So many people in their twenties have to live with parents now - or in houseshares. You see a standard three bed terrace for example and you might imagine that there is one family of four there. In reality there's likely to be four single adults (one in a converted lounge), or a family with two grown up children and quite possibly their own partners in there too.
The four single adults would likely be happy to have a one bed flat to themselves - or a two bed with a single friend. Not all mind, but plenty would.
Then the 3 bed terrace they live in frees up for one of the couples living with a parent and siblings to have - they can then have room for children.
We need far more smaller places to live for childless couples, single people, single parents with one child, elderly people etc. These are people who also need affordable homes.
Take a look on Right move for one or two bed flats/small houses near you. I guarantee the vast majority are either expensive apartments in complexes that lots can't afford, or cramped rooms above shops etc which people can afford but most won't prefer. That's certainly the situation round me.
We need more houses that are fit for different purposes to enable every adult in the UK to choose to live however they prefer affordably - alone, with a partner, friends or family. Right now there aren't enough for most to choose.