The collection of sheds and sound stages is where the eight films were shot over the course of a decade, and soon they will be home to the official "Making of Harry Potter" studio tour.
With more than five months to go until the tour's March 31 opening - advance tickets go on sale Thursday - stonemasons in hard hats are busy laying the (real) flagstone floor of the Great Hall at Hogwarts School of Witchcraft and Wizardry. Even half-finished, its Gothic arches, gargoyles and huge fireplace are an impressive sight. When it's completed, studio Warner Bros. hopes it will be, well, magic.
Movies are all about illusion, but creators of this tour are keen to stress its authenticity. The 150,000-square-foot (14,000-square-meter) site will include only authentic sets, props and costumes, on the original studio site 20 miles (30 kilometers) northwest of London.
For the movies' cast, who spent a decade working here - the younger ones growing up on set - it can still evoke powerful feelings of nostalgia.
"I get shudders down my spine every time I walk back in there," said Tom Felton, the 24-year-old actor who played Harry's Muggle-hating Hogwarts rival, Draco Malfoy. "Immediately, as soon as you go back it just fires up a decade's worth of memories.
"I remember the first time I went in there - it was on camera. (Director) Chris Columbus specifically didn't want us to see it before filming, because we were only 11-year-old kids. So, our reaction when we walked in there was pretty much genuine."
The vast Great Hall, where hundreds of Hogwarts pupils dined, celebrated, and were divided into houses by the mysterious Sorting Hat, will be the centerpiece of the tour, but there will be plenty more to delight Potter fans.
Re-erected sets will include the cupboard under the stairs where Harry was forced to sleep by his miserly relatives, the Dursleys; the imposing Ministry of Magic; headmaster Albus Dumbledore's book-lined office; and Hogwarts' classrooms, common room and a dormitory,
The tour is spread across two soundstages - stages J and K, a pleasing but accidental tribute to Harry's creator, J.K. Rowling. The existing stages here at Leavesden Studios are A through I.
As well as the sets, visitors will learn how the series' magical creatures were created in the studios' workshops, and see some of the 200 shipping containers full of props that producers have kept from the films.
The eight Potter films made here between 2000 and 2010 were a mini-industry in themselves, employing both the cream of Britain's acting talent and hundreds of craftspeople and technicians. Part of the tour's aim is to show off the behind-the-scenes skill that went into creating the spectacle.
The level of detail is impressive. Dumbledore's bookshelves are lined with individually titled books. His desk drawer opens to reveal quill-written letters and parchments that no moviegoer would ever have seen. The Weasley family kitchen will include a self-washing frying pan, enchanted knitting needles and other ingenious supernatural gadgets.
"The attention to detail and the care and the thought is breathtaking, and still is to us, even after eight films," said actor Mark Williams who played Arthur Weasley, father of Harry's best friend Ron. "You'd go on set and go, 'Bloody hell, it works!'
"I think people will be amazed about what was created as a physical prop rather than fixed later in the computer," added Warwick Davis, who played Hogwarts charms master Prof. Filius Flitwick and the goblin Griphook.
"Certainly for me, the filming experience on these was quite different to the work I'd done on 'Star Wars,' in the sense that stuff was here and real," said Davis, who appeared in both "Return of the Jedi" and "The Phantom Menace."
"George Lucas would've built the first six feet of wall and left the rest to the computer."
Filming on the final Potter movie, "Harry Potter and the Deathly Hallows: Part 2," finished last year, and it was released in July, to a global wave of emotion from fans. The studio tour is a way to keep the Harry Potter machine running - but to be a success, it must avoid feeling like a cynical cash-in.
"I hope people will come on the sets and feel the warmth on the sets, and the experiences that have been here," said Bonnie Wright, who played Ginny Weasley in the films. "They're really lived in, all the sets. They don't feel just like a studio, they do feel like a world."
It will also be a working movie studio. The facility - for years a ramshackle collection of aging buildings and temporary structures on the site of a former aircraft factory - is being turned into Warner Bros' British base. The company says it will be the biggest studio complex in Europe when it opens next year.
Many people feared the end of the Potter series would bring job losses in Britain's movie industry, but Warner Bros.' investment - which will make it the only U.S. studio with a permanent base in Britain - should bring a big boost.
"It's lovely to see the redevelopment," Davis said. "I just wish they'd done it before we filmed them. We spent years here in the damp and cold, and now I see these beautiful studios, with roofs."
Felton says he hopes to return one day to shoot a new film here.
"And if the work dries up," he said, "we can always come back and be tour guides."