The Northern Virginia suburbs and Montgomery County have continued to gain a large number of newly built condominium units, many of themplanned during better times for the housing industry.

The increases may come to an abrupt halt in those areas soon, however, as future construction reflects plans made during a more depressed period, industry spokesmen say.

The Metropolitan Washington Council of Governments recently reported that from July 1980 to July 1981 the Washington area had registered the highest annual increase in condo construction since COG started keeping records in 1977.

Most of the new projects have only a small number of units each, but there are some major exceptions. The four-building, 15-story Montebello on Rte. 1 in Fairfax County (not included in COG's 1981 numbers) will have 1,000 units when completed, for example. Delivery of units in the first two buildings there is expected this year.

Other Virginia high-rise complexes that opened for sale in the first part of 1981 include the Barkley on Columbia Pike in Arlington, with 125 units; Lakeside Plaza in Fairfax County, with 384 units; Waterford Square in Fairfax County, with 144 units. Among other openings were Burke's Cove Landing, a low-rise community with 448 units and Court Bridge town houses in Arlington with 190 units.

For the 12 months ended in July 1981, a total of 4,138 new units were opened up in the Washington area, up from 2,975 in the previous 12-month period and about triple the number three years earlier. The total inventory for the area in mid-1981 was 40,066, up 11 percent from a year before.

Half of the added inventory in 1981 was in Northern Virginia. Montgomery County's number jumped the most of any one county, however, rising by 1,351 over the year. In the District, the number of new units was 577, less than a year earlier when 679 were added.

Elizabeth Irwin, co-author of the COG report, pointed out that the significant increases in Northern Virginia and Montgomery reflected many projects that were in the pipeline for two years or so and only came on line in early 1981.

"The market has really crashed since then," she said, indicating that building permits for new condos in the last half of 1981 may well show a sharp decline.

Mike Brenneman of Brenneman & Associates, condominium specialists, agreed that the increased activity reflects the boom market of two or three years ago.

"It's not that anyone in the past 15 months has decided they would like to build a lot of condominiums," Brenneman said. "Some of them condo developers are sorry about it now."

The sharp rise also in part shows the shift in recent years from the overwhelming predominence of detached homes to smaller and less expensive condominiums throughout the country but particularly in the Washington area, housing experts say.

Housing Data Reports, local new-home specialists, recently reported that in 1981 condominiums continued to grab a growing share of the area market, representing 45 percent of the market compared with 39 percent in 1980.

"That's what's available, affordable and achieveable now, even if not desirable," said Robert Koury, president of the Northern Virginia Builders Association, about the rising number of condominiums.

Whether these will sell as well as in the past is another question. At the 1,000-unit Montebello complex, the first building sold out before completion, and the second, due for delivery this fall, already is 80 percent sold, said a spokeswoman for the developer, International Developers, Inc. Prices there range from $64,900 to $138,100.

Others may not have that kind of success, however. COG found that of a total of 94,989 new and converted condominiums in the Washington area, there were about 27,448 units available for sale. Of these, about 42 percent were in Northern Virginia, 27 percent in the Districtand 31 percent in suburban Maryland.