Slash the costs of your home addition

(Money Magazine) -- When your family outgrows your house - or you just can't live without a master bath or family room anymore - your options are clear: trade up or add on. But these days, given the difficulty of finding qualified buyers and nabbing a decent price, adding on has gained a distinct edge. Plus, it usually costs less than moving, you remain close to neighborhood friends, and you get to keep using your favorite dry cleaner.

If only you didn't have to live through months of chaos and dust to get the job done. Well, you might not have to, thanks to a relatively new choice - buying your addition straight from a factory - which compresses the on-site construction process to as little as two weeks and can knock significant dollars off the job too. You've probably got questions. Here are the answers.

Unlike a contractor who builds your addition on your property, a factory assembles it to your exact specifications, then transports it to your home on the back of a flatbed truck. While the addition is being built, a local contractor prepares your house by pouring footings and opening up walls or removing the roof. The next day, your addition arrives and a crane sets it in place. Because the module comes with the roof, siding, windows, insulation, wallboard, wiring and plumbing all in place, the contractor can make it weather-tight within a day or two and complete the connections and final details two to four weeks after the job begins (that's a quarter of the time an on-site builder needs).

The idea of factory-built housing probably makes you think of double-wide trailers, and it's true that most manufacturers started out in low-cost construction. But over the past decade, many have retooled their operations for the high-end residential market. They now use the same construction materials and methods as site builders, follow the same local building codes and offer the same menu of fixtures, finishes and amenities. No, the factories aren't as good as the finest local craftsmen money can buy, but they do beat the average contractor's results, says architect Michael OBrien, a professor of architecture at Texas A&M who has studied this kind of housing. "In the factory, the materials never get wet and there's a lot more quality control than you get on a job site," he says. Plus, the factory builds a beefier structure - with plywood underneath the wallboard, for example - which is necessary to accommodate transport.

The factories are located in rural areas where tradesmen are paid less than workers in well-heeled metro areas, so project costs typically come in 15% to 20% below what a site builder would charge. And that includes the truck, the crane and the architect. But if you live more than 500 miles from the factory, the price of hauling the modules can wipe out that cost advantage (to find manufacturers in your area, go to modularhousing.com). Also, in some regions the construction business is so bad that builders are slashing prices and may come in below the factories'. "How much you'll save really depends on how the industry is faring where you live," says Steve Scharnhorst, the CEO of Excel Homes, which has factories in Pennsylvania and West Virginia.

There are two types of additions that work best for modular: a "pop top," where a single-story home gets turned into a two-story - and the building is without a roof for less than a day - and a bump-out, in which the new space is set next to the existing building. Because of the cost of the truck and the crane, there's an economy of scale to consider as well. "To make modular worthwhile, your addition should be at least 500 square feet," says Jim Perella, executive vice president of Haven Custom Homes in Linthicum, Md.

As with any home improvement hire, get referrals and check references. "Tour some of the factory's houses that are a few years old and talk to the owners," says Mark Blanke, assistant director of New York's codes division, which oversees factory-built homes in that state. His office hears few complaints about the quality of modular construction, he says. What problems do occur usually involve the on-site installation. Old houses aren't perfectly level, so getting the addition to align properly requires a top remodeling pro with modular experience.

And consider leaving a few features for the on-site builder to install. "The typical modular project is 90% complete when it leaves the factory," says Darrell Hoss, a Stamford, Conn. home builder. "For a high-end job, I get 75% done at the factory and finish the rest myself." The features he installs: siding, wood flooring, trim, stone countertops, porches and decks. True, the more you do on-site, the less time you'll save, but it's worth an extra week or two to hedge your bets and get results you'll be happy with for years to come.