A lot of the open concept houses around here use enough space for four rooms for one big one. Usually kitchen + dining area + a single living room easily large enough to be two. If you're lucky you have one other room in the public area, designated by design as a dining room (distinct from the merely-yards-away dining area!), and if you're super lucky it's at least got three walls rather than just being designated by flooring and maybe like one pillar.
More rooms is nice if you don't like being on top of every single other person in the house all the time, without having to go to your bedroom (aren't we supposed to only go there for sleep, for sleep-hygiene reasons?) to escape. In recent houses this means using ~3x the space you actually need to accomplish that, mostly by adding a room or two in a finished basement, because the main public area's gigantic, yes, but also entirely open.
It's also very nice to be able to contain messes. So nice. One large shared living space plus kids means no part of your house ever doesn't look like shit without heroic efforts or paid help.
More rooms is nice if you don't like being on top of every single other person in the house all the time, without having to go to your bedroom (aren't we supposed to only go there for sleep, for sleep-hygiene reasons?) to escape. In recent houses this means using ~3x the space you actually need to accomplish that, mostly by adding a room or two in a finished basement, because the main public area's gigantic, yes, but also entirely open.
It's also very nice to be able to contain messes. So nice. One large shared living space plus kids means no part of your house ever doesn't look like shit without heroic efforts or paid help.