A giant "porous cage" is to be built around a crumbling architectural masterpiece while conservationists work to protect it in a £4m project.
The Hill House in Helensburgh, Argyll, was built as a "home for the future" by Charles Rennie Mackintosh between 1902 and 1904.
His choice of Portland cement for the render led to problems as it has allowed water to soak in for more than a century.
Decades of driving west coast rain have saturated the walls, threatening the long-term survival of the property.
It features bespoke interior designs that Mackintosh and his wife Margaret MacDonald created for his client, publisher Walter Blackie.
Famous visitors to the building include Brad Pitt and Angelina Jolie, who saw Hill House privately in August 2011 while Pitt was shooting the film World War Z.
Hill House owner the National Trust for Scotland (NTS) now plans to build a huge transparent cage, designed by architects Carmody Groarke, around the building to keep the elements out while conservationists come up with ways to protect it for the long term.
Simon Skinner, NTS chief executive, said: "As our president, Neil Oliver, put it, the Hill House is in danger of 'dissolving like an aspirin in a glass of water'.
"We are building what amounts to a shield around and above the Hill House to keep wind and rain out, and give the building a chance to dry.
"The structure is effectively a porous cage, albeit a beautifully designed one, that still allows some movement of air and a degree of moisture penetration - this is essential to ensure the walls do not dry out too quickly and crumble as a result."
He added: "While the Hill House is being protected from the elements, our conservation and architectural heritage teams can start work to find solutions that will respect the historic and design integrity of the building, meet the standards and obligations required by its listed status and ensure that this precious place will survive to inspire future generations.
"The temporary enclosure is see-through, which means that the building will still be visible from the outside, despite its respite from the elements after a century of being drenched."
'The National Trust for Scotland are adopting a very bold approach to the conservation of the Hill House.'Andy Groarke, architect
The enclosure is expected to go up in 2018 and could be in place for a number of years.
NTS will launch one of the biggest fundraising drives in its history early in the new year to raise money for the £4m project.
Born in 1868, Mackintosh trained as an architect and went on to create much admired buildings including the Glasgow School of Art and Scotland Street School in Glasgow.
The building will remain open to the public while conservationists are at work, and visitors will be able to climb stairs and gangways to gain a bird's eye view of Mackintosh's masterpiece and watch the restoration work as it progresses.
Andy Groarke, of architects Carmody Groarke, said: "The National Trust for Scotland are adopting a very bold approach to the conservation of the Hill House, one that is radical and experimental in seeking new methods to extend the lifespan of our heritage, and one that invites public interaction and interpretation of these processes."