The plan to buy a house was born during an Easter Shore party last summer and nurtured over pork and enchiladas at a popular Cuban restaurant in Adams-Morgan. Individually, the three knew their salaries would pay for tiny efficiency condominiums in Washington or small detached homes in Maryland's suburbs. They wanted neither.

So Bill Kalhkof, Jennifer Owens and Caroline Kmitch dressed up in their best clothes last year and applied to buy a house together. Their combined money enabled them to buy a large row house near downtown Washington.

If the trio had strolled into a mortgage office 10 years ago and asked for a loan, "everybody's chin would have dropped on their chests," one local loan official said.

After all, even though they have steady, professional jobs and down-payment money, they are three single people -- Kalkhof and Owens, an unmarried couple who may someday marry, and Kmitch, a divorcee.

But lenders, realtors and housing experts say that such sales have become more and more common throughout the Washington area over the past few years. Unwed couples and groups of unmarried persons are successfully bidding for town houses in Springfield, condominiums in Silver Spring and row houses near downtown Washington. In some city neighborhoods, many of the couples are homosexual.

In a survey of home buyers in 20 metropolitan areas, the U.S. League of Savings Associations found that nearly 41 per cent of the buyers last year in the Washington area were unmarried -- a greater percentage than any other city except Denver.

An economist with the savings league estimated that 12 per cent of all home buyers in the Washington area last year were couples of two unmarried unmarried people -- about one out of every eight buyers who bought a home last year through a savings and loan.

"There's just been a tremendous increase in unmarried people buying homes," said Howard Orebaugh, executive vice president of Washington Federal Savings and Loan. "I've been here for 20 years and I've seen the trend within the last five years . . . It's a market we're really going after."

Like married couples who jointly buy a house, unrelated persons who sign mortgage documents are all held liable for the montly note. But Orebaugh said unmarried couples and groups -- whether combinations of two males, two females, or a male and female -- appear to be no more delinquent or tardy in their monthly payments than married couples.

The trend toward unmarried home buyers has emerged for many reasons. In the mid-1970s, the federal Equal Credit Opportunity Act was amended to prohibit lenders from discriminating against loan applicants on the basis of marital status and sex, thereby giving couples a legal basis for fighting bias.

In addition, there is more public acceptance of different lifestyles throughout the country. add to this the increasing number of women who earn good incomes, and the growing sophistication of buyers who look at houses as investments rather than homes.

With the sales price of houses averaging more than $100,000 in many neighborhoods, the trend toward unmarried couples as homeowners explains how many people are able to afford property.

Kalkhof, Owens and Kmitch together earn more than $60,000 a year, enough to pay $152,000 for their house.

"We put our salaries together and ended up with this," said Kalkhof, waving his arm expansively toward the high ceiling in the four-bedroom, two-unit house on Corcoran Street NW.

Kalkhof and Owens share the top two floors, and Kmitch has her own one-bedroom unit below with a backyard. she used money from the sale of the house she shared with her former husband to make her down payment.

Kalkhof said they were surprised at how easy it was to get a mortgage loan. Their lender asked no personal questions about their living arrangements, they said.

Other unmarried couples, despite the federal and local laws, have encountered difficulties, however.

Jerry W. Markham and Marcia J. Harris applied for a loan to buy a Capitol Hill house more than three years ago, anticipating no troubles. The couple was unmarried at the time but formally engaged.

But their lender rejected them. The couple fought the decision in court. Last year, the U.S. Court of Appeals in Washington, citing the Equal Credit Opportunity Act, ruled that bank must base its mortgage loan decisions on the combined income of a couple applying jointly for the money, even if the two aren't married.

Markham and Harris bought their home in early 1977 through another lender, and were married a month later.

One unmarried couple said that when they bought their house two years ago, their insurance agent told them he had to say on their homeowners, policy that they were engaged to be married in order to get coverage for them.

Such practices vary, however. some agents said it is common to say the couple is engaged or else to suggest that the policy be written under the name of one owner only. Other agents said their companies have no problems insuring unmarried couples.

Nationally, the number of unmarried couples, both renters and homeowners, tripled during the 1970s, from half a million to more than a million and a half, according to census information. Households with non-related people of the same sex also increased dramatically -- from 430,000 to about 600,000.

Signing a 30-year deed seems to contradict a lifestyle that supposedly thrives on "no strings attached" relationships. But buying a house today isn't like in the past, when Mom and Dad bought homes for their growing families with the intention of living there until they died.

"We live in a society that's very keen on investment," said Arthur J. Norton, assistant chief of population for the U.S. Census Bureau." ". . . . A 30-year mortgage represents a short-range commitment because you can sell at a profit in one or two years."

Unmarried couples who sign deeds become liable for a monthly note. If one owner leaves and the monthly payments aren't made, foreclosure could be sought against both. However, lenders said that in most instances the partner left behind usually will sell the property or refinance it.

Many unmarried couples draw up their own legal agreements outlining the rights each party has. One couple who recently bought a home after living together for six years has an agreement that will give the woman 70 percent of the profit from any sale because she made 70 percent of the downpayment.

Kalkhof, Owens and Kmitch have a written contract that says they agree to keep the house at least two years.

After that, the three will sit down at six-month intervals to debate whether to sell the house. when they sell, the profit will be proportioned according to the amount of money each put into the house. If one person wants out, the other two have "first right of refusal" to buy the deserter's share.

Sometimes the best of agreements overlook an important detail, however. Clark Hand and Ted Saks bought a Capitol Hill row house three years ago. The two signed a 16-page, single-spaced, triple-witnessed and notarized agreement that spelled out what would happen if, for instance, one of them died or defaulted on the loan, Hand said.

But then Hand got married last year -- an occurrence they hadn't included in those hundreds of words in their legal document. After an unsuccessful attempt to sell the house last year. Hand and his wife bought in their names. "I changed partners," Hand said.

Shirley Burruss, a real estate agent with Long & Foster, bought a house with a male friend in 1978. Their relationship ended and he moved out a few months ago -- but he continues to pay half the mortgage for the tax benefits, Burruss said.

Several unmarried couples interviewed said the biggest obstacle they've had is getting their parents to support heir home-buying efforts. There are warnings of being deserted and left with huge unpayable mortgages, they say.

Jennifer Owens solved that problem simply -- she didn't tell her parents until after she had bought the house.