Prefabrication is the method — and often the business — of making building parts in standard sizes so they can be put together anywhere later. It is the noun for the idea behind prefabricated: the pieces are made first, then assembled on site. A company may specialise in prefabrication; a school may be built by prefabrication. The word names the process (and the industry), not one finished hut — that hut is a prefab, or a prefabricated building. Register is technical / construction, a little formal.