After the second raid in 1385, St Giles’ was rebuilt and over the next 150 years, side chapels were added by prominent members of the community, both by individuals, such as the Duke of Albany, son of King Robert II, and by the prosperous Merchant Guild and the craft incorporations. When Sir William Preston bequeathed to the church an arm bone believed to be a relic of St Giles, an additional aisle was built in his memory in 1455. The relic has long since vanished, but Preston’s carved coat of arms of three unicorns’ heads can be seen in the aisle. By the middle of the sixteenth-century, there were about 50 side altars in thee church, in addition to the High Altar at the east end, dedicated to St Giles’ himself.
As said in the main seciton, St Giles’ became a Collegiate Church in 1466. In future, the Collegiate Church of St Giles’ would have increased facilities for holding Masses for the dead, which were very much in demand at that time. The St Giles’ vicar would now become its provost, similar to the dean of a cathedral. There would be fourteen prebendaries, who were like cathedral canons. They would provide daily Masses for the souls of King James III and his ancestors, as well as for all the other people commemorated at the many altars in the church. A sacristan would be responsible for the church plate and vestments and for making sure that the church bells were rung and the organs played. There would be a leader of the choir and four boy choristers. Not long afterwards, a song school was established in the churchyard, to train the choristers.