A debate is raging over London's Royal National Theatre building. It adjoins the South Bank Centre, though it is to a degree separated from it by Waterloo Bridge. It is under its own board of trustees, also distinct from the South Bank Centre's administration.

It is this board that wants to make alterations to the building. The proposals have aroused vigorous protests from various quarters, including respected architectural historian Mark Girouard, and - more sensitively - the architect Sir Denys Lasdun.Lasdun, who is now 80, is vociferous in defense of what he sees as his masterpiece. The cry of "vandalism!" is heard in the land - and in numerous letters to the London Times and articles in the Independent.

The theater trustees have justified most of its required alterations, prompted by changing needs, by calling them "reversible."

English Heritage, the body designated by Parliament to rule on planning applications to alter officially protected historic buildings, agrees. The theater was given protected status (unusual for post-war buildings) only this year - a high status called Grade II and Lasdun uses this in his defense.

There is one requested alteration, however, that English Heritage has recommended should go to a public inquiry, so that all concerned parties can express their reservations.

This concerns removal of the building's lower terrace or walk-way, "a substantial element of `hard fabric.' "

Like the walkways that surround the neighboring South Bank Centre buildings, this terrace has been frequently criticized for making it difficult for people to find a main entrance, and for creating a gloomy negative space underneath it.

The lower terrace is, however, a link from the theater to Waterloo Bridge. And it is arguably different from the South Bank Centre walkways because it does not, like them, make the buildings look as if they are knee-deep in horizontal concrete. It is an integral part of the building, rather than part of its surroundings.

Lasdun and his allies say that to remove it would be "crude" and "damaging."

The architect himself, in an article in the Independent, cries indignantly that it would diminish "the integrity of the building and the experience theatergoers and passersby have of it." It would destroy his design, a building in which "overhanging terraces . . . descend in a series of rhythmic layers down to the river," he writes.

Lasdun points out that there are now plans to return the Royal Festival Hall to its original 1951 state. By saying this, he is hinting that the cost of returning his theater, at some future and more enlightened period (should the proposed "vandalism be allowed), to the condition he intended, may be considerable.

The real issue seems to be the current distaste for 1960s concrete architecture, which this monumental but highly imaginative theater strikingly epitomizes.

Lasdun's supporters see this attitude as the driving force behind the desire to destroy the lower terrace. They consider his theater as a potential victim of the arrogance and partiality of changing taste.