The mustard-yellow, turn-of-the-century row house sits sandwiched in the middle of a long block of homes in the Woodley Park neighborhood of Northwest Washington, just off Connecticut Avenue. Inside, it is a typical week night.

John, the economist, and Sally, the attorney, are out of town on business trips. Jane, the statistician, is home, but she is leaving town tomorrow. Debbie, the mathematician, arrives home shortly before 9 with a friend. Charles, the nuclear physicist, has just prepared 2 1/2 pounds of swordfish and a pound of brussels sprouts for anyone who is hungry. He's leaving town next week on a skiing trip.

The five of them live in a four-story, five-bedroom house. None are related, nor were any of them friends when they moved in together.

They are among the hundreds, perhaps thousands, of Washington-area residents who live in the modern version of the 1960s hippie communes, households of mostly young professionals who live together -- not out of love or as a political statement, but to beat the high cost of housing in expensive neighborhoods.

Housing experts say their numbers are growing -- for those having a tough time finding an empty or affordable apartment, for divorced men and women who no longer can afford their mortgages, and for single newcomers to town who find themselves looking at houses that cost more than $150,000.

Their ranks include:

A recently divorced research psychologist who shares her five-bedroom home off Columbia Road with three men. By charging rents ranging from $100 for an unheated attic room to $275 for the basement, she has been able to hang onto the house since her marriage ended. "It's bucks," she said of the arrangement. "It's too expensive to live in Washington."

Jack Day, a middle-income health administrator, who shares a town house in Columbia with two other men and, until recently, a woman. On weekends, Day drives three hours to spend his time with his "real" family on a farm in the Shenandoah Valley. "To carry both houses would have been impossible," he said, without taking in roommates.

The 64-year-old mother of a real estate agent who had been living in a $575-a-month Arlington apartment -- with financial help from her sons. The agent bought a house in Falls Church for his mother and found her a 75-year-old roommate to share the $700-a-month mortgage payment.

"She needs companionship, and so does the other lady," he said. "From an economic standpoint, she just couldn't continue to live by herself . . . Now she can share the rent, and it's working out good for everyone concerned."

The group houses are not always welc omed by their single-family neighbors, although most are allowed under local zoning codes. Most jurisdictions permit home owners to rent to up to two boarders. And most consider certain numbers of unrealted persons to be a family, as long as they form one household and share the house equally.

In the District of Columbia, up to six persons can form such a household, in Montgomery and Prince George's counties, up to five, and in Arlington and Fairfax counties, up to four in most cases.

The arrangements have their disadvantages. A lack of privacy, for one; personality conflicts, for another. One housemate called a roommate referral service to complain that the new housemate it had sent over was crazy -- he had just taken a motorcycle apart in the middle of the living room and did not know how to put it back together.

The group homes may or may not have a family atmosphere. Some may share the buying of food, others do not. Some housemates seek the companionship of the people they live with, but for others it's strictly a business arrangement and they may go for long periods of time without even seeing their housemates.

Charles Warr, the owner of the Woodley Park row house, said his home is like a cooperative apartment. The residents share it without merging their finances. With salaries totaling between $100,000 and $150,000 a year, they each pay about $225 a month for rent, and an average of $24 monthly apiece for utilities.

The five also share the cost of food under a sort of "whatever-you-see-is-yours" policy. Warr said there normally are two or three nights a week when at least two of the residents are home to eat dinner together. But the chance of all five sitting down to a meal at once is rare, he added.

Warr has structured responsibilities formally to keep the household running smoothly. One resident pays the maid each week and sees that she gets a vacation and a Christmas present. Another serves as house treasurer. A third takes care of the garbage, the fourth the backyard. Warr takes care of odds and ends such as utility bills, calling the plumber when needed and organizing the search when they need to find a new roommate. Usually, new housemates come to them through Roommates Preferred, one of the area's oldest roommate referral service agencies, located on Capitol Hill.

Since organizing his group home here, Warr, who has lived in group homes in other cities, has come to enjoy the living immensly. To live alone, he said, would be "creepie, eerie."

Group living offers many advantages over living alone, housemates say. If you have to leave town on the spur of the moment, there's someone else to pay the bills and bring in the newspaper. If you want to go out at night, you don't have to call around to find a friend.

Housing demogapher George W. Grier said the entire Washington area saw a remarkable increase in its singles population -- including people who were never married, divorced, separated, or widowed -- between 1970 and 1975. Figures from a study that he conducted in late 1974 showed the number of such people jumped from 870,000 to 928,000 during those years.

Grier said the number of group houses, as well as the number of adult married or single children who live wit h their parents, appears to be increasing. The trend is a function of changing lifestyles and rising housing costs, he said.

"It's a way of adapting to changing pressures," he said. "You make do with what you have."

The boom in shared houses -- and the need to find compatible housemates -- has created a new industry of roommate referral services, some specializing in particular jurisdictions or types of people. Their fees average about $30.

Metro Roommate Referral Service, for instance, specializes in Montgomery County and Northwest Washington and has about 1,000 clients, both houses looking for roommates, and people seeking residences. Roommate Referrals of Columbia specialized in that area and Roommate Exchange is in Alexandria. The Christian Oasis referrel service, meanwhile, matches up people who usually want Christian roommates.

Needless to say, the referrals are not always successful. Angela Batson, of Roommates Preferred, recalls one young woman who came to her office searching for a male roommate.

Batson thought she had just the person, a man who had recently come in, also looking for a roommate. Batson began to describe him in great detail while the woman listened silently. Finally, the woman cleared her throat and said, "I don't think so; I've just broken up with him."