The lavatorium is an octagonal building, built against the eastern cloister. It dates from the 12th century and has a dome-shaped vault, the ribs of which rest on corbels in the shape of human heads (added 14th century). The upper level was a treasury. In front of the lavatorium is a water well.

The abbey of St Bavo was founded in the 7th century AD outside the walls, and it was destroyed in 1540 when Charles V built the new citadel (now destroyed). In 1539 the chapter had been moved to the parish church of St Jan (St. John), which was rededicated to St Bavo (now the cathedral). At the destruction in 1540 the sanctuary of the abbey church (Romanesque, consecrated 1148) was preserved as a garrison chapel, but even that was soon demolished by the Calvinists. Of the abbey buildings there survive only the cloister with its lavatorium, the adjoining north wall of the church and the surrounding conventual buildings, dating essentially from the 12th century.