The '60s building, credited to Francis Pym, is subject to a planning application for a series of hugely controversial changes to its fabric.
Unusually, this includes the Arts Council of Northern Ireland, which normally refuses to comment on planning applications.
The proposal includes changes to the building's entrance, which has a celebrated canopy, and the multi-level interior.
Other opposition to the alterations has come from the Ulster Architectural Heritage Society and Docomomo.
The planning application is expected to be decided in March, with a recommendation from Northern Ireland's Planning Service expected imminently.