In addition to the relation between luxury housing market and affordable housing market that others are calling out, regulations and other hurdles to build are

In addition to the relation between luxury housing market and affordable housing market that others are calling out, regulations and other hurdles to build are a reason expensive units are built rather than cheap ones. If you need 15 permits to build in SoMa and changes requested by one council can require going through the process for another permit again, your fixed cost gets so high that you really have no choice but to build expensive units. I'm sure housing cost would quickly get under control of SF for example were to eliminate any height and other zoning constraints and have as the only requirement that you are able to get insurance for your building.