Adding a nonbuilding in Williamsburg. Walking into this art gallery is like stepping into an architectural time machine. Its entrance, a 1773 Colonial building, leads to a skylit contemporary museum underground By Carleton Knight III, Special to The Christian Science Monitor

ADDING on to a building, either to an existing older structure or an integrated complex, has vexed architects for years. How do you do it? Some architects try to closely replicate the original, while others opt for a design that is sympathetic, but more modern, reflecting the change in times. Still others offer a totally contrasting approach, a mirror glass wall, for example, to set old and new apart. A recent solution to this problem in Virginia is worth examining because of its demonstrated concern for the existing architecture.

Colonial Williamsburg has a growing collection of magnificent decorative objects -- some 8,000 examples of furniture, silver, pewter, textiles, porcelain, and prints -- that until recently the public could never see because of a lack of display space. Spurred on by the gift of $14 million from the late DeWitt Wallace, founder of Reader's Digest, and his wife, officials of the 173-acre living museum began to look for solutions.

In keeping with its traditional use of historic buildings, Colonial Williamsburg considered reconstructing the nation's first facility for the mentally ill as a decorative arts gallery. But it soon became evident that the 1773 Public Hospital, whose meticulous re-creation was undertaken by the Colonial Williamsburg architectural staff, would not be large enough. Thought was given to building the museum underground, beneath the hospital, but the hospital would then have appeared to be merely a cupola for the necessarily huge structure below, an inappropriate solution intellectually and aesthetically. There was also a significant restriction: Colonial Williamsburg allows only historic buildings within its boundaries.

Architect Kevin Roche of Hamden, Conn., a prot'eg'e of Eero Saarinen and one of the nation's most talented designers, came up with a typically bold but understated solution -- a thoroughly modern building that does not violate the architectural proscription. He suggested that the Public Hospital be rebuilt and used as the entrance, via a 65-foot-long tunnel, to the new facility that would be situated just outside the historic precinct. That way, the gallery could be of contemporary design.

But visitors would hardly know it from outside appearances. Roche partially sank the new, 62,000-square-foot structure and surrounded it with a 12-foot-high brick wall of the kind found around 18th-century gardens in the area. Thus, all one sees is the Public Hospital and a wall behind it. The 465-foot-long wall serves a valuable function, screening out a visually intrusive, 1960s-modern criminal-justice facility right next to the historic sector. Construction on the $17 million complex, which started in November 1982, was completed last June.

After entering the Public Hospital, visitors descend to the lower level and walk through the entry tunnel, which serves as an introductory gallery. This enclosed space opens stunningly into a large two-story stairwell flooded with natural light from skylights overhead. The most important works in the collection are displayed around the balcony, which is walled in latticework to give the space a domestic sense appropriate to the objects. The surrounding galleries are divided by subject, and additional rooms will permit Colonial Williamsburg to play host to traveling shows for the first time.

The exhibit spaces unfold dramatically from one to another and are filled with surprises. An indoor courtyard replete with plants, vine-covered trellises, and places to sit is hidden behind several galleries. Next fall a large garden, designed by an English landscape architect, Sir Peter Shepheard, and featuring appropriately historic plantings as well as a fountain, will open in the west end of the building. Notes Roche, who won the Pritzker Prize (a sort of Nobel Prize for architecture) in 1982, ``One of the problems in all museums is fatigue. Relief spaces are very necessary and very welcome to visitors.''

Roche says there is nothing special about his deceptively simple, non-monumental solution. He declares, ``The only real buildings are nonbuildings. The rest is theater.'' The DeWitt Wallace Decorative Arts Gallery may be theatrical, but in the way it fits unobtrusively into Colonial Williamsburg, it is assuredly not a scene-stealer.

This project shows once again that the best solutions to adding on are those that make an architectural statement, but not at the expense of what came before. The architect has avoided an ego trip, and modesty may be the best route to follow when dealing with this often-delicate problem.

Carleton Knight III is contributing editor of Architecture magazine and reviews architecture regularly for the Monitor.