In keeping with the historic character of the Church's original buildings, the Classroom Annex is built of the same Rhode Island granite found on the Church and Parish House, but uses readily available techniques to give texture and shape to the stone. The painted wood trim and details paraphrase the adjacent buildings, including a simpler version of the classical/Gothic mullions of the original church. In keeping with the Unitarian view of the universe, five phases of the moon are illustrated by mullions in the round windows.

The new lobby between the Classroom Annex and the existing Parish House has triangular skylights that allow light into the Parish House auditorium and views from the auditorium into the lobby. The ceiling lights show the Big and Little Dippers pointing to the North Star.