A new £20 million conservation studio - the largest in the National Trust’s history - has opened its doors at Knole near Sevenoaks to restore and conserve historic treasures.

The studio, which has been created on site inside a former medieval barn, will conserve many of the treasures housed at Knole and also take in works from other trust sites and other organisations.

As part of the unveiling, four of Knole’s important showrooms are being re-opened this weekend following completion of the first phase of building and restoration work at the house, which has included new heating systems and lighting.

They include the King’s Bedroom, with its suite of silver furniture and 17th century State Bed, and the Cartoon Gallery, with six full-height restored copies of working designs for tapestries by the Renaissance artist Raphael.

Since the National Trust took on the house in 1946, it has faced a battle with leaking roofs and windows, damp, moths and woodworm which have put its collection of furniture, paintings and textiles at risk.

The new conservation studio will rescue the house and its contents from centuries of decay, and upgrade, repair and re-present its grandest rooms and inside spaces that have never been seen before.

The collection includes pieces of rare Stuart furniture, some acquired from royal palaces by Charles Sackville, 6 Earl of Dorset when he was Lord Chamberlain in the 17th century.

A permanent team of specialist conservators will be based there year-round to clean, repair and treat objects, the first time this has happened.

Items being conserved include more than 40 portraits including Henry VIII and Elizabeth I, a famous 17th century Knole sofa, originally designed as a throne and still upholstered in its original red velvet, and the Royal "Stool of Easement", an early form of toilet from the French ‘lieux d’aisance’, thought to have been used by Charles II.

Dame Helen Ghosh, the National Trust’s director-general said: “Knole is one of the trust’s most important houses and this is an exciting moment where we can begin to open new spaces within this vast property to tell the story of its occupants and outstanding collections.

“The new conservation studio is a first-class space for our expert conservators to work on collections from Knole and across the trust, and share their expertise with our visitors."

The project has been partly funded with a Heritage Lottery Fund grant of £7.75 million.

Knole's general manager, Hannah Kay, added: "We are in the final stages of the biggest building and conservation project that Knole has witnessed in the last 400 years.

"It is an enormous but exciting challenge and we are thrilled that we can now share the next chapter in the story of this fascinating house with our visitors and supporters.

“The new conservation studio will allow us not only to care for our own collections but to take in work from other trust houses and external organisations."

The four completed showrooms open on Saturday, March 18. For details visit nationaltrust.org.uk/knole or call 01732 462100.