in the new building nor a house planned to be too small in the long term, and as a side effect, possibly additional subsidies and of course rental income.

Completely stupid idea or possibly sensible?

Completely stupid idea or possibly sensible?

Completely stupid idea. Build the house the way it is supposed to be used. Everything else is, in my experience, pointless.

The idea of giving the young generation their own domain is of course charming but far from affordable or even feasible free of charge. You have to be able to afford a granny flat. Can you? You should not believe that it will become "cheaper" in any way, despite subsidies. Well, and whether you want strangers living in the house or not, you have to decide for yourselves.

The idea of giving the offspring their own domain is of course charming but far from affordable or even feasible for free. You have to be able to afford a granny flat. Can you do that? You shouldn't believe that it will become "cheaper" in any way, despite subsidies. Well, and whether you want strangers living in the house or not is up to you.

Basically, the main idea would rather be not to build two rooms for the offspring and leave them empty at first, but rather to rent them out profitably in the meantime. The "own domain" would only be a bonus for the future >16 years, when you would swap back.

profitable

Find the mistake. It doesn't pay off. Instead, it is pure luxury, the investment of which can be reduced by "rental income". If you don't want to do anything good for your children with it: you certainly won't do yourself any favors with it. Of course, if money is abundant, you can do it. But then you might as well build your children their own house. ;)

Find the mistake. It doesn't add up. Instead, it is pure luxury, whose investment can be reduced by "rental income". If you don't want to do something good for your children: you certainly don't do yourself any favor with it. Of course, if money is abundant, you can do that. But then you might as well build the children their own house. ;)

I am confused. The only "luxury" would be an additional small kitchen. The rooms are exactly the number we would actually like to have. Only that they are not just empty until the second child arrives. 4 rooms = granny flat + guest room + home office = 2x children’s rooms + guest room + home office. Where is the rest of the "pure luxury" then?