Consumer advocate Ralph Nader yesterday urged President Reagan to convene a conference with D.C. officials to discuss the "accelerating foreign takeover" of office buildings in the city, saying the trend could raise commercial rents and pose a threat to the financial ability of nonprofit groups to continue to rent office space here for lobbying operations.

Nader said Washington, as the nation's capital, "should be economically accessible to citizen, labor, trade and other groups who need to have offices here. No more than Bonn, Paris or Tokyo, it should not be turned into an absentee-owned office metropolis that is prohibitively priced and controlled by foreign corporations."

Foreign investors own 75 Washington area properties, including 64 District office buildings that account for one-fourth of the city's office space -- 13 million out of 52 million square feet, according to a recent study commissioned by the National Association of Realtors and conducted by the Massachusetts Institute of Technology's Center for Real Estate Development.

Foreign investors, finding relatively cheap deals in the United States because of the decline in the value of the dollar, have increasingly bought multimillion-dollar office buildings in Los Angeles, San Francisco, New York, Boston and Washington, among other cities. These investors have said they view the United States as a stable haven for their investments and often reap a substantially higher return on their U.S. investments than they do in their own countries.

In Washington, the Japanese, British and Canadians have been the major foreign real estate investors, with the Japanese leading the buying binge. Two years ago, the Japanese did not own one building here; now they own 12 with more than 3.6 million square feet of space.

Nader, who did not offer Reagan specific suggestions as to how foreign ownership of office buildings should be limited here, said he was not concerned about heavy foreign investment in such places as Hawaii and Los Angeles. Los Angeles' downtown office district is more than 50 percent foreign-owned.

"It is quite another matter to reign passively while the nation's capital becomes foreign-owned," Nader said. " ... Going down in history as the president who presided over the biggest transfer in history of Washington, D.C., real estate to foreign ownership is certainly not what your friend, John Wayne, had in mind for you."

The White House did not immediately comment on Nader's letter.

Nader said trade associations and corporate law firms, regardless of the extent of foreign ownership, would continue to be able to afford downtown office rents because they would pass on the increased costs to their members or clients. But Nader questioned whether nonprofit groups would be able to bear the burden of higher rents generated by escalating prices paid by foreign firms for buildings here.

He said such groups "have a stake in not being driven away by the sale of Washington, D.C., in order to make profits for foreign currency-driven companies."