The term “offsite construction” generally applies to any construction process whereby a portion of the construction occurs at a location other than the building site. About ten percent off all buildings and homes in the United States are constructed offsite, while a much higher percentage have some elements of prefabrication or offsite components (roof trusses, HVAC ductwork).
The term offsite construction can apply to volumetric modular construction, wall panels, bathroom pods, shipping container conversions, and pre-cast concrete construction. Specifically, the term offsite is defined as:
Any module building or modular component which is designed and constructed and is wholly or in substantial part made, fabricated, formed or assembled in manufacturing plants for transportation and installation on a building site, and has been manufactured in such a manner that all parts or processes cannot be inspected at the installation site without disassembly, damage to, or destruction thereof.
There are several key points in the definition that might need further clarification. For example, when a component is built offsite and cannot be readily inspected on site without disassembly or damage, that process is referred to as “closed construction.” This simply means that there are concealed elements to the component when it left the factory. Common examples of concealed elements include plumbing and electrical systems.
Because these concealed elements cannot be inspected on site, there must be a process or system to inspect them at the factory. Thirty-five states have an administrative program that sets for the rules and requirements for how these concealed components will be inspected and approved. Common among all state agencies is the requirement for quality control programs in the factory.
These state agencies have review and oversight authority on the offsite components themselves. Once the approved components leave the factory, arrive at the site, and assembly begins, authority for conde compliance shifts to the local code official. In states where no program exists, the local code official where the building is located has oversight.