The history of Stavanger cathedral starts at the turn of the twelfth century. These are the days when Sigurd the Crusader is a joint king in Norway with his brother Øystein. He appoints a bishop called Reinald to the new Diocese of Stavanger (see Store Norske Leksikon). Quite likely, this Reinald commissions the build of the church and also sees it consecrated in 1125.
Excavations inside the cathedral continue to reveal more about the site before the church was built. In 1968, burnt wood was discovered under the altar. And now this year, layers of dark soil containing animal bones are found elsewhere in the building. The bones date to the Viking Age. That is why archaeologists tentatively assume the cathedral was built on top of a settlement. More graves were found, too, dating from the Viking Age to the later Middle Ages. There is not much reading on the topic yet, see Life in Norway and HeritageDaily.