Wait with the building application because of the child construction allowance

We bought our house last year. Therefore, we do not qualify for this subsidy anyway (which we do regret, because the grant for 2 children would have been very appealing to us), but we would have actually met the requirements (even the latest one with a maximum of 120m2, since we have only 100). With a full basement and an outbuilding, we have more than enough space. I'm just curious how they are thinking about this... what happens if you have only 1 child at first, but actually want 3, which could quickly turn into 4 unexpectedly (multiple births), where do the ridiculously small children's rooms (10m2 more per child) come from, etc. And here, practically no one builds under 130m3, usually it's 140-150, and old buildings are often bigger too, but on tiny plots, because those are the price drivers. Our house is more of an exception here. Little living space with a lot of land. I also think that the income limit should be set so that support is given up to the middle class, and otherwise no restrictions should apply. But then it becomes too expensive, they have noticed.^^.

It's only bad for those whose family planning is not yet complete with the construction, which is said to happen. But that's probably how it is, hardly anyone has more than two children anyway, and one child on 140m2 is manageable.

Is there already something concrete to read about how the areas are calculated?