Renoir’s once shocking La Loge is back from Belfast. Van Gogh’s show-stopping Self-portrait With Bandaged Ear has arrived from Amsterdam. Seurat’s Young Woman Powdering Herself from Edinburgh. A long white wall will soon have eight Cézannes, once art lovers in Bergen have finished admiring them.

“It is so thrilling. We’ve missed them,” said Ernst Vegelin van Claerbergen, the head of the Courtauld Gallery in London. “It is wonderful that other people have enjoyed them but it is time for them to come home.”

In November the gallery will reopen after a three-year closure. Vegelin gave the Guardian an early, exclusive tour of transformed gallery spaces which he believes will have visitors’ heads spinning.

The changes are enormous but the star of its redevelopment is the restoration of the Great Room, which will now display the Courtauld’s collection of impressionist art – one of the finest in the world.

Previously they hung on chains with picture lights in worn-out gallery spaces which had reproduction chandeliers hanging in them. “It was a 60- to 70-years-old approach to the collection,” said Vegelin. “It was oppressive and just not suitable for works of this calibre.”

The impressionists are gradually returning to the gallery to be hung in a space which was originally the Great Room of the Royal Academy in the late 18th and early 19th century but had been subdivided into four rooms. Back then, thousands of people would stream in to see new works by Gainsborough, Reynolds, Turner and Constable in the summer exhibition.

The newly refurbished but not yet finished Great Room, where the Courtauld’s collection of impressionist paintings will hang.

The redevelopment brings back the spectacular grandeur of the room and the impressionists will sing like never before, Vegelin believes. “They feel like new pictures to me. I had become so used to seeing them in that very historical setting downstairs. The experience of them was so conditioned by the architecture and the decorative setting, they felt old in a way, they felt like old masters. Here I think people are going to get a real charge from them.”

The Courtauld’s closure was planned to be two years but delays were caused by the pandemic and the discovery of a medieval cesspit in the basement. “Not just a small one … huge,” said Vegelin. “They dug and they dug and they dug. I think it was three or four metres deep, so obviously all the work stopped and that set us back.”

The transformation of the gallery is top to bottom and includes using backroom space. For example, what was once a toilet, security control room and store will now be the medieval and early renaissance gallery.

For the first time, the refurbishment allows wheelchair access to the main entrance, which has been made bigger to reduce congestion and queues that used to stretch to the branch of Greggs on the Strand outside.

New temporary exhibition spaces have been created and named after donor Denise Coates, the betting billionaire who founded Bet365 from her father’s Stoke-on-Trent bookmaking business.

The upside of the long closure was sending works around the world. The Van Gogh self-portrait is particularly fragile and would never normally travel but, rather than put it in storage, it was loaned to the Van Gogh museum in Amsterdam – “a one-off opportunity for it to live with its brothers and sisters”, said Vegelin.

There were tours of works which went to Tokyo and Paris. “It has been absolutely wonderful,” he added. “To eavesdrop on people as they stand in wonder in front of a painting like Manet’s A Bar at the Folies-Bergère, you just feel a great sense of pride.”

The Courtauld Connects programme sent works around the UK to places that, in the first half of the 20th century, helped generate the fortune of the textile industrialist Samuel Courtauld, allowing him to buy his collection. It included lending a Boudin to Preston, drawings to Coventry and Monet’s Antibes to Hull.

The redevelopment cost an estimated £57m but the gallery said that includes elements such as the loans programme, the temporary move of students to King’s Cross, digitisation and the creation of a learning centre.

Paintings and works of art are gradually returning before the grand reopening, which promises to be one of the UK’s cultural highlights of the year.“To have them now come home is really quite emotional,” said Vegelin.