What's built in a factory? Everything from roof trusses (now used in 95 percent of site-built homes), floor trusses and pre-hung doors to wall panels and nearly complete houses. According to Automated Builder magazine, about 64 percent of houses and low-rise apartments now are built in whole or part with factory materials.

Here's the range:

* Modular homes are built and assembled in sections by about 200 manufacturers. Most come in boxlike sections that can stack. According to Automated Builder magazine, modulars are "the strongest of all frame homes built." They are sold direct or through local builders or builder/dealers. Modulars are built to local building codes and can cost between 10 percent and 30 percent less than stick-built homes. The average time to complete them is four to eight weeks. About 2 to 3 percent of new homes are modular.

* HUD-Code (mobile) homes or manufactured homes are made by about 90 companies operating about 350 factories. They've traveled far from the aluminum trailers of the '50s, with multiple sections and design features meant to erase that stereotype. Recently, new technology has permitted more attractive two-story options. Assembly is similar to that of modular but with generally lighter construction and always with a metal chassis built into the floor of each story. The wheels are removed at the site.

Construction must meet a national HUD code, which modular manufacturers and stick-builders contend is less stringent than state and local requirements. Automated Builder says since the HUD Code took effect in 1976, exterior frame construction of mobile units is "on par with site-built homes." In the early '90s the code was revised to improve wind resistance of homes in areas prone to hurricane-force winds. The big selling point has always been price, between 20 to 50 percent less than stick-built (roughly $43,000 for a multi-section house excluding land costs in 1997, compared with $136,400 for a new site-built home, according to HUD). Multi-story houses come closer to the costs of modulars. The average time to complete them is one to three weeks.

Because of the savings in labor costs and time, Automated Builder says "double-section HUD-Code homes which look like site-built homes will account for most low-cost housing in the future." Manufactured houses have been on a roll through most of the '90s, with almost 25 percent of the market.

* Panelized homes are sold in panels, some as packages through builders and builder/dealers, some as log homes or dome homes, and some in pieces through lumber yards and home centers. The houses must meet state or local building code. According to the NAHB councils, between 11 and 12 percent of houses built in America use panelized construction for all or part of the house. Automated Builder says completion of a panelized house takes six to eight weeks.

* Factory-fabricated parts include roof trusses, floor trusses and pre-hung doors and windows. Production builders sell directly to end buyers.