The General Accounting Office yesterday urged a sharp cutback in plans for a new $70 million downtown campus for the University of the District of Columbia, charging that enrollment projection soon which the building program is based are far too high.

Over the last three years, enrollment at the new public university has dropped by about 10 percent, GAO said.

University officials have said they expect enrollment to go up again by 48 percent between now and 1985, but the GAO said the projection is suspect because of declines in the city's population and school enrollment and nationwide trends in college attendance.

If the new campus were built as proposed just north of Mount Vernon Square, the agency said, the college would have space for about 3,400 more full-time equivalent students than it is likely to enroll.

Instead, the agency called for renovation of buildings on Georgia Avenue and Harvard Street NW, which used to house D.C. Teachers College. It also suggested using space in under used public school buildings for adult education classes.

Some construction still might be done on the Mount Vernon site, the GAO said, but it suggested that the buildings be erected only as they become necessary, instead of constructing a large campaus all at once.

The GAO study was released yesterday by Sen. Patrick J. Leahy (D-Vt.), chairman of the Senate D.C. approprations subcommittee.

In a statement, Leahy said he concerned that the university may be overbuilding and said he wanted assurances that the Mount Vernon campus is needed before the city begins construction.

University officials could not be reached for comment.

UDC president Lisle C. Carter Jr. cancelled a press conference that had been scheduled for midafternoon. A spokesman said Carter planned to delay comment until Thursday.

In a related development, the U.S. House of Representatives a's 1978 budget yesterday with an amendment that none of the $56.7 million it authorized for the mount Vernon campus can be spent until a "consolidated master plan" for the university is prepared.

The amendment, backed last week by a House-Senate conference committee, requires that the plan be endorsed by Washington's mayor and City Council and also by both the Senate and House appopriations committees.

The city has spent $9.7 million buying and clearing two square blocks for the proposed campus, plus about $3 million in planning and architectural fees.

The proposed downtown university center would be built in an area bounded by Seventh and Ninth Streets NW on the west and est and by K and M Streets on the north and south.

The convention center is proposed for an earea munded by New York Avenue, H Street, Ninth Street and 11th Street NW, just southwest of the square.

The university site originally was scheduled to the permament campus of the Federal City College, which last year was merged with Washington Technical Institute and D.C. Teachers OCollege to form the new city university.

Classes mf the university's Federal City College division are still scattered in 15 downtown office buildings.

However, since 1972, Washington Tech, now called the Van NEss campus of the university, has received $68.3 million for buildings at Connecticut Avenue and Van Ness Street NW. By the time they are completed in 1981, they will be able to accommodage 7,307 full-time equivalent students.