Kinloch Castle on the Isle of Rum - which Prince Charles wants to restore - needs new slate tiles, the removal and replacing of lead guttering and other work.
Owners Scottish Natural Heritage have advertised for tenders with a deadline of May 26 and said that “erection of a scaffold will be required.”
Known as The Forbidden Island because of its remoteness, Rum is a national nature reserve off the west coast and the largest of the Small Isles. It has a population of around 40 - most of whom work for SNH.
Ferries only run four times-a-week in winter, and in bad weather the island can be cut off for days on end. In good weather, in the summer, it is infamous for its fearsome biting midges.
Prince Charles previously led attempts to restore the 118-year-old castle, which was once the pleasure palace of a couple whose affairs rocked Edwardian society - including one that involved a fling with the prince’s great, great grandfather Edward V11.
SNH says it has invested more than £1m in the castle over the past few years, including repairing the castle’s turrets last year, but say it has not been easy to find the many millions needed to fully restore the Edwardian masterpiece.
Islanders also received a £700,000 Big Lottery Fund grant in 2013 to help build a new bunkhouse which recently opened.
Kinloch Castle was built in 1897 by the wealthy Lancastrian industrialist, George Bullough. Around 10,000 people visit Kinloch Castle every year.
After falling into disrepair, Kinloch was highlighted on the BBC2 Restoration series in 2003 but despite attracting 143,000 votes, it failed to win the £3m prize towards the cost of restoring it.
But it caught Prince Charles’s interest and following the programme he called a meeting at Birkhall, his Deeside retreat, of interested parties, to investigate a possible restoration package.
He was captivated by the history, architecture and location of Kinloch.
Lancastrian industrialist Sir George Bullough and his wife Lady Monica - a beauty who had many affairs among London society, and was Edward V11’s lover - turned Kinloch Castle into a centre for extravagance and partying.
Kinloch cost pounds 250,000 - equivalent to pounds 15 million today - to build for Sir George.
The castle became the first home in Scotland to have an internal telephone system - there was even a direct line to Newmarket so Sir George, who died on a golfing holiday in France in 1939, could monitor his horses - as the Bulloughs created Belgravia in the Hebrides.
Lady Monica, who last visited Rum aged 85, bequeathed in her will that the castle’s collection should be left in place - except for restoration. She is interred beside her husband at the family mausoleum on the island.