OLYMPIA, WA — A new Washington state law aimed at expanding affordable housing options will make it easier to build small “kit homes” and backyard units across the state.

Gov. Bob Ferguson signed Engrossed Substitute Senate Bill 5552 into law over the weekend, following four years of work by Sen. Jeff Wilson of the 19th Legislative District. The measure directs the Washington State Building Code Council to develop building code rules specifically for kit homes that are 800 square feet or smaller.

Supporters say the legislation is intended to help address Washington’s housing shortage by reducing costs and simplifying the construction process for smaller homes. Wilson said standardized kit-home designs could allow plans to be approved once at the state level rather than requiring separate design reviews for each project.

Kit homes — popular in the early 20th century — typically include precut lumber and other materials delivered as a package that can be assembled on site. Modern versions often use prefabricated wall and roof panels to speed construction, and some kits can start at less than $10,000.

Under the new law, the state Building Code Council must complete rulemaking for kit-home construction by March 31, 2027. The legislation defines kit homes as prefabricated residential structures assembled on site that are 800 square feet or smaller.

Wilson said the goal is to create a simpler and more affordable path for homeowners interested in building accessory dwelling units or small homes on their property.

State officials estimate Washington will need more than 1 million additional homes by 2044, increasing pressure on lawmakers to find new housing solutions.

The governor also signed another Wilson measure adjusting the state’s definition of rural counties to ensure Cowlitz County retains its rural designation when seeking funding for public facilities and economic development projects.