I don't know why everyone is saying this is crazy, I know several families who have adapted their large family homes to make them into small apartments or bedsits for their adult children. I know one family who converted several garages in their rural property, one for each child, and a couple that have created independent bedsit type arrangements, one of those is now 40 years old and still living there.
Personally I won't be doing that as I want my children to move off and away from me, for their sakes and my sakes, not immediately, but in their twenties. I strongly encourage them to do this, I don't think in most cases it works well at all if they go straight into this type of living arrangement and I think it undermines their sense of them being the new family unit (and not the original family).
I also know quite a few adults living at home who have never moved out because of illness, not living separately but in the more conventional one bedroom per person type of arrangement.
Again, I'm happy for my children to come home for a few months to live between houses and jobs and countries, but not to permanently live with me. I've seen so many examples of where it hasn't worked and the adult concerned, who probably has issues either with ME, mental health or some other reason isn't going to ever leave now, even though they are in their late thirties up to mid-forties. It doesn't also provide people with an incentive to earn more money if they can exist on a small part-time wage living in their parents house forever.
Mine are moving out and they know that! I do help with living costs though as an alternative.