Dramatic increases in interest rates, coupled with generally rising prices, caused the monthly cost of buying a home in this area to more than triple between the late 1970s and the early 1980s, according to a study done by the Washington office of the Department of Housing and Urban Development.

Even buyers using the relatively low-interest FHA loan program faced costs that increased 187 percent between 1977 and last year, according to a second HUD study released this week.

The principal results of the cost increases have been the emergence of an "affordability gap" here, excluding many would-be buyers from the market, and a "major" shift away from single-family detached homes, especially among purchasers of new housing, HUD concluded.

The conclusions of the two studies--one focused on housing market changes and the other on the relationship of FHA interest rates and home buying activity--are not likely to surprise anyone who has followed the local situation. But they provide an unusually detailed collection of statistics to show the impact of high interest rates on home buyers here.

The effect on sales of single-family detached homes is most striking. In 1978, when "total home sales reached an historic high," the sales report says, detached houses accounted for 60 percent of the new housing market. Town houses accounted for 22 percent and condominiums for 17.

By last year, detached houses' share had declined to 52 percent, while town houses had climbed to 23 percent and condos to 25. Among newly constructed homes, detached houses dropped from 44 percent of the market in 1977 to 25 percent last year.

"These significant shifts within the sales housing market have occurred primarily because of the affordability problem," the report said. " . . . Because of the relatively low prices of newly converted condominium units, the converted condominium was the growth leader within the sales market in each year of the 1977-to-1981 period."

If it is assumed that a family's monthly payment should not exceed 30 percent of its income, the income required to buy an average-priced new detached home rose from $20,000 in 1977 to $50,000 in 1981, the study said.

The report did not address the question of whether the shift to attached housing is likely to become permanent, but Charles Langpaul of the Ryland Group told a National Association of Home Builders conference this week that his firm, at least, feels that "the single-family detached house is still the American dream." The move to town houses and condos is "temporary," he said.

Among the reports' other findings:

* From January of 1977 to January of 1983, 337,831 housing units were sold in this area, representing "approximately half of the total owner inventory that currently exists" here.

* Existing homes dominated the market during that period, accounting for 64 percent of sales. Last year, however, new housing rose to 51 percent of the market.

* Northern Virginia is by far the most active of the local housing markets, both in absolute numbers and as a share of its housing stock. The close-in Virginia jurisdictions accounted for 41 percent of all sales in the 1977-83 period; Prince William, counted separately, had 10 percent, and Loudoun 1 percent.

* The average price of a house bought with an FHA loan climbed from $41,600 in 1977 to $78,400, an 88 percent increase, but the average monthly principal and interest payment on a new FHA loan jumped from $295 to $846, a 187 percent rise.

* The average FHA price here was 52 percent above the national average last year, total family income of FHA buyers was 21 percent above the national average, and total housing expense was 44 percent above the national average.

* During the past 6 1/2 years, the FHA interest rate has changed 33 times, ranging from a low of 8 percent to a high of 17 1/2.

The FHA report also examined buyer attitudes and found that "the interest level has become the critical factor which prospective buyers consider when contemplating the purchase of a house." It found that consumer views of whether it is a good or bad time to buy correlated closely with whether rates were rising or falling.

It also found that buyers' new attention to interest rates means that FHA can expect a wave of new loan applications with every downward click in its rate.

"Because of the steadiness of the FHA interest rate prior to the summer of 1977, prospective home buyers were at first cautious and slow to react to the new realities. . . . As the interest rates started ascending and descending with great frequency (during 1980 and 1981) the number of applications received in the Washington area office increased to new highs whenever the FHA rate eased," it said.

FHA applications here were running at 81 a month in early 1977; for the first half of this year, the number was 2,054. The report noted that one local lender who does a lot of FHA business reported that while most are processed in 45 days, "it is not unusual for 60 days to pass between submission of the loan application and its approval or disapproval.