The National Capital Planning Commission approved a revised U.S. Army plan for a $25 million library and academic facility next to historic Fort McNair last week, but delayed the approval until it accepts a new master plan for the fort, which the Army is expected to submit within several months.

The NCPC had rejected the Army's original plan for the building as too prominent for the fort's turn-of-the-century landscape design. The commission found that the revised plans for the structure are in keeping with District policy within historically protected areas.

The Army had originally planned to tear down 400 feet of a brick wall surrounding the fort to build the new library. The new master plan to be submitted by the Army will call for tearing down 150 feet of that wall, officials said.

The Army also had originally proposed closing part of C Street SW for the new facility. The commission directed that the Army's plans retain the street.

If the new master plan is submitted promptly, the Army could begin constructing the building in October, according to the commission's staff report.

Col. Michael Alba, manager of the project, said the Army's architects are revising the building plan and the Army foresees construction beginning in the fall and being completed by the middle or the end of 1988. "I feel more optimistic than I have ever been before with this project. It's very sorely needed and it's time we get on with it," he said.

The Army is currently seeking funds from Congress to build the planned facility, which would be used for research and instruction by the National Defense University, under the direction of the Joint Chiefs of Staff.

Fort McNair, designed by Pierre L'Enfant in 1791 and redesigned in a Beaux Arts style by the McKim, Mead and White architectural firm in 1903, is designated a historic district under the Federal Historic Preservation Act and a "special place" in the preservation and historic features portion of the District's Comprehensive Plan. The Theodore Roosevelt building, the fort's largest, is both a District and a national historic landmark.

The Army says it needs the proposed building, called the National Defense University Library/Academic Center, to serve as a library and staff center for the university, which offers advanced training in military strategy to military officers and State Department employes.