The grandiose dome , building starting in 1733, was designed by Filippi Juvarra (1730), then architect of the Savoia House. With its height, it is the optical fulcrum of the town seen from the hills and the characterizing motif of its skyline, seen from the lake. The very large windows, typical of the style of the architect from Messina, express the climax of the increasing light which the interior of the church manifests proceeding from the naves to the transept to the apses.
Its planning occupied architects from the Lombard and Roman milieu for three centuries. About thirty very valuable drawings are conserved. Before Juvarra, other architects planned the dome: Giovanni Antonio Piotti from Vacallo (1590-94), Andrea Biffi (1683, wooden model in the Museum), Francesco Castello (another wooden model in the Museum), Carlo Fontana (1688).
The stucco decoration of the dome with rosettes in the eight vaulting cells and in the small dome is by Gaspare Mola from Coldrerio (Ticino CH), who made the reliefs with the Evangelists in the pendentives.
Dangerous stucco falls from the dome began in 1757,
Giulio Galliori, architect from the Fabbrica del Duomo of Milan, planned and realized a raised counter-cup on a plinth intended to defend the dome against the seepage of rainwater.
The counter-cup disfigured the architectural shape as period prints and subsequent photos show. However, the problems of the small dome, subject to frequent repairs, remained unsolved.
A very grievous fire broke out during the restoration works of the copper roof on 27 December 1935: the timbering superimposed by Galliori to the dome of Juvarra burnt. Gilio Verga, engineer of the Fabbrica died after a few days, having taken care of the cathedral for forty years.
There was no other victim. The restoration commission was composed of Attilio Terragni “podestà” of Como, Camillo Silo engineer of the Fabbrica and architect Federico Frigerio who published a preliminary study. The hypothesis of arch. Frigerio to restore the dome in the original Juvarrian lines caused controversies in local newspapers.
The Sovrintendenza designated as restorer Pietro Gazzola. The restoration reinstated the original lines of Juvarra and took place between June and September 1838. The external illumination of the Cathedral was made during the opening ceremony on Christmas night, but also because of the war, only in 1950 the internal decoration was restored.
In 1964 static problems appeared and were solved by Politecnico of Milan with the restoration of the marble arches, a zinced iron scaffolding on the apsidal cup, a reinforced concrete chain at the vault masonry (1966).