The giant box was constructed around the property in 2019 to protect its saturated walls from the elements and help it to dry out as part of a long-term conservation programme.

The NTS said new funding from the National Lottery Heritage Fund will now enable it to fully remove the existing render from the building, which was completed in 1904, and find a replacement that will resolve a long-standing damp issue.

The conservation charity added that once the metal structure is removed in 2028 it plans to add updated permanent visitor facilities to the site – including a cafe, a shop and toilets at the site on Upper Colquhoun Street in Helensburgh.

Philip Long OBE, the Trust’s chief executive, said: “As the charity privileged to care for some of the finest remaining examples of the couple’s work, we strongly feel their contribution should be better understood and celebrated. With the Hill House and the Mackintosh at the Willow at the centre of all our work, we will engage with more people in person and online to tell the story of their creation and the inspiring people involved in those visionary ventures.

“And as a conservation charity, we will protect both buildings to be enjoyed and experienced long into the future. At the Hill House, this means continuing our work to solve its century-long issue with damp, both preserving this masterpiece and pioneering methods that add to the global conservation sector’s knowledge of how to care for and repair 20th-century buildings, and helping train new craftspeople who can carry out work like this in the future.

“At the Mackintosh at the Willow, that means ensuring that this excellent experience thrives under the Trust’s care and continues to grow and contribute to a much-needed revitalisation of Sauchiehall Street and Glasgow’s heritage as a whole. Just a year ago, the future of the tearoom was in doubt. This new funding means the future looks bright, both for the Mackintosh at the Willow and for the appreciation of Mackintosh and Macdonald and their unique take on, and place in, nature, beauty and heritage.”

Caroline Clark, The National Lottery Heritage Fund director for Scotland, said: “Conserving the architectural masterpiece of the Hill House and enabling the National Trust for Scotland to tell the story of Charles Rennie Mackintosh and Margaret Macdonald’s creative collaboration is hugely important. Thanks to National Lottery players, the Hill House and the stunning Mackintosh at the Willow on Sauchiehall Street will enable visitors to explore the work of these two artistic virtuosos whose influence stretches around the world.”