Bathrooms at Hogwarts have water closets and porcelain faucets, which are relatively modern inventions, considering Hogwarts was founded over 1000 years ago. Are the toilets magical, and upgrade themselves to modern standards, or have they been renovated (by house-elves, I suppose)? How often do the renovations happen?
Modern-style toilets were only added to Hogwarts during the 18th century:
When first created, the Chamber was accessed through a concealed trapdoor and a series of magical tunnels. However, when Hogwarts’ plumbing became more elaborate in the eighteenth century (this was a rare instance of wizards copying Muggles, because hitherto they simply relieved themselves wherever they stood, and vanished the evidence), the entrance to the Chamber was threatened, being located on the site of a proposed bathroom. The presence in school at the time of a student called Corvinus Gaunt – direct descendant of Slytherin, and antecedent of Tom Riddle – explains how the simple trapdoor was secretly protected, so that those who knew how could still access the entrance to the Chamber even after newfangled plumbing had been placed on top of it.
(Pottermore, Chamber of Secrets)
Whoever did the upgrade is unknown.