Can you name an example of where this actually happens? I've never seen affordable housing built unless it was mandated by law or had some sort of government subsidy.

Even out here in tiny flyover midwest cities, where developers have no restrictions of any kind , there's no affordable housing being built that isn't government mandated or subsidized.

Lots of new developments, all 100% luxury apartments, or luxury homes. I wish it was as simple as "let them build it", but experience has taught me that if you let developers "just build", the only thing they ever build is impossibly expensive luxury units.

But even barring that, you can still do it with only limited mandates. You can put a limit on a property that says the rent can only be X% of the median, and as long as it's possible to build housing for that price, someone will build it and make a profit on it.

One example of no restrictions working out is (was) the city of Berkeley. In 1997 they got rid of rent control. All of a sudden, tons of new buildings went up, and at first rents were very high, but then they stabilized. It all got ruined in the last year when people in SF started to move to Berkeley in droves and shove out the students, but that's really SFs fault, not Berkeley's.

When I went and saw the new Wrath of Khan with my brother (now in Portland, at the time he was the Inner Richmond in SF), he surprised me when he told me what was most fantastic to him about the movie. It was not the space aliens, the faster than light travel, the giant spaceships, Cumberbatch's acting, no. To him, the most fantastic part was that there were highrises in SF. Space monsters? Sure. A sensible housing board? Woah buddy, you just crashed my suspension of disbelief!