House Bill 2001-A requires certain localities, cities of 25,000 or greater,  to allow for the development of at least one type of "middle housing" per lot, in areas zoned for single-family dwellings within their UGBs. 
The term “middle housing” refers to a variety of housing designed to accommodate more occupants than single-family homes, but less than large multifamily complexes, such as duplexes, row houses, cottage clusters, stacked flats, and accessory dwelling units.

The Legislature adopted the change to zoning laws to address the need for more affordable housing. Single family residence zones may continue to have such houses, but some multifamily types will be permitted in those zones. 
The law establishes that local government can allow middle housing types in addition to duplexes. It does not apply to cities with a population of 1,000 or fewer; and lands not within an urban growth boundary (UGB); unincorporated lands and those lacking sufficient urban services; lands not zoned for residential use or unincorporated land and zoned under a designation that maintains the land's potential for planned urban development.
It allows local government to regulate siting and design of middle housing under certain conditions. and establishes that local government is not prohibited from permitting single-family dwellings in areas zoned to allow for single-family dwellings or middle housing in areas not required in measure