(This article is humorous, not factual.)
The answer can be found in the revised standard edition of Hogwarts, A History. I have manage with great difficulty to tear out the page to share with you… just don’t tell Madam Pince. You heard what she did to those second years last week? They’re terrified to be on the same floor as the library now, let alone in it.
HOGWARTS, A HISTORY (PAGE 729)
THE ADVENT OF MODERN PLUMBING
There is more to Hogwarts than meets the eye dear readers. The castle is a magical wonder of the wizarding world. Who knows exactly how the four Founders constructed it? There is some light to be shed on this topic however so let us tell you about it. (Tap the image with your wand for a five minute photograph lecture.)
[THERE WERE SOME TECHNICAL DIFFICULTIES GETTING THE IMAGE DIGITAL SO I’VE TRANSCRIBED THE LECTURE BELOW]
Each of the founders had their own specialties. It was Salazar Slytherin who preferred cleanliness above all the rest. He could not abide by filth, which would lead one to wonder why he chose the dungeons as his domain (more on this in another photograph lecture.)
When Slytherin lived he worked hard to create new ways to stay clean. These ways have gradually been adopted by wizards and muggles alike over the centuries and been adapted and improved upon to be the modern plumbing we all take for granted today.
The first thing that Slytherin did was create a way for waste to move out of the castle. With the help of 780 Hogwarts House Elves they created drainage and a sewer system.
When Hopjinx, the Head House Elf, came up with the idea to bring fresh water into Hogwarts by similar methods, Slytherin was quick to pick up the plan. It took more finagling than the waste system, but they did it.
Over the years, as the magic involved evolved and expanded to include modern toilets and showers, the rooms of the castle devoted to chamber pots and baths were converted by crews of House Elves and overseen by Headmasters.
There are three bathrooms in Hogwarts that surpass all the rest. The first is the Headmaster’s bathroom with gold fixings that change to reflect the Headmaster’s Hogwarts house affiliation. Professor Dumbledore has opted to add a talking mirror of his creation that reflects on his appearance.
The second is the professor’s spa area. Each professor has added onto it. It currently operates under five enlarging spells and features a steam room and sauna. Professor Snape has added a detox chamber for pesky potion removal and Professor McGonagall has added a small space for automated pet grooming.
The last bathroom of note is the Prefect’s Bathroom, which can also be accessed by the Head Girl and Head Boy. It sports a large swimming pool size bathtub with over 400 kinds of bubblebath. Readers beware, the ghost of Moaning Myrtle particularly likes to hang out here in the stalls when she thinks no one is watching.
If modern plumbing is not your thing, there is still one room in Hogwarts that can scrounge you up a chamber pot if you know where to look.
Find answers to other Unanswered Harry Potter Questions