The president of the University of the District of Columbia yesterday described as "seriously deficient and oversimplistic" a report by the General Accounting Office that asserted that proposed $70 million downtown campus is much larger than is needed.

In its report, released on Tuesday, GAO said that the university's forecast of a 48 percent enrollment increase by 1985 was far too high.

But Lisle C. Carter Jr., the university president, said GAO's forecast of only a 10 percent increase "deviates from the standard planning techniques . . . normally used by higher education planners throughout the country."

In fact, Carter added, a poll taken by a university consultant last month indicates a "potential population of 33,900 new students" in the university over the next two years.

The university, formed by a merger of Federal City College, Washington Technical Institute and D.C. Teachers College, now has 8,725 full-time equivalent students, down about 10 percent since 1975.

The planned new campus, just north of Mount Vernon Square, would boost capacity to 13,000.

"I think the whole campus will have to be built to meet the needs of the people," Carter said. "To build it piecemeal means that costs will escalate that much more."

Carter said the new campus was planned to replace the 14 old rented office buildings in which the university's downtown division, formerly Federal City College, now is housed.

"Limping along without adequate facilities," Carter said "would be to continue to accept an educational environment which robs D.C. citizens of the opportunity to receive a public higher education that is equivalent to the kind" offered in other states.

In addition to the proposed Mount Vernon campus, the university already has received $68.3 million for buildings under construction on its Van Ness campus, formerly Washington Tech, at Connecticut Avenue and Van Ness Street NW.