A Elvaston Castle needs nearly £6.5m in repairs to secure its future, according to a new report.
Elvaston Castle was acquired by the county council in 1969 but is now only rarely open to the public.
A survey by building specialists, funded by the council and English Heritage, said the roof of the main building was in a poor condition.
A council plan for a hotel and golf course is being opposed by groups who claim the house should remain public.
The castle – mainly a 19th Century country house – stands in more than 200 acres of parkland and gardens and attracts about 300,000 visitors a year.
The survey also covered the grounds and found lakes were silting, unstable walls and general poor landscape conditions due to erosion and vandalism.
It is some 15 years since the council first mooted the sale of Elvaston Castle and 11 years since they had it on the market. 11 years later and still nothing has been signed by anyone and the repair bill continues to rise each year.