State Building Code. 
The State Building Code (Code) establishes minimum performance standards and requirements for construction and construction materials in the state, consistent with accepted standards of engineering, fire, and life safety.  The Code comprises a number of model codes and standards, developed and published by international and national organizations, which are adopted by reference in the State Building Code Act (Act).  Model codes and standards adopted in the Act include the International Building Code, International Residential Code, the International Mechanical Code, the International Fire Code, and the Uniform Plumbing Code Standards.  The State Building Code Council is responsible for adopting, amending, and maintaining the model codes and standards adopted by reference in the Act.

Factory-Assembled Structures.
The Department of Labor and Industries (L&I) regulates factory assembled structures, which include manufactured and mobile homes, recreational vehicles, and factory-built housing and commercial structures.  State law defines a manufactured home as a single-family dwelling built in accordance with the United States Department of Housing and Urban Development (HUD) Manufactured Home Construction and Safety Standards Act (HUD Code), which is a national, preemptive building code enacted in 1974.  Under the HUD Code, a manufactured home means a structure that is:

• designed to be used as a dwelling;
• with or without a permanent foundation;
• built on a permanent chassis;
• able to be transported in one or more sections;
• at least 8 feet wide or 40 feet long in traveling mode, or at least 320 square feet when erected on-site; and
• inclusive of plumbing, heating, air-conditioning, and electrical systems contained in the structure.

  
A structure that meets the HUD Code for a manufactured home except the size requirements may be considered a manufactured home if the manufacturer voluntarily files a certification and complies with the construction and safety standards in the HUD Code.  

L&I enforces the HUD Code in the State of Washington.  In addition, L&I is responsible for conducting in-plant inspections of factory-assembled structures, including manufactured homes and other factory-built housing.  L&I also offers a certification program for manufactured home installers, and only trained and certified individuals may install manufactured homes.

Federal and state law limit the ability of local governments to regulate manufactured housing.  A city or county may not enact construction, safety, and energy standards stricter than those established by HUD.  However, cities and counties are responsible for inspecting a home's installation and utility hookups.  Cities and counties also may apply local land use and zoning requirements, building setbacks, rear and side yard size requirements, site development, and property line requirements to manufactured housing.   

Modular housing and other types of factory-built housing are excluded from coverage under the HUD Code and must comply with the State Building Code instead.

"Kit homes" are defined as structures designed and constructed in a factory to sufficient life, health, and safety standards as outlined in the United States Department of Housing and Urban Development code and inspected at the factory by L&I for in-state factories or inspected by the functional equivalent of L&I for out-of-state factories.

Kit homes under 800 square feet on private property, or on public property within a planned unit development, are exempt from the Code subject to permitting by cities and towns and local fire safety regulations.