The basilica, perhaps to avoid the unstable and marshy ground, was built on an artificial hill not far from the walls, along the Via Ticinensis, the main access route to the city, and not far from the Imperial Palace and the amphitheatre, from which were taken some of the materials used in constructing the temple itself.
The complex was surrounded by various waterways, coming together to form the Vettabbia, the canal that takes away the waters of Milan, which still flow towards the agricultural areas to the south of the city.

The building had a central plan approached by a four-sided portico and surrounded by two connected structures.
Access to the portico was through a colonnade which in turn gave access to three portals leading to the main body of the building.
This consisted of a square hall inscribed as a building with four apses, whose semicircular hollows overhung by semi-cupolas were articulated by four columns.
Around this space ran the ambulatory surmounted by a space later used as a women's gallery.
Towers rose at the four corners of the square building.
The whole was topped by a dome of which we know little, this having been lost.
The interior was lit by large windows, and probably decorated with marble in the lower parts and with mosaics in the vaults and arches.
Of the two side buildings, the smaller was in the east, opposite the entrance: a chapel in the shape of a Greek cross, later on octagonal, dedicated to St Hippolytus.
The larger building was to the south, having the function of the imperial mausoleum: tradition attributing its foundation to Galla Placidia, which is why the sacellum took on the name of the chapel of the Queen.

Between 489 and 511 Bishop Lorenzo had a third structure built to the north, a chapel dedicated to St Sixtus, to be used for the burial of metropolitans.
Perhaps in this period, when Roman imperial authority in Italy had diminished, the mausoleum to the south of the basilica was transformed into a chapel dedicated to St Genesius the martyr.
By the sixth century, on the east wall, opposite the entrance, two portals were opened giving access to two local apses.

In the tenth century, probably in the Ottonian era, reconstruction took place possibly involving the participation of a Byzantine workforce who had retained knowledge of the classical techniques of construction and decoration.
Little is known regarding these restorations, but it is assumed that the cupola (dome) had been reconstructed using pipes made of terracotta, making it lighter than the previous one, perhaps already damaged to the extent of justifying a reconstruction.
After the disasters of the eleventh century, the restorations of the twelfth and thirteenth centuries focused on providing stability to the whole complex, rebuilding the pillars that supported the cupola and carrying out other interventions on the load-bearing structures (columns, towers).
 In this period, a lantern was added above the dome, supported by flying buttresses leaning against the towers.
In the fifteenth century, the Chapel of the Citizens was created from the hall of the apse in the southeast, which had already been refashioned in the eleventh century.

After the collapse of 1573, a new cupola was commissioned from the architect, Martino Bassi.
His collaboration with Rinaldi, Meda and Trezzi resulted in numerous changes and was only completed in 1619.
Archbishop Borromeo had the chapel of St Genesius rededicated to St Aquilino, whose relics were placed in the chapel; to its sides were added two chapels, dedicated to St John the Baptist and the Holy Family.
In 1623, at the wish of Archbishop Federico Borromeo there began the construction of the chapter house to the side of the courtyard, a project carried out by the architects Aurelio Trezzi and Francesco Maria Richino; construction was completed in 1626.

In 1713 at the initiative of Francesco Croce, the Chapel of Redemption was inserted between those of St Aquilino and the Holy Family (now a sacristry).

In 1894, the engineer and architect Cesare Nava built a vestibule in front of the church,
The basilica, perhaps to avoid the unstable and marshy ground, was built on an artificial hill not far from the walls, along the Via Ticinensis, the main access route to the city, and not far from the Imperial Palace and the amphitheatre, from which were taken some of the materials used in constructing the temple itself. The complex was surrounded by various waterways, coming together to form the Vettabbia, the canal that takes away the waters of Milan, which still flow towards the agricultural areas to the south of the city.
The building had a central plan approached by a four-sided portico and surrounded by two connected structures. Access to the portico was through a colonnade which in turn gave access to three portals leading to the main body of the building. This consisted of a square hall inscribed as a building with four apses, whose semicircular hollows overhung by semi-cupolas were articulated by four columns. Around this space ran the ambulatory surmounted by a space later used as a women's gallery. Towers rose at the four corners of the square building. The whole was topped by a dome of which we know little, this having been lost. The interior was lit by large windows, and probably decorated with marble in the lower parts and with mosaics in the vaults and arches. Of the two side buildings, the smaller was in the east, opposite the entrance: a chapel in the shape of a Greek cross, later on octagonal, dedicated to St Hippolytus. The larger building was to the south, having the function of the imperial mausoleum: tradition attributing its foundation to Galla Placidia, which is why the sacellum took on the name of the chapel of the Queen.
Between 489 and 511 Bishop Lorenzo had a third structure built to the north, a chapel dedicated to St Sixtus, to be used for the burial of metropolitans. Perhaps in this period, when Roman imperial authority in Italy had diminished, the mausoleum to the south of the basilica was transformed into a chapel dedicated to St Genesius the martyr. By the sixth century, on the east wall, opposite the entrance, two portals were opened giving access to two local apses.
In the tenth century, probably in the Ottonian era, reconstruction took place possibly involving the participation of a Byzantine workforce who had retained knowledge of the classical techniques of construction and decoration. Little is known regarding these restorations, but it is assumed that the cupola (dome) had been reconstructed using pipes made of terracotta, making it lighter than the previous one, perhaps already damaged to the extent of justifying a reconstruction. After the disasters of the eleventh century, the restorations of the twelfth and thirteenth centuries focused on providing stability to the whole complex, rebuilding the pillars that supported the cupola and carrying out other interventions on the load-bearing structures (columns, towers).  In this period, a lantern was added above the dome, supported by flying buttresses leaning against the towers. In the fifteenth century, the Chapel of the Citizens was created from the hall of the apse in the southeast, which had already been refashioned in the eleventh century.
After the collapse of 1573, a new cupola was commissioned from the architect, Martino Bassi. His collaboration with Rinaldi, Meda and Trezzi resulted in numerous changes and was only completed in 1619. Archbishop Borromeo had the chapel of St Genesius rededicated to St Aquilino, whose relics were placed in the chapel; to its sides were added two chapels, dedicated to St John the Baptist and the Holy Family. In 1623, at the wish of Archbishop Federico Borromeo there began the construction of the chapter house to the side of the courtyard, a project carried out by the architects Aurelio Trezzi and Francesco Maria Richino; construction was completed in 1626.
In 1713 at the initiative of Francesco Croce, the Chapel of Redemption was inserted between those of St Aquilino and the Holy Family (now a sacristry).
In 1894, the engineer and architect Cesare Nava built a vestibule in front of the church,
Basilica of San Lorenzo, Milan