What is Off-Site Construction?

The use of off-site construction is expected to increase significantly as the building industry and society struggles to address key challenges including the availability of affordable housing, a lack of skilled workers, material use and sustainability, job site safety and industry productivity.

Off-site construction (also called pre-fab or modular construction) is defined by the National Institute of Building Sciences Off-Site Construction Council as, “the planning, design, fabrication and assembly of building elements at a location other than their final installed location to support the rapid and efficient construction of a permanent structure. Such building elements may be prefabricated at a different location and transported to the site or prefabricated on the construction site and then transported to their final location.” Often, the broad concept of off-site construction is broken into additional characteristics including open versus closed construction, permanent versus relocatable and manufactured housing versus modular housing versus tiny homes.

Closed construction means that the final product cannot be inspected at the installation site without disassembly, damage or destruction. Open construction still allows for inspection of the product at the installation site. The fact that a closed construction product cannot be readily inspected at installation has caused the regulatory process to move into the factory. Examples of open construction include mechanical racking and certain panels (without embedded MEP for instance). Examples of closed construction include volumetric modules, pods, and some panelized systems.

Relocatable buildings include job site trailers and similar buildings intended to be reused and transported to different sites. They are covered by Chapter 31, Section 3113 of the International Building Code. In contrast, permanent construction is intended to remain at its initial place of assembly.

Manufactured housing is built on a permanent chassis and is subject to the requirements contained in the Manufactured Home Construction and Safety Standards developed by the U.S. Department of Housing and Urban Development (HUD). Modular housing is subject to the code requirements of the final jurisdiction — typically based on the International Residential Code. Tiny homes are typically less than 400 sq.ft. and are covered by Appendix Q of the International Residential Code.

Because off-site construction components are designed to be transported, in some cases they can economically be constructed in one country and transported to another country. As with all off-site construction, the components still need to comply with requirements in place where they will be installed. So, factories in these countries are still subject to plan review and factory inspection requirements. NTA currently provides such services for factories in Canada, Mexico, South Korea, Poland and China that are building modules for the U.S. market.

Most state programs require closed construction components to include a label indicating that they have been designed and fabricated in conformance with the requirements of the state program.

Because they are open, these components can be readily inspected at the job site to assure that they comply with requirements of the local code. However, these components could still be unfamiliar to a code official or contain products or materials that must meet specific requirements. In this case, a product evaluation from a recognized source like the ICC Evaluation Service can help instill confidence.

Without a state program, regulation of off-site construction falls to the local jurisdiction where the project is located. This can certainly present challenges as the factory may be located outside the local jurisdiction but still require inspection prior to final assembly. In most cases, the local AHJ can allow third-party plan review and inspection agencies like NTA to fulfill this role, limiting the AHJ’s expenditure of resources for inspecting these projects.

While requirements vary from state to state, there are two main processes for the plan review and inspection of off-site closed construction. Either the state conducts all plan review and inspection internally or they allow the use of recognized third-parties like NTA to provide these functions.

ICC ES typically provides Evaluation Service Reports (ESRs) for open construction products that do not require separate plan review or for closed construction products that are highly replicable or commoditized. NTA provides third party plan review and inspection services for closed construction products that are subject to the HUD Manufactured Home Construction and Safety Standards, state modular or industrialized building programs or would otherwise be difficult to inspect outside of the manufacturing facility.

With the exception of manufactured housing which is subject to the Manufactured Home Construction and Safety Standards developed by the U.S. Department of Housing and Urban Development (HUD), all off-site construction must meet the code requirements in place in the jurisdiction it is being assembled. There is not a separate code for off-site construction. ICC/MBI Standard 1200: Standard for Off-Site Construction: Planning, Design, Fabrication, and Assembly and ICC/MBI Standard 1205: Standard for Off-Site Construction: Inspection and Regulatory Compliance are being developed to help streamline the use of off-site construction.

The HUD standard covers requirements for the design and fabrication of the unit itself. All other aspects of the project including utility hookups, add-on or accessory buildings and structures, foundations, and site conditions are subject to local codes and 24 CFR 3285 – Model Manufactured Home Installation Standards. These are the responsibility of the local AHJ and should be inspected by the local AHJ or qualified third party.