Declining interest rates are opening up the housing market to more buyers, including first-time purchasers who have had increasing difficulty buying homes during the past several years of rising house prices and interest rates, several economists said this week.

Since last October's stock market plunge, mortgage interest rates have dropped about two percentage points to about 10 percent this week and may inch down by another quarter point to half a point, the economists predicted. Expectations of an economic slowdown have sent all interest rates down recently, and major banks lowered their prime rates to 8 1/2 percent this week, a drop of one-fourth of 1 percent.

Most analysts expect fixed-rate mortgages to stay around 10 percent for most of the year, unless the economy slides into a recession. New single-family home sales dropped 6 percent in December, the Department of Commerce reported this week. Most of the unexpectedly large decline alarmed many home builders, but they believe sales will "bounce back" in the coming months, said David F. Seiders, chief economist for the National Association of Home Builders.

James Christian, chief economist for the U.S. League of Savings Institutions, said, "I'm not looking for a plunge" in mortgage rates but a decline by another quarter of 1 percent is probable. Movement of interest rates in the next few months "will be heavily influenced" by economic indicators reported by the government, he said.

A similar dip early in 1987 brought interest rates down to about 9 and 9 1/2 percent during the first three months of the year, drawing more home buyers into the market. Rates began rising in April and soared to levels of 11 percent and 11 1/2 percent by autumn.

There is little chance rates will go up dramatically later this year, according to Robert Van Order, chief economist for the Federal Home Loan Mortgage Corp., commonly known as Freddie Mac. He said his best guess for the 1988 average is about 10 percent, but "if there is more of a recession, they will go lower."

Prospective buyers benefiting the most from the lower rates will be those looking for their first homes. Nationally, the number of first-time buyers has dropped in the last decade, bringing calls from many industry groups for government action that would help young Americans buy houses. Compared with first-time buyers of a decade ago, Americans buying their first homes now are older, have saved longer and are more likely to buy an existing house than a new one.

The U.S. League of Savings Institutions said this week that the number of first-time buyers dropped to 35.1 percent last year from 39.1 percent of all purchasers in 1985. The organization surveyed 11,000 home buyers nationwide to reach its figures.

Washington area buyers reversed the apparent national trend, however, with first-time buyers rising from 30.4 percent of all buyers in 1985 to 34.9 percent in 1987.

In another national survey, however, the Chicago Title Insurance Co. found that the number of first-time buyers actually increased last year, largely because of the dip in interest rates during the first quarter of 1987, according to James Pfister, vice president and market research manager for the company. Chicago Title's figures showed first-time buyers accounting for 36.8 percent of the market last year, up from 35.6 percent in 1986.

But Americans buying their first homes in 1987 had to save for an average of two years and four months before making their purchases, compared with an average of one year and eight months for 1986 buyers, the Chicago Title survey showed. These purchasers paid 29 percent more for their homes in 1987 than in 1986, with the average price rising to $106,450 from $82,510. Average income of the buyers rose to $43,800 in 1987 from $38,920 in 1986. The Chicago company surveyed about 500 buyers in 15 metropolitan regions, although the Washington area was not included.

With periods of lower interest rates and new types of mortgages, there is a growing inclination of first-time buyers in the Washington area to buy what they can afford instead of waiting until they can make a down payment on their dream home, said Kenneth M. Tuchtan, head of Coldwell Banker residential real estate services.

"We lived and died on the move-up market here in the last couple of years, but more people have gotten into the market now," he said.

District of Columbia real estate agent Linda Low, who specializes in houses in the Mount Pleasant area, said a number of her clients are first-time owners who usually buy the area's smaller homes or houses that need rehabilitation. They nearly always are two-income couples, and frequently get financial help from relatives, she said.

The U.S. League survey found that first-time and repeat buyers in 1987 were older, more often single and had smaller households than their counterparts a decade ago. The median age of buyers increased to 37 last year from 32.4 years in 1977, and the number who were married dropped to 74 percent from 83 percent during the same period.

Households became smaller during the decade, and the number of one- and two-person households buying homes increased by nearly eight percentage points to 53.6 percent.

The median purchase price of a home more than doubled, growing to $95,000 last year from $44,000 in 1977 , the study showed. The nationwide median of combined monthly mortgage, tax, utilities and insurance costs also doubled, to $822 in 1987 from $400 a decade ago. But at the same time, buyers' incomes grew from a median of $22,700 in 1977 to $45,996 last year.

Washington area home buyers had higher incomes and paid more for the houses they bought than most Americans.

Three metropolitan areas -- San Francisco, Los Angeles and New York -- reported higher median purchase prices for homes than Washington's 1987 median of $137,500. Only the San Francisco area reported higher monthly housing expenses, including mortgage, taxes, utilities and insurance costs, than Washington. The San Francisco median for these expenses was $1,320 a month, while the Washington area median was $1,178 last year, the U.S. League reported.

A total of 67.6 percent of Washington households reported paying more than $900 each month for various housing expenses last year. In contrast, 35.7 percent reported paying more than $601 a month in 1977. The number of Washington households paying less than $600 in total housing expenses dropped to 13.3 percent last year from 64.3 percent 10 years ago.

The median income of Washington home buyers rose to $62,160 in 1987 from $32,718 a decade ago , and a larger percentage of the buyers were single. The unmarried buyers rose to 32.1 percent last year from 20.7 percent of the total in 1977.

Patterns of income growth among Washington buyers surveyed were remarkable. A decade ago, 42.2 percent of the households reported annual incomes of $25,000 or more. By last year, 57 percent of households covered by the study had incomes of $55,000 or more.

Ten years ago, 30.5 percent of the buyers surveyed earned from $25,000 to $34,999; last year 9.3 percent reported incomes in that range. Households with incomes of less than $25,000 dropped to nine-tenths of a percent last year from 27.3 percent of the buyers studied in 1977.