Just "adequate" by modern but still simple standard would do. Think tiny home, 1-bedroom apartment, or Japan style capsule hotel with some shared amenities. But you know what? Even that isn't built.
In the NL, around ~1960, many families had like 4, 5 children. Yet managed to live in 1- or 2-bedroom apartments.
Fast forward to today: Many people live single, pairs, or 1-child families. And population has grown. Building on average more compact housing would seem appropiate. But what happens?
In small town where I lived before, there were 5 recent home construction sites. Only one of those is modest sized social housing. How many? Around 30 apartments, in a ~80k municipality.
The other 4 sites (several dozen homes each) are all large, freestanding or 2-under-1-roof homes with like 4 or 5 bedroom, large yard, garage/carport etc. You can be very sure that if you'd check a random house, chances are you'd find a single well-earning person, or a couple living there. Not rarely elderly & retired. Perhaps a 1-child family if lucky.
Not big families with 4, 5 kids that you'd think is a logical match for homes like that.
It's like that almost everywhere. What's built is not what's needed to house a growing population of singles or small families. Instead, what's built is built to serve the well-off, use home (or 2nd, or 3rd) as investment part of the population.
Never mind professional landlords & real estate investment funds that drive up the price of housing for everyone.
This is a political problem first & foremost. Solutions exist if the political will were there. But political establishment won't fix it until things get untennable for a large enough % of the population that it threatens their re-election.