Some Laurel residents drove past two largely vacant lots on their way to work one recent morning. When they returned in the evening, two largely completed houses stood on the sites.

The houses--a two-bedroom, bi-level home and a three-bedroom, two-bath split-level--had left the factory in six large pieces on flat-bed trucks earlier that morning. They were put in place by crane in one day on already-laid below-ground foundations.

One house, which came in two sections, took two hours to erect; the other, in four sections, took three hours. Almost everything, including the kitchen cabinets, appliances and bathroom fixtures, was already built into the house.

These structures are called modular homes. Together with "manufactured" or mobile homes, they represent the fastest growing segment of the housing industry.

The houses put up in Laurel last week were manufactured by The Ryland Group Inc. in a $4.2 million, 88,000-square-foot plant opened last July in New Windsor, Md., northwest of Baltimore. The Columbia-based Ryland established its modular division last year after 15 years of building houses in large subdivisions the old-fashioned way--on-site construction.

"We really feel modular is the wave of the future," says Ryland official Nancy Smith. The company's modular homes division now is able to serve the smaller builders who build in smaller subsidivisions and communities that wouldn't be served normally by the company, she said.

The New Windsor factory has the capacity to produce three homes a day, each one taking between four and six days. The plant was profitable in six months, and ground has already been broken for a second factory in North East, Md.

Ryland isn't the only builder moving away in some degree from traditional on-site home construction. Late last year, U.S. Home Corp., the nation's number one builder, moved to acquire Brigadier Industries Corp., one of the country's largest producers of mobile homes. Noting that more than half of U.S. housing starts are in non-metropolitan areas, officers of U.S. Home told their stockholders recently that "the new off-site manufacturing capacity will enhance U.S. Home's ability to meet the demand for affordable, quality housing in these areas."

From the builder-developer's point of view, the advantages of the modular homes are obvious: they save time and money. A modular house can be completed in less than a week; after the set-up by crane, the finishing touches usually take between five and ten days.

In contrast, a "stick-built" house takes between 68 and 72 days to build on-site, plus two to three months' lead-time, according to Ryland marketing representative George Kalivretenos. The time can be shorted when the builder uses prefinished or panelized components, something more and more builders are doing.

"You have the quality control and expertise in the factory; you also have cost control; you don't have weather to contend with; you don't have to worry about materials being stolen or the house not coming in to code," says Cyrill W. Martin, president of Cy-Nan Corp., the independent builder-developer that took delivery of the two Ryland houses on May 12 and plans to put up four more on Talbott Avenue in Laurel, next to the first two.

"This is the wave of the future," Martin says, echoing Ryland's Smith. "I think this industry is where Ford was when it started out with the Model A. There's no restriction on style or anything else you want with a custom home."

The Ryland houses shipped to Laurel literally came C.O.D. to Cy-Nan; a check was brought over to the site by Michael H. Hall, vice president of John Hanson Savings & Loan Inc., which is financing the mini-development.

The homes appear to be well-insulated, and came with a General Electric heat pump, stove and dishwasher; a Carrier air conditioning unit; Andersen insulated windows, Moen plumbing fixtures; solid oak kitchen cabinets with brass hardware, and Armstrong carpteting and no-wax floors.

Two sides of each house were already covered with aluminum siding, with the siding for the remaining sides of the house shipped inside the rooms, along with everything else necessary to put the finishing touches on the homes.

Everything needed to complete the house on the site was contained inside. Tied down for shipping inside the living room, bedrooms and kitchen were the water heater and air conditioning units, the gutters, the doors, moldings, the carpeted stairs, long bolts for fastening the modular units together, windows for the lower level, caulking, nails and even a can of touch-up paint.

Totally self-contained, even the wiring and pipes were already in place behind room walls, ready to be hooked up to the proper outlets in the basement. The houses are built to satisfy FHA, VA, and local building code requirements.

"Kind of makes you wonder why anyone builds from the ground up," said Jack Armentrout, a plumbing contractor who watched the two-hour "construction" of the first house. Armentrout was hired to connect the plumbing.

According to Ryland marketing representative Kalivretenos, the builder has responsibility from the floor joists down--putting in the foundation and running the water and sewer, gas and electricity lines in; from the joists up, Ryland is responsible. The erection crew takes the house off the trailer with the crane, raises the roof which has been flattened for traveling, sets the house on the foundation, bolts it together and weather-proofs it. A second crew comes in to finish the house--put up the siding and shutters, smooth over the places where walls from the sections come together, finish the trim, seam the carpets.

A builder can decide to finish off the house but most elect to have Ryland do it, Kalivretenos said.

The Laurel houses will take a little longer than normal to complete--almost two weeks--partly because Martin is finishing off basement rooms, adding a bathroom to each house, putting in patios, oversized driveways and landscaping. Also, inclement weather delayed construction for a day and a half.

The houses on Talbott Avenue will also be a little more expensive than some modular houses because of extra work Martin is doing to make them "turnkey operations," enabling the purchaser simply to take the key and move in, Martin said. The Talbott Avenue houses will start in the mid-$70,000s, Martin said. The same stick-built house would cost at least 15 percent more, possibly higher, he added.

A potential homeowner who already has a lot would pay about $50,000 for the modular houses, which wholesale for about $25,000 each, he suggested. The cost of each house varies, depending on what improvements are made and what options are taken. "It's like buying an automobile and adding options; it takes the price up," Martin noted.

The smaller of the two houses already erected has 852 square feet of habitable space, not counting the lower level, while the larger house had 1,026 square feet of space, also not counting the lower level which Martin is finishing off.