The number of condominiums throughout the metropolitan area rose by 21.8 percent in a single year -- the largest increase ever -- with the greatest jump taking place in suburban Fairfax County, according to a new study by the Metropolitan Washington Council of Governments (COG).

The increase was largely attributed to the continuation of the housing trend that swept the Washington area in the 1970s: the conversion of rental units to condominium ownership. Of the 15,102 condominium units added to the Washington market during the period, the study said, 11,922 were rental units that had been converted. The remainder were newly built.

The most striking finding in the survey -- which covers the period July 1, 1979, to July 1, 1980 -- was that the construction of new rental apartments fell to virtually nothing while conversions were eating into existing rental stocks at a higher rate than in any previous year.

In contrast to the large number of rental units that were converted, only 2,862 new rental units were constructed throughout the metropolitan area, according to COG's William Davies. Of that, about 70 percent was government-subsidized, low-income housing.

"As far as the individual is concerned who needs to rent but does not qualify for subsidized housing," Davies said, "you can honestly conclude that nothing was added to the market" in the study year.

Generally, the conversion of former rental apartments to condominiums carries much greater social impact than new condominium construction because in many cases conversions squeeze out the relatively poor, the elderly and those on fixed incomes.

The conversion phenomenon, once characterized as "condomania," as it swept through formerly rent-controlled apartment buildings in upper Northwest Washington and other rental apartment bastions, still is one of the hottest issues on the local housing scene. Landlords contend that the economics of the market make the rental business unprofitable; tenants and their political champions say that landlords' "greed" is forcing growing numbers of tenants who cannot buy their dwellings onto the street.

Even where they are restricted by tough regulations, as in the District of Columbia, landlords are continuing to press for converting their buildings. "A few years ago," Davies said, "it used to be that conversions and new construction shared the condominium market about 50-50." Today, he noted, conversions easily have surpassed newly built condominiums.

In the last decade, according to the survey, the construction of new apartments outdistanced conversions throughout the region, 65,398 to 47,880. Most of that apartment-building -- 70.3 percent -- occurred between 1970 and 1974. Since 1974, only 19,380 rental apartments have been built; 30,344 apartments were converted.

The study shows that Fairfax County experienced the highest growth rate in rental apartments converting to condominiums -- 60.7 percent -- while in the District of Columbia the increase in conversions was 54.4 percent. In absolute terms, though, the District of Columbia converted the largest number -- 4,319 apartments -- while 2,390 were converted in Fairfax.

As of July 1, 1980, the metropolitan Washington area contained 84,240 condominiums -- 21.8 percent more than existed in the region the year before. Of these, 48,011 were a result of rental apartments being converted and 36,229 were new construction.

Noting that data for July-to-July periods prior to 1977 did not exist, the COG survey found that a comparison with previous calendar years "shows that this 12-month period surpassed all other annual periods of conversion activity [and] that the conversion trend is continuing to increase its momentum." The study also noted that "not since 1975 has the Washington metropolitan area experienced a decline in conversion activity from the previous year's level."

Fairfax County led the metropolitan area in new condominium construction during the period of the COG survey by adding 987 units to its previous stock of 11,387 units built as condominiums. The new total, 12,374, gives Fairfax County 34 percent of the region's new condominium construction. gCombined with the 6,328 units converted in the county to date, the total of 18,702 condominium units puts the county first in the area.

The District of Columbia added 680 new condominiums. The city now has 3,571 units built as condominiums -- since the first one ever seen in the metropolitan area was built here in 1968 -- and 12,253 converted units. The total of 15,824 puts the city in second place behind Fairfax County and just ahead of Montgomery County, which has 15,588 total units.

According to COG's study, 2,225 units were built in Montgomery during the 1979-1980 period and 312 new units were built. The overall condominium growth in the county represented a increase of 29.7 percent over the previous one-year period.

Arlington County experienced a total growth of 37.5 percent: 1,872 units converted and 680 new units built.

Throughout the metropolitan area, according to the study, condominiums continued to sell very well. Of the converted units, 70.2 percent had sold, as had 78.5 percent of the new units. The study found that the most popular form of new condominium construction was the "single family attached structure," or town house, which accounted for 45.1 percent.