Dumbarton Oaks, the historic Harvard University-owned mansion and grounds in Georgetown, will seek initial approval this week to build a two-story underground library at the rear of the house.

The project will tear up the lawn terraces known as the North Vista, leaving a large hole for about 18 months while the 25,000-square-foot annex is built.

Most of the garden will remain open to the public during the project, as will the museums of Byzantine and pre-Columbian art.

Director Edward L. Keenan said the annex is needed to house and consolidate the thousands of volumes used by scholars at the research institution. Keenan pledges a faithful restoration of the terraces once the work is finished, mindful that a similar expansion proposal in the 1970s was withdrawn in controversy. The old plan would have altered the look of the garden.

In spite of Keenan's assurance, however, opposition is gathering to the new plan. A prominent landscape architect already has written to Keenan asking him to reconsider, and others have joined the opposition.

With its much-copied turf steps, squeezed perspective and extraordinary stone carvings of urns and swags, the North Vista is considered one of the signature elements at the 16-acre garden and a masterpiece of its chief designer, pioneering landscape architect Beatrix Farrand.

A conceptual plan will be presented to Georgetown's Advisory Neighborhood Commission today, followed by scheduled hearings before two historic review bodies, the Old Georgetown Board on Thursday and the Fine Arts Commission Nov. 18.

Elizabeth K. Meyer, assistant professor of landscape architecture at the University of Virginia, said the world-famous gardens are part of the "humanist, academic" environment at the Dumbarton research center, where scholars research the history of landscape architecture, Byzantium and medieval societies as well as the pre-Columbian Americas.

Moreover, Meyer says, the institution hasn't done the scholarly groundwork to proceed with the plan, and needs to prepare a history of the garden's development and then a master plan of preservation based on it.

"Here this proposal is being made without the basic process being followed," she said.

James Urban, the landscape architect for Dumbarton Oaks, said the garden's mass of documentation, including drawings and photographs, as well as correspondence between Farrand and her patrons, Robert and Mildred Woods Bliss, form their own master plan. In addition, "I don't think it ever should be considered a frozen landscape," he said. "It has changed dramatically" since the Blisses gave it to Harvard in 1940.

Keenan said the North Vista and adjoining spaces--the copse veiling Philip Johnson's pre-Columbian wing and the Rocaille Fountain terrace--will be more faithfully returned to Farrand's vision of them. He said the North Vista's decorative masonry, including the brick steps, would be numbered, lifted, stored and rebuilt once the library is finished. The side walls of brick, stone and carved limestone would be supported with sheeting and shoring as the structure is built.

Keenan finds it perverse that the scholars studying at Dumbarton Oaks work in the basement while the books occupy much of the mansion's first three floors, including a loft area. Space is so short that some of the books are kept in closets.

The new climate-controlled library would allow a better environment for the collections of volumes and extensive archives while allowing refurbishment of the house and better working conditions for staff and scholars--called fellows, who typically study for a year and number around 18 at a time.

Under the plan, a suite of offices would be built on one level above the music room and a new entrance built on 32nd Street. The estimated cost for the work is $12 million to $15 million. Exact costs of tunneling beneath the copse, providing access to the handicapped and building climate control systems haven't been nailed down yet, Keenan said.

Architect Richard Williams said he expects construction to take 18 months, and Keenan hopes work can begin in about 12 months. Urban said most of the public grounds will remain open, except the North Vista, Star Garden and Serpentine Stairs leading to the Rocaille Fountain and swimming pool terrace.

Two deodar cedar trees, replanted several times over the years, will be lost, though one is distinctly stunted as a result of bad drainage, which will be corrected. The work will allow the cedars to be replaced with the cedars of Lebanon that Farrand wanted, Keenan said.

Of greater concern, Williams said, is the effect on the copse. Williams's design calls for a corridor beneath the copse and Urban is looking at the feasibility of tunneling beneath the woods to save the old trees. Smaller trees and shrubs will be removed to restore the copse closer to Farrand's version.

Meyer worries that the copse and other areas of the grounds will suffer as the library and its elaborate drainage systems affect ground water levels.

Urban and Keenan said water levels will be monitored so soil moisture can be regulated.

Meyer and Florence Everts, a landscape designer in Northwest Washington, want the institution to build the library above ground on surplus land it owns off S Street, several hundred feet outside the garden. But Williams said the annex must be part of the scholars' environment of mansion, collections and garden. "If any one of those legs is pulled away, the richness of the culture begins to unravel," he said.

CAPTION: Approval is being sought for an underground annex at the Georgetown mansion.

CAPTION: Most of the garden at Dumbarton Oaks would remain open during construction of a proposed subterranean annex.