Prefabrication means manufacturing building components away from their final construction location. It may involve individual elements or larger assemblies. The term describes production rather than automatically defining materials, completion level or the entire project duration.
How this term relates to your project
For an extension, ask what happens before delivery and what remains on site. Foundations, installation, connections and finishing may be separate packages. Component dimensions affect transport and positioning. Assess the location before selecting the production approach.
Establish when measurements become fixed and which changes remain possible afterwards. A brief installation stage does not explain design or manufacturing duration. Compare a prefabricated offer with site construction using equivalent complete scopes so omitted operations are not mistaken for advantages of the method. Record the boundary between supplier and site contractor to avoid responsibility for an important connection falling between their separate packages.