The main building was built as an extension by the FCJ sisters following their purchase of Upton Hall in 1863. When built, it housed the entrance hall, senior study, dining hall, a classroom, bonnet room and toilets on the ground floor; two dormitories and a classroom on the first floor; a further dormitory for pupils, a nuns dormitory, an infirmary and a small chapel on the second floor.

Each of the rooms in the building was named, for example the main reception was called St Aloysius' Hall, the classroom next to the dining room was called St Anne's, the dormitories on the first floor were called The Immaculate Conception Dormitory and The Infant Jesus Dormitory, the classroom on the first floor was called St Zoe's and the pupil dormitory on the second floor was called the Sacred Heart Dormitory.

The Main Building around 1910

The only name still in common use is St Anthony's which is the passage which runs alongside the old dining hall, now the staff room. Until the dining hall was converted into the staff room, the room at the end was still known as St Anne's.

In 1939 an air raid siren was mounted on the roof of the building, this was controlled from Upton Police station. The air raid siren remained until the 1960s.

The building is designed in the Italian style. A cross is mounted on the apex of the roof, this is five feet tall and is inlaid with copper gilt bands and Bohemian prismatic glass. Below the cross are the letters I H S, these were originally painted white, edged with vermilion. The background was painted ultramarine. People often ask what I H S means.

The Cross and IHS on the Roof of the Main Building

The letters are often seen in Catholic and other Christian Churches and, according to the Catholic dictionary, IHS are the first three letters of the name 'Jesus' in Greek.

Sometimes the letters are interpreted as standing for the Latin 'Iesus Hominum Salvator' (Jesus, saviour of men), but there is no foundation for this in fact.