Complex sentence: Sponsorship from Anne de Bretagne was behind the addition of the south porch in 1498-1509 and the bell-tower porch on the western side as well as the start on the neighbouring chapel of Sainte-Barbe.
Split into simpler sentences: Sponsorship from Anne de Bretagne was behind the addition of the south porch in 1498-1509 and the bell-tower porch on the western side. This sponsorship also funded the start on the neighbouring chapel of Sainte-Barbe.