The Washington area now has more than 100,000 units of condominium housing, up from about 94,000 in 1981 and from only 1,672 in 1970, according to a study by the Metropolitan Washington Council of Governments.

The findings mean that condominiums now account for about 9 percent of the area's housing stock, which currently totals about 1.2 million units, the report said.

But the dramatic growth in condominium building and conversion, which produced a record 15,716 new units in 1981, seems to be coming to an equally dramatic halt as nearly two years of sky-high interest rates have depressed sales, discouraging new projects and driving some existing ones into foreclosure see related stories on Page F1 .

The COG study, written by Elizabeth Irwin and Nancy Brown, calculates that only about 9,700 new condos came on the market in 1982, the lowest total in four years. And the authors found "a new phenomenon," the "reversion of a substantial number of units to rental status."

This was most pronounced in Arlington, where almost 9 percent of the units surveyed were in reversion. The trend was also notable in the District, where 7 percent of the units were in reverted projects.

The study pointed out that these figures cover projects that have reverted entirely to rental and do not include partially-sold-out projects that have some rentals. They also do not cover units that have been sold and put up for rent by the buyer. A 1980 COG study concluded that nearly one-quarter of the condos in the Washington area were renter-occupied.

Experts in the field say that condominiums are becoming increasingly popular among investors who buy them to put on the rental market for tax benefits and potential appreciation. G.V. (Mike) Brenneman Jr. of Brenneman Associates Inc., a firm specializing in condominium and cooperative housing, notes that syndicates are buying large blocks of condos specifically for that purpose.

Brenneman goes so far as to predict that there will be a shortage of condominiums for sale by the middle of this year.

He notes that, in addition to syndicate and other investor purchases, sales to owner-occupants have been picking up in the past month. "That, plus the fact you've got nothing coming through the pipeline," adds up to a shortage, he said. "Two plus two equals four."

Brenneman said it takes a year to two years to bring newly constructed condos to market--longer if rezoning is involved--and six to 12 months for a conversion, assuming there is an existing conversion certificate. In addition, financing is currently difficult to get from lenders "still smarting" from the recent poor market.

The COG report found that, as of mid-1982, there were no new condominium projects for which permits had been obtained but for which construction had not begun. "Thus all permitted units are either under construction or complete," it said. "It is therefore likely that . . . new construction condominium production will significantly lag single-family housing production as the housing market begins to recover from the recession."

Among the study's other findings were that:

* Fairfax County, with 23,654 units, accounted for 22 percent of the area's condominiums, the largest share of any jurisdiction. Close behind were Montgomery County with 20,026, or 18.7 percent, and the District of Columbia with 19,467, or 18.2 percent.

* Alexandria had the most condominiums per capita--107 units for every 1,000 people. Arlington was second, with almost 70 per 1,000. At the other end of the scale, Prince William and Prince George's counties had fewer than 25.

* Conversions continue to outnumber newly built condos, as they have since 1979. By mid-1982, when this survey was taken, conversion units totaled 64,157, or about 60 percent of the area total. When the first COG survey was taken in 1977, conversions represented only 44 percent of the total.

* The District has the most of conversions, 14,897, or 23.2 percent of the area total. Montgomery is second with 12,016, or 18 percent, and Prince George's is third with 9,796, or 15.3 percent.

* Fairfax dominated the new construction market with over one-third of the area total--14,348 units. Montgomery was a distant second with 8,010 units, or 18.7 percent.