- A reference to a construction, usually a dwelling, that is built, or at least partially preassembled, in a factory rather than on site. Most factory-built units are constructed in two or more modules, often complete with plumbing, wiring, etc. The modules are delivered to a building site and assembled there. finish work is then performed. Factory-built houses are generally less expensive to build and quicker to erect because of savings gained through mass production and factory efficiencies.