Beneath the streets of Waterford, a few metres east of where St Peter's church once stood, the remains of a medieval undercroft quietly outlasted the building it once supported. An undercroft is a vaulted or semi-subterranean ground-floor chamber, typically used for storage, and this one is substantial: roughly 25 metres north to south and just over 8 metres wide, with walls between 0.7 and 1 metre thick and footings a full metre wide. Three relieving arches, structural openings built into the outer face of the west wall to distribute the weight of the masonry above, were visible during excavation, as was a doorway with two steps descending from it at the eastern end of the south wall. It is the kind of structure that suggests a prosperous merchant household, built to last and built with some ambition.

The undercroft may have formed part of a house constructed in 1470 by a man named John Collyn, on land granted to him by Thomas Rice. The grant is recorded in a register connected to the chantry of St Saviour in Waterford, placing this otherwise anonymous building within the documented social and ecclesiastical life of the late medieval city. At some later point the structure was enlarged considerably: a stone-lined pit lying 9 metres to the south was absorbed into the expanded plan, and a new building was added to the south and west, its interior measuring approximately 13 metres by 11.4 metres. The east wall of this later addition butted directly against the south wall of the original undercroft, a detail that archaeologists can read as evidence of phased construction rather than a single build. The whole complex was examined during excavations carried out between 1986 and 1992, part of a major programme of urban archaeology in Waterford that transformed understanding of the city's Viking-age and medieval layers.