Officials of the University of the District of Columbia have sharply scaled down their estimates of the size the new university will reach in the next six years, and have told Mayor Washington that they do not need extra new buildings costing $115 million that had been in their long-range plans.

Even so, the university trustees said they want to go ahead with other new buildings, now awaiting approval by Congress, which will be able to accommodate the equivalent of about 13,000 full-time students, about 45 per cent more than are currently enrolled.

In its previous plans, the university had forecast an enrollment of 20,000 full-time equivalent students by 1983.

Last week university planners said that high figure was "unrealistic."

Instead, they said, they expect enrollment to reach about 13,000 by 1983, just enough to fill all the new buildings that the city government has asked Congress to approve this year.

The university is being created by a merger of the District's three low-cost public colleges - Federal City College, Washington Technical Institute, and D.C. Teachers College. Although their programs are still separate, the three schools have been governed by a unified board of trustees since May 1976. They are scheduled to be placed under a unified administration on Aug. 1.

The university's previous enrollment forecasts and building plans had been made by the separate colleges, but they were included in city budget documents as the basis for overall planning.

Last week, Claude Ford, the acting president of WTI who has also been that college's chief planner said the old forecasts contained "some duplication" and were "unrealistic."

The size of the student body at the three colleges - 8.939 full-time equivalent students last fall - was down about 1,000 from the year before. The decline was caused mainly by a sharp drop in the number of veterans enrolled after eligibility rules for GI Bill bebefits were tightened.

The decline brought enrollment down to what it had been in 1971, and was so severe at one of the colleges, WTI, that a special late registration was held to try to get enough students to meet its budget quota.

In addition, the number of students graduating from D.C. public high schools has dropped by about 500 since 1974, and it is these schools that supply most of the students for the public university. Last year the D.C. public schools had 5,041 graduating seniors.

Despite this decline which is expected to continue as the city's population falls, Ford said he expects that the university's enrollment will grow because an increasing proportion of high school graduates will probably attend college.

Overall, the three city colleges had a head count of 13,133 students last fall. About 60 per cent of them were part-timers, and for planning and budget purposes the university uses a formula of full-time equivalent students to show what its needs and operations are.

Under the formula, which is used by most colleges in the country, the total number of credit hours that students are taking each term is divided by 15, which is considered a full-time program. Thus, five students, each taking one 3-credit course, would be considered one full-time equivalent.

Since 1972, Congress has appropriated $68.3 million for buildings for WTI at Connecticut Avenue and Van Ness Street, NW, the former site of the National Bureau of Standards. Two large modernistic buildings have already been completed there, and construction is under way on the second phase of the project. By the time it is ready in 1961. Ford said the Van Ness campus will be able to accommodate 7,307 full-time equivalent students.

In additon, the city has cleared and assembled land downtown - on two blocks north of Mount Vernon Square - for a permanent campus for Federal City College, which is now scattered in 15 old office buildings.

Including architect's fees, the District already has spent $12.7 million on this project. Its 1978 capital budget, now before Congress, includes $56.7 million more for construction.

According to the university planners, these buildings at Mount Vernon are supposed to be ready in 1983, and will be able to house 5,627 full-time equivalent students.

When they open, Ford said, the university will be able to close the two old buildings which now house D.C. Teachers College on Georgia Avenue, NW, and on Harvard Street, NW.

In its old master plan, the university has proposed a second group of buildings at Mount Vernon Square, costing $70 million, and a third group at Van Ness, costing $45 million.

Ford said both these building projects are not needed, and the trustees have dropped them from the university's long-range plans.

In a revised master plan, presented to the trustees last week, all science programs - two-year, four-year, and graduate level - would be taught on the Van Ness Campus. All business and education programs would be on the Mount Vernon campus, while other liberal arts programs would be given at both campuses.