A lavatory, also known as a bathroom or toilet, is a common facility found throughout the wizarding world. While functionally similar to Muggle lavatories, they are often sites of significant magical activity. At Hogwarts, the introduction of a more elaborate plumbing system in the eighteenth century was a notable development. Before this, wizards were less concerned with sanitation and simply used magic to vanish their waste. The new plumbing was installed, in part, to be more discreet around Muggle observers of the castle. This modernization had a significant, unintended consequence: a descendant of Salazar Slytherin, Corvus Gaunt, took advantage of the construction to incorporate the hidden entrance to the Chamber of Secrets into the pipework of a new girls' lavatory. (Pottermore) Lavatories within the wizarding world can be enchanted for various purposes. The entrance to the Ministry of Magic for most employees involves flushing oneself down a toilet in a public lavatory in London, and the Prefects' Lavatory at Hogwarts features an enchanted, swimming pool-sized bathtub.
Despite their mundane function, lavatories serve as the backdrop for several pivotal events in the series. In their first year, Harry Potter, Ron Weasley, and Hermione Granger battle a Mountain Troll in a girls' lavatory, an event that solidifies their friendship. In their second year, the same lavatory becomes their base for brewing the Polyjuice Potion in an attempt to uncover the Heir of Slytherin. This lavatory is also the location of the secret entrance to the Chamber of Secrets. In Harry's sixth year, he and Draco Malfoy engage in a violent duel in this lavatory, which culminates in Harry grievously injuring Malfoy with the Sectumsempra curse. Albus Dumbledore also mentions that he first discovered the Room of Requirement one morning when he was in desperate need of a lavatory, and the room appeared for him filled with a collection of chamber pots. The lavatories at the Ministry of Magic serve as a clandestine entrance, which the trio uses during their infiltration of the Ministry in their search for Salazar Slytherin's Locket.