New single-family house 190 m² for a 4-member family

That children continue to live with their parents or move back in is, to my knowledge, extremely rare. Of course, this can change in some cities due to the housing shortage. But usually, a house is rarely actually divided later to be used twice. The builders just have to ask themselves whether they would want or could move back in with their parents instead of building... Once you live in your own house with a garden, you no longer want to tolerate a foreign family, especially not above you.