The fringes of metropolitan Washington will experience the area's fastest population growth during the 1980s, while a new mixed-use business core east of the White House is expected to emerge to keep downtown thriving through the decade.

Commercial realty executive James B. O'Brien told more than 200 area business executives and public officials yesterday that the D.C. area's growth "will be the equivalent of adding a new county the size of Montgomery County today."

Total population growth in the '80s here is expected to be 600,000 at a 20 percent rate, O'Brien said.

While the outlying suburban counties are growing, the metropolitan statistical area will be growing too, said O'Brien, an executive with Coldwell Banker.

It will take 247,000 new units to house the added population of a total cost of at least $20 billion this decade, according to research done by Coldwell Banker in conjunction with several area statistics gathering bureaus and organizations. O'Brien told the business executive that the 20 percent increase in population with a 3.5 percent increase in real per capita income will result in a 62 percent increase in retail spending by 1990.

Retail expansion, according to O'Brien's interpretation of researched statistics, "will translate into a demand for 87 million square feet of new retail space in addition to the current base of apporximately 125 million spare feet." That's an increase of 44 percent.

To serve the growing metropolitan population, the area can expect 250 new supermarkets and neighborhood shopping centers, 935 new restaurants, 10 regional shopping malls and 37 million square feet of new office space, O'Brien said.

Countering the expansion of fringe counties will be a growth in close-in areas, helped along by a viable Metrorail system and the rehabilitation of city residential and commercial properties.

"The renaissance of the city is well under way, tending to counter the outer migration," O'Brien said.

Examining the prospects for a new downtown based on a viable Metro system, O'Brien cited the area east of the White House as ripe for rejuvenation, offering the possibility for 30 million square feet of new space under existing zoning for offices and retailing hotels and some residential properties.

Some of the impetus for the expansion of downtown will come from the convention and civic center, the convergence of Metro lines, the new District of Columbia College near Mount Vernon Square and activities generated by the Pennsylvania Avenue Development Corp.

O'Brien cautioned the executives however, that the "rich-poor tension" could become more apparent by 1990. "Will housing, jobs or education in the poor community be sufficiently improved to forestall social unrest?"' O'Brien asked.