It takes way too long to build new housing in expensive cities
Zoning that prevents missing middle housing or makes it difficult to build is one cause of housing supply not meeting demand. However, it isn't the only factor-- simply updating the zoning won't solve the problem. Another area that should be kept in mind is the amount of time it takes to approve permits for new buildings. The Washington Post discusses the connection between the length of time it takes for permits to be approved and home supply in this article. Eugene's current permitting time is a bit over a month for new single family homes and duplexes, but jumps to almost six months for new commercial buildings (which includes multi-family dwellings.)
“This last factor — building permit delays — turns out to be a significant one, according to an analysis by Trulia chief economist Ralph McLaughlin. New York, Boston and San Francisco all take much longer on average than Atlanta or Charlotte to grant building permits to developers. That means new housing takes much longer to create. Building it costs developers more. Less of it ultimately gets built. And what does get built doesn’t respond very quickly to pressure in the market.”