When Richard and Terry Jeng bought their tiny bungalow in 2001, they kept their clothes in the attic since the closets in their first-floor bedroom were practically nonexistent.

“We would sleep downstairs and go upstairs to get ready for work,” Terry Jeng says. “We often talked about getting a ‘real’ house, but it wasn’t until our daughter was born in 2008 that the house really felt too small.”

The Jengs thought about moving, but they love their neighborhood and can walk to the subway and shopping areas. They love the character of their home, which is on a large corner lot.

After interviewing several contractors who told them they’d need to move out for six months and quoted them extremely high prices, they opted to expand their home with a customized but modular addition.

Modular construction, in which a complete home or an addition is built in a climate-controlled factory and then delivered to the home site, has been around since the early 1900s, when Sears and other companies offered “house kits” that were then put together by homeowners or contractors.

“Until recently, modular additions were one size fits all, but through design and manufacturing innovations they have become highly customizable,” says Michael Winn, owner of Winn Custom Modular, a division of Winn Design + Build in Falls Church, Virginia. “Anyone who’s picked up a copy of Dwell magazine has quickly discovered that modular construction has become very cutting edge. The modern approach is highly automated, customizable, precise, sustainable and fast.”

The customization possible with today’s modular units means that each addition can blend seamlessly inside and on the exterior.

“The homes look better when we use premium finishes like cedar shake roofs and Hardiplank siding,” Winn says. “We usually put on a front porch or portico to blend the two sections together, too.”

Modular additions generally cost $200,000 to $300,000, but high-quality exterior materials and interior fixtures and finishes can increase the cost to $400,000.

The Jengs, who added about 1,200 square feet on top of their 1,500-square-foot home, including three bedrooms, a master bath, a second bath and a laundry closet, opted for fiber-cement siding, a sun tube to add light in the hall bath, and reconfiguring the stairs from the main level to the upper level, spent about $300,000 on their addition.

Modular additions can be designed to blend with almost any architectural style. The goal is to have a seamless transition so that the new space doesn’t look like it’s modular and doesn’t look like an addition at all. However, modular additions work best as an upper-level expansion rather than on the main level because of the difficulty in aligning the floors in a side-by-side addition, Winn says.

The modular process is also environmentally friendly since a computer is used to cut materials rather than an on-site construction worker. “About 20 to 30 percent of the cost of materials ends up in the dumpster at traditional sites because of waste,” Winn says.

Locally sourced materials are another tenet of green building. Winn’s modular additions are built in Liverpool, Pa., or Rocky Mountain, Va., both within the typical 500-mile radius used to define local materials.

“The raw building materials come in one end of the factory and in as little as 10 days the modular addition is produced,” Winn says. “The factory provides a controlled setting so there’s no risk of weather damage or theft of the materials.”

In order to ship the units safely and lift them by crane for installation, they are typically built with 20 percent more framing material, which makes them stronger.

Winn has been building custom modular additions for about five years.

Winn and his team take about one week before the unit is delivered to remove a home’s roof and prep the residence for the modular unit.

“The unit arrives and is put in place in one day with all the pre-wiring, light fixtures, plumbing and tile work in place,” he says. “It looks finished at that point from the outside, but there’s still work to be done inside to complete the customization.

Winn clients and Bethesda, Maryland, homeowners Vin and Kerry DeSomma purchased a 1950s-era home that had remained in the same family for more than 60 years and planned to expand it. The couple increased the size of their previous home with the help of a design-build firm, but this time they knew they could only build up given the size of their lot. They say they also dreaded the lengthy process of a traditional remodeling project.

“A neighbor at the local pool told us about modular units, which we had never heard of,” Vin DeSomma says. “We have three school-age kids and a dog, so the most important thing to us is that the modular addition meant we only had to move out of our home for six weeks instead of nine months. We still had the benefit of the design-build process since we could customize the modular addition, but it was much faster.”

Winn says a traditional custom addition typically takes at least four months or longer, while a custom modular addition typically less than half the time of a site-built addition. Once the design process and permitting are done, it could take as little as four to six weeks to get the addition completed. He says it’s common to have to move out for as little as two weeks.

“You don’t have to move all of your belongings, either, the way you often do with a traditional remodeling project,” he says. “The ceiling joists stay in place and the home is exposed for only a very short time.

The shorter time frame helps keep labor costs down and limits how much homeowners must spend on alternative living arrangements.

“In general, modular additions cost about 10 to 15 percent less than additions that are built on-site as long as you’re comparing an apples-to-apples remodel with the same fixtures and finishes,” said Winn.

The DeSommas increased the size of their home by about 30 percent, to 3,500 square feet, adding four bedrooms upstairs, each with a walk-in closet, a master bathroom, a laundry room and a hall bathroom. They turned the first-floor master bedroom into a den and knocked down a wall to create a great room. The entire project cost less than $400,000, including upgrades such as hardwood flooring, a frameless glass shower door and tile flooring in the laundry room.