It may have been a little optimistic of Roald Dahl's relatives to expect an outbreak of public philanthropy when they launched an appeal on Tuesday to raise £500,000 to renovate the contents of the author's dilapidated Buckinghamshire garden shed, when the books he wrote there continue to sell at the rate of 12 a minute every day of the year.

Instead, the Save the Hut campaign, launched on what would have been the author's 95th birthday, was immediately greeted on the internet with snorts of derision – and disbelief that the family, which has benefited from the sale of 100m books, were not meeting the full cost themselves. There was also incredulity that removing the hut's contents and preserving them in the local museum dedicated to Dahl's works would cost so much.

One person wrote on Twitter: "I love Roald Dahl but half a million quid to relocate a shed? Really? And the continuing royalties won't cover that anyway?" On the Daily Telegraph website, commenters called the Dahl family "stingy" and "greedy" and said: "Let them restore their own bloody shed." The author's granddaughter, the model Sophie Dahl, who said the family wanted to share his "palpable magic and limitless imagination", found herself described as the Big Stingy Giant.

Amanda Conquy, chairman of the Roald Dahl Museum in Great Missenden, Bucks, and director of the author's literary estate, said: "Some people are asking me questions which are not centred on the real good news, that the contents of the hut are going to be preserved and people will be able to see them."

The hut's contents include: the armchair in which Dahl sat as he wrote books such as Charlie and the Chocolate Factory, Matilda, James and the Giant Peach and The BFG; the yellow legal pads imported from the US on which he wrote; his extracted hip bone; and a large ball of foil wrappers, evidence of the boiled sweets he sucked for inspiration.

They will be reassembled at the museum precisely as they were left in the hut when Dahl died in 1990, though the building itself, built in the late 1950s by his friend and snooker partner Wally Saunders – who was also the model for the BFG – will now be left to fall down. Built of a single layer of bricks, insulated by polystyrene blocks, it is thought unlikely to survive another winter.

In a statement, the campaign said: "Ramshackle and dangerous perhaps but there is no question that Roald Dahl's writing hut is a unique cultural icon, of both national and international importance.

"Roald Dahl was a highly disciplined writer and ensconced himself in the hut … every day for 30 years.

"During his lifetime he was the sole person to go in and out of the hut [and] it has remained a private place entered only by friends, family and visitors … Now, that is poised to change and the public at large will be able to experience its magic."

Conquy added: "It seems like a lot of money but filleting and removing the contents and renovating them to put on display is almost a forensic exercise.

"I know it's only a small room but, to put it in context, recreating Francis Bacon's studio in Dublin cost £4m."

She insisted the Dahl family had already contributed to the project and fundraising was aimed mainly at educational trusts rather than individual donors.

Dahl's literary estate, of which she is one of two directors, had a turn-over of £938,562 last year and an operating profit of £299,985. The estate gives 10% of the profits, after expenses, to the museum and to Dahl's children's charity.