Fewer new houses and apartments are under construction in the Washington area this year than in any year since 1976 and those that being built are smaller and more expensive, according to two new reports.

One of the more telling of the reports' findings was that the detached house -- the mainstay of the American Dream -- slipped to a much smaller share of the single-family market. During the years 1970-1979, 61.5 percent of all single-family houses built in the area were of this type. Last year, the figure dropped to 51.2 percent.

The number of construction permits granted for all types or residential housing -- houses, town houses, apartments and condominiums -- in the metropolitan area by the end of 1980 was donw 5.5 percent from the year before, down 15.1 percent from 1978 and 20.5 percent from 1977. The permits are a handy tool for measuring new construction.

The median price of all types of new housing at the start of this year was up more than 9 percent, from $88,990 to $97,220. But, while prices did go up, the rate of increase was less than half what it was for the preceding two years.

These findings, contained in two reports by the Metropolitan Washington Council of Governments, point to a number of housing trends in the area, nearly all of them a result of financial pressures on buyers and builders:

Cheaper, smaller condominiums and attached houses are accounting for a greater share of sales of new homes than ever before.

Construction of traditional, detached single-family houses is at one of its lowest levels since World War II.

Attached residences -- called town houses, even if they're built in the middle of a pasture in suburbia -- are becoming the most popular, or at least most affordable, form of single-family houses.

Families that once moved up from apartments or small houses to larger detached houses are increasingly moving into smaller, but more luxurious, versions of town houses, often located near Metrorail stations.

Multifamily condominium buildings are becoming just about the only type of new apartment living available to persons not receiving some form of government assistance.

Virtually all rental apartments being built are subsidized by government programs while existing rental units continue to be eaten up by condominium conversions.

COG gathered its statistics on new construction from local government records and its figures on prices from Housing Data Reports Inc., a Washington firm that does marketing research on new housing in the metropolitan area, according to W. Bruce Steele, who wrote both reports.

The report on new construction said that the marked decline in numbers of new single-family detached houses was "undoubtedly reflective of a continually shrinking market for traditional detached units due to cost and affordability factors.

"I wouldn't be surprised if within a year town houses take over the single-family market," Steele said in an interview. The explanations, he said, were that town houses enable bvuilders to achieve higher density along with lower costs, which can be passed on to buyers, and that buyers wanting to move up to better housing increasingly prefer the luxury features of many new town houses, as well as their reduced maintenance.

By January of this year, according to the report on prices, the median sales prices -- the price at which half the sales were above and half below -- for all new, detached, single-family houses in the area was $110,300, up 12.6 percent from the beginning of 1980. But, the report noted, this rise was nearly half what it had been the preceding year, January 1979 to January 1980, when the increase was 16.8 percent.

COG attributed this proportional decline in part to a slackening of the amount of space in these houses, following relative increases in size each year since 1978.

The average size of these residences was 64 square feet smaller than those built in 1979, while the cost per square foot was up 15.5 percent: less space for more moeny.

Average price increases, rather than median, were nearly as dramatic, rising 11.2 percent to $112,546 from $101,238. Steel noted that average figures tended to be skewed by the highest-priced units, even though these were relatively few in number. The median reflected the true picture more accurately, he said.

The least expensive detached, single-family houses in the area at the start of this year were in Prince George's County, with a median sales price of $87,900. The most expensive were in the District, at a whopping median $492,600 -- but only three new houses were included in the survey of the District.

By way of comparison, the median price of detached, single-family houses throughout the United States was $66,700 at the start of the year, according to Michael Sumichrast, chief economist of the National Association of Home Builders. This represented a 6 percent rise in prices since the start of 1980, he said.

The basic reason for the drop in numbers of new housing units being built in the Washington area, according to Steele, is that with interest rates now in the mid teens, buyers can't afford to buy and therefore builders are cutting back on their production. He also said that because single-family house construction can be halted more readily than large, multifamily projects, the biggest cutbacks were in the single-family market.

The median sales price of traditional attached housing was $88,490 in January, up 15.8 percent from January 1980. Other types of attached housing, such a "piggy-back" and "back-to-back" town houses, dropped 2.5 percent in January, from $71,250 to $69,400.

The report termed this price decline "a significant reversal" from previous years' trends and attributed the drop to a corresponding 5 percent decrease in unit size.

The 5.5 percent decline in the umber of all housing units being built in the metropolitan area -- 19,710 units in 1980 compared to 20,861 in 1979 -- was "consistent with national trends," the report on construction permits said. This overall decline was attributed largely to a 23.9 percent drop in single-family permits.

The decline was partially offset by a "substantial" 33.7 percent increase in multifamily housing, up to 4,498 units in 1980 from 3,363 in 1979. "At the same time," the report stated, "the total was below the 6,411- and 5,054-unit figures documented for calendar years 1977 and 1978, respectively."

The increase in multifamily construction was largely a factor of a sharp 56 percent rise in condominium units to 2,077. "This was the highest annual level of permit activity" for condominiums since 1974, the report said.

The number of rental apartment units was up slightly in 1980, according to the report, but well below the levels of 1976 through 1978. The bulk of rental units -- 1,248 -- were in Montgomery County; 590 units were located in the District, and 302 in Alexandria.

By jurisdiction, only the District, Montgomery County and Alexandria showed increases in overall construction. The District had its largest increase in more than a decade the report said, from 1,742 units in 1979 to 2.642 in 1980 -- entirely due to the rise in multifamily condominium units.

The increase in Alexandria, to 568 units in 1980 from 330 in 1979, was mainly a result of a 302-unit federally subsidized housing project being licensed.