“ The Washington state House of Representatives late Monday passed a bill that would legalize duplexes or fourplexes in almost every neighborhood of every city in Washington, potentially bringing an end to local zoning rules that limit large swaths of cities to only single-family homes.”

This is to increase the housing supply. It looks like it is simply going to erase the distinction between single-unit and multiple-unit residential zoning so that developers could put in either in any area designated as residential.

this would still need to pass the Senate and similar bills have failed to pass in recent years.