There's lots of data out there to suggest that there is a correlation between urban density and housing unaffordability. Take Hong Kong. It is very dense, and also one of the most expensive housing markets in the world. But I think the real question is: does urban density actually cause housing unaffordability, or do the two simply tend to be correlated when you plot a country's biggest cities?

One the one hand, there are factors that do drive up home prices when you build more densely. Building a reinforced-concrete high-rise is always going to be more expensive on a per square foot basis than building a wood-framed bungalow. But of course, the former also uses land a lot more efficiently, which is what you need to do in big and supply-constrained cities.

Michael Lewyn's view (credit to Robert Wright for sending me the article) is that density is incorrectly used as a scapegoat to fight compact development. It does not actually cause higher rents. One counter example he gives is that of Manhattan, which went from 2.3 million people in 1910 to just under 1.7 million in 2020. In other words, it got less dense, while at the same time its rents grew exponentially.

Like most important city matters, the answer is complicated. But this is an interesting topic that I think we should spend more time on here.