Local developer Ray F. Smith Jr. knew he was on to something in 1979 when houses with double master bedroom suites were the "best seller" in Wingate, a South Arlington project of the Anden Group.

"It was a shock because we didn't think the market was that deep," Smith said. Of the first 100 homes sold in Wingate, a dozen went to married couples, 44 went to single purchasers and 44 went to the growing segment of house buyers that the housing industry is calling "mingles," two unrelated singles who pool their resources to buy a condominium or house and live there together.

Smith, who left Anden to form Sequoia Building Corp. last summer, now offers double master bedrooms as an option in every house he builds. The concept, which allows two housing partners to have individual but virtually identical bedroom, bathroom and storage space, is tailored to the large number of unmarried individuals in the nation who want to have the benefits of home ownership but may not be able to afford it on their own.

"It's one of the largest markets in the Washington area, but has been relatively untapped until the last couple of years," he says.

"We're trying to customize our houses to the life style people have rather than force people into the old style of house built for husband and wife with three or four kids," Smith said. The traditional house had one master bedroom, possibly with its own bathroom, and two or three other smaller bedrooms. If two "mingles" bought such a house to live in, one gets a large bedroom and bath and the other gets "short shrift," he said. With two master bedrooms, each gets the identical space, and they share communal areas--the living room, dining room, kitchen and den.

In Wingate, Smith said the mingles could be two men, two women or a man and a woman. "In the vast majority of the cases, it was a business arrangement with a friend," he added. The large number of young professionals--almost 40 of the first 100 had a lawyer in the house--made him think that there was a large group of individuals who worked in Washington and would consider buying a house that was not too far away from their work, their friends, their nightlife.

"They want a different house than married couples with children, and they tend to want to live close in," Smith contends. Out of the first 250 houses sold at Wingate, he said, there were 12 children. "If you took 250 homes in Oakton, you'd probably have 750 children," he noted.

Double master bedrooms are available in Sequoia's first project--Cameron Knoll at 4000 Duke St. in Alexandria, which opened five models a week ago--and will also be available at two more projects to be opened this year within the Beltway in Fairfax County, he said. "We're even offering triple master bedrooms," Smith added.

There are at least a half dozen other area builders who offer houses geared to appeal to "mingles." However, many are being sold to one individual who may rent out the second master bedroom so that the expenses are shared; in that case, of course, the first individual is the mortgage holder and the one who receives the tax benefits of home ownership.

Rick Sullivan, president of Porten Sullivan Corp., another company started last year, also is gearing his development toward mingles by building town houses with double master bedrooms in its first project called Farmingdale in Germantown. Since Feb. 1, about 75 homes have been sold, he said, a few of them one-bedrooms houses. Of the remaining, about 25 percent are going to "mingles," another 25 percent are going to young married couples, and 50 percent are going to singles. In the case of the single buyers, they are often sharing the expenses with another person, he noted. The houses are in the $50,000 to $60,000 range.

According to Jane Lyons, with Gerstin and Zaslow, an advertising agency that specializes in real estate clients, condominiums being built by Charles E. Smith Cos. at Skyline City in Falls Church have two master bedrooms that are separated totally by the joint living areas. "The living room is in the core; on the right is one bedroom and bath and on the left is another bedroom and bath," she said.

"This is not really a new idea, just an application of an old idea," Lyons said. "It was introduced at Rossmoor, designed for older people, years ago. They had two master suites."

The "mingling" concept has been spreading nationwide; according to a report on housing trends published by Advance Mortgage Corp., about 200,000 households doubled up last year.