The National Trust for Scotland says it needs to raise £2.5million to protect one of Charles Rennie Mackintosh's finest buildings from the elements.
Hill House, near Helensburgh, remains under threat from longstanding issues from "excessive water ingress" according to the conservation body.
The house, built for publisher Walter Blackie in 1902, is regarded as the architect's most accomplished domestic endeavour.
The trust, which has cared for the building for more than 30 years, said Mackintosh made use of "innovative" building techniques to create the beautiful, minimalist exterior design.
Because as he did not use traditional weathering material, over the years damps has caused damage to the 113-year-old structure.
Director of Conservation Terry Levinthal said the trust had agreed a strategy for repairing the building, admittedly at a "significant cost."
He is now launching formal fundraising plans and is asking for wealthy donors to step forward and help.
Mr Levinthal said: "The Hill House is clearly a place of the highest cultural significance, and must be well looked after.
"Caring for our heritage can be very complex and this project is a prime example of that.
"The trust has been exploring how best to resolve the longstanding issues of water ingress. We have now concluded what our repair strategy is going to be, after considerable research investigation and consultation with stakeholders.
"This is time-consuming and, of course, the costs are significant too.
"We are in the early stages of fundraising in support of this major project to protect the Hill House, Mackintosh's finest example of domestic design, for future generations."
Among the building's many international admirers is actor Brad Pitt who visited the site four years ago during the filming of World Ward Z.
The star later agreed to become patron of the appeal to restore Mackintosh's iconic Glasgow School of Art building following last year's blaze.
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