Apartment construction here declined by 20 percent last year to 5,016 units, the third-lowest number since 1960. Apartment units counted for only 22 percent of the building permits issued in this area last year, the Metropolitan Washington Council of Governments reported.

COG researcher Elizabeth Irwin said a total of 23,173 permits for houses and apartments were issued, a decline of 3 percent from 1977, the area's most active housing year since 1973. She noted, however, that housig activity in recent years has been below the levels of the 1960s and early 1970s.

Permits for detached houses accounted for 48 per cent of last year's total.

Housing activity increased in Alexandria and in Prince William and Montgomery countries but declined in Arlington, Fairfax, Loudoun and Prince George's counties. Permits in the District were roughly the same in 1978 and 1977 - slightly below 2,200 each year.

Despite a slight percentage decline in 1978, Fairfax County led with 8,212 residential permits, 35 percent of the area's total.

The report, based on information from the jurisdictions, did not include Charles County, Md., which has been part of the standard metropolitan statistical area since 1974 but is not a member of COG.