Trustees of the University of the District of Columbia have adopted a new master plan, requested by Congress, to support the trustees' proposal for a $70 million downtown campus just north of Mount Vernon Square.

If the campus is built, the plan estimates the university will have space for about 13,000 full-time equivalent students and can expect that many by 1983, when it hopes the buildings will be completed.

Even if enrollment does not grow that fast - a 48 percent increase in five years - the master plan says the new buildings are justified as replacements for the 14 old rented office buildings scattered across downtown in which half the university now operates.

The plan says the city university wants tob build "not just for the (next) five years . . . (but) for the 20 years that remain in this century."

This past year the university had a full-time equivalent enrollment of 8,725, down about 10 percent from the total enrollment in 1975 of the three institutions which were merged into the university.

However, university officials said they are confident that enrollment will increase steadily because of untapped demand for higher education, especially among city residents in their late 20s and early 30s and school drop-outs who return for high school equivalency diplomas. The master plan states that the "lock-step approach," which expects most college students to come directly from high school, is out-moded.

Full-time equivalent enrollment is computed by adding the total number of credit hours that students are taking and dividing by 15, the number of credit hours considered to be a full-time course, load. Thus, five students, each taking one course of three credit hours, would be considered one full-time equivalent.

Last fall the University of D.C. had a headcount of 13,471 students. About 80 percent attended part time.

The D.C. university was formed last year in a merger of the three public colleges in the city, Federal City College, Washington Technical Institute and D.C. Teachers College.

Last month the General Accounting Office said the large downtown campus, which had been planned for Federal City, probably was not needed. GAO noted that not only has university enrollment declined, but that the District has lost population rapidly since 1970 and public school enrollment has dropped.

In addition to the proposed Mount Vernon campus, the university has received $68.3 million for buildings already under construction on its Van Ness campus (formerly Washington Tech.) at Connecticut Avenue and Van Ness Street NW. That campus will accommodate 7,307 full-time equivalent students.

GAO suggested that in addition to the Van Ness campus, the university should renovate the old D.C. Teachers College buildings (instead of abandoning them) and place adult education classes in under-utilized public schools.

Overall, GAO said, these arrangements would accommodate about 9,600 students, the enrollment it projects for the university by 1983.

The new master plan does not deal directly with the GAO suggestions but stresses that university buildings should "not only be efficient, but dignified, humane and appropriate to the enterprise of higher education."

About $12.7 million already has been appropriated to buy land and pay architects' fees for the downtown campus.

Another $56.7 million for construction is included in the 1978 city budget, approved by Congress last month. Because of an amendment, however, the money cannot be spent until the new university master plan is approved by the mayor and City Council and by the House and Senate appropriations committees.

After the trustees approved the master plan last Thursday night, board member Marjorie Parker said she expected it would "move ahead routinely" in the city government. However, university officials are less sure of how it will fare in Congress.

The proposed downtown campus would be built just north of Mount Vernon Square, the site of the old D.C. Public Library, which already is being renovated for a university library and president's office. The campus would be bounded by 7th and 9th streets and K and M streets NW.

A new D.C. convention center has been proposed for an adjacent area - bounded by New York Avenue, H Street and 9th and 11th streets - just southwest of the square.

In its report, GAO said even its projection of 9,600 university students may be too high if the college raises academic standards and flunks out more students and places more on probation than it has in the past.

The new master plan notes that the university already has a high drop-out rate - almost 20 percent of its students do not return each semester. It said improved counseling and remedial programs now being planned probably would lower the number of drop-outs.

Like the old city colleges, the new university admits all high school graduates, regardless or grades or test scores.

Under the master plan, two major university divisions would be housed on the Mount Vernon campus: liberal arts and business administration. Three others would be based at Van Ness: life sciences, physical science and education."