The United States for the past 15 years has "consistently underestimated" Soviet intelligence capabilities and wrongly assumed that superior American counterintelligence technology would thwart Soviet efforts to penetrate the new embassy building in Moscow, a State Department study released yesterday said.

A Soviet attempt to plant electronic "bugs" in the building was "both foreseeable and foreseen. But as a nation, we have failed to anticipate the boldness, thoroughness and extent of the penetration," said the report written by James R. Schlesinger, the State Department-appointed security consultant.

A former CIA director and defense secretary, Schlesinger was named in January to assess the security damage done by the Soviets to the unfinished, eight-story main embassy building, or chancery, in Moscow.

The techniques used by the Soviets to bug the building "with a full array of intelligence devices" are so sophisticated that the United States even today does "not yet understand either the technology or the underlying strategy" behind them, according to the 14-page report, which was released yesterday. Some conclusions were made public Tuesday when Schlesinger testified before Congress.

In his report and comments at a subsequent news conference, Schlesinger echoed earlier criticism that the Reagan administration particularly and U.S. government generally have not taken the Soviet intelligence threat seriously enough.

Schlesinger said it had been "uphill sledding" for such critics and conceded that once "a gradual dawning" of the extent of the Soviet threat permeated the government, the State Department "was one of the last to get on board."

But he also said that there had been "no standout performances" among any of the U.S. intelligence agencies and departments, adding "the blame is upon all of us as a nation."

Schlesinger's recommendations to Congress included rebuilding the top three floors of the chancery and construction of a new six-floor annex for top security functions at a cost of $35 million to $40 million beyond the $192 million already appropriated for the new embassy complex.

Schlesinger said the Soviets were "imaginatively exploitive" of eavesdropping technology used by intelligence services around the world. The United States had "no comparable advantage" in this field, he said, and should be "learning from the Soviets."

The United States had been unable until very recently to detect even with X-ray machines the sophisticated Soviet devices implanted in prefabricated concrete pieces used in building the chancery's walls and floors.

Since agreement was reached in 1972 between Washington and Moscow providing for the simultaneous construction of new embassies by both governments, "the United States government consistently underestimated the determination and ability of Soviet intelligence organizations to penetrate the new chancery building," the Schlesinger report said.

"To counter Soviet efforts, heavy reliance was placed upon what was then seen as superior U.S. technology and ingenuity to counteract Soviet efforts," it said. "In retrospect, it is evident that U.S. confidence in its early ability to neutralize that effort was misplaced."

The report also took issue with those urging the administration to oust the Soviets from their new embassy site on Mount Alto off Wisconsin Avenue.

The Soviet Embassy, it said, is "undeniably well-sited" and the view is "simply terrific," allowing "a significant improvement in line-of-sight access to strategic U.S. government buildings." The Soviets' present building is on 16th Street several blocks north of the White House.

However, given extensive Soviet eavesdropping facilities elsewhere, including those in Cuba, the advantages of the Mount Alto site "are less in reality than in public perception," the report concluded. "Indeed, the question of the height of Mount Alto has become a symbol of our national frustration over the whole Moscow embassy issue." Excerpts from the Schlesinger report appear on Page A19.