To access the Hufflepuff Common Room, students must go via the Hogwarts kitchens, where they will find a ‘pile of large barrels.’ Of these, ‘The barrel two from the bottom, middle of the second row, will open if tapped in the rhythm of ‘Helga Hufflepuff’.’
Suitable for the incumbent Hufflepuff head of house during Harry’s time (Professor Pomona Sprout, Herbology teacher), it’s certainly the most green of the four house common rooms. According to J.K. Rowling, ‘various cactii stand on wooden circular shelves (curved to fit the walls), many of them waving and dancing at passers-by, while copper-bottomed plant holders dangling amid the ceiling cause tendrils of ferns and ivies to brush your hair as you pass under them.’
Unlike Harry in the books, you can actually explore the common room in the mobile game Harry Potter: Hogwarts Mystery, showcasing the squishy, comfy, ‘badger’s sett’ design notes that J.K. Rowling describes in her piece on the subject. Yellow is a great colourway for a common-room, FYI.
You can also get an idea of what this cosy common room looks like via Bloomsbury’s beautiful 20th anniversary house editions, which contain illustrations by Levi Pinfold. The Hufflepuff sketch showcases the porthole-type windows and the almost subterranean allure perfectly.
This, from J.K. Rowling herself, explains that although it didn’t crop up in the books themselves, the common room was still a fully imagined place for Hufflepuffs:
‘When I first planned the series, I expected Harry to visit all four house common rooms during his time at Hogwarts. There came a point when I realised that there was never going to be a valid reason to enter the Hufflepuff room. Nevertheless, it is quite as real to me as the other three, and I always knew exactly where those Hufflepuffs were going when they headed off towards the kitchens after lessons.’
J.K. Rowling, writing for Pottermore