Earlier quoted context omitted.
I think you're missing the point. For example, when my siblings were kids we all lived in 1 house. I have 3 siblings so there were 6 of us including my parents. In 1 house. As we graduated from college we moved out. Now there were 6 of us in 5 houses. Beyond the raw house count, you're going to need more space. Multi-bedroom homes have shared common space. The house we grew up in had 1 dining room, 1 kitchen, 1 livin…
Yes, I think I was missing your point. I think that your thrust was that smaller average household sizes means more dwellings are required. This is a good point. Shrinking average household sizes could be explained by the shrinking size of the average family, or it could be explained by shifting expectations, by people wanting more. If you take a population where all single people are willing to live in share houses,…
(Disclaimer, I'm American, so can't speak directly about the Portuguese experience.) But as an example, at this point I don't remember the last time I saw a bunk bed in a non-dorm room setting. This used to be extremely common in low-income housing in particular [1] but as an example I have never, ever seen anyone share a bunk bed with a stranger.
One other effect is location, location, location. In Lisbon in particular, like most European cities there is generally a height limit imposed on most of the city (higher than most places in the US, but not so high that tall buildings are allowed). There is only ten buildings above 80 meters in all of Portugal. If your building is so short, there is only going to be so many ways to subdivide it before you can't split it more to accommodate people.
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[1] - a historical photo of an NYC tenement: https://media.gettyimages.com/id/640482741/photo/bunkrooms-i...