Thieves have stolen up to £10,000 worth of lead from Seaview House in Monifieth.
Police are investigating the theft after owners reported a break-in.
The raiders used a vehicle to transport their haul.
The A-listed building, which was once used as a primary school, is one of the town’s most prominent Victorian-era mansions.
It was purchased from Angus Council by developers Tayfield Investments with a view to putting the mansion back into use.
Steve Cummings, the company’s director, said the theft had dealt a serious blow to the building’s restoration.
“I just can’t believe the audacity of these people,” he said. “This is a secured property but they came in with a vehicle, broke into the estate and proceeded to move between £5,000 and £10,000 worth of lead from the house.
“What worries me more than anything is they will have had to do this in broad daylight with ladders, because some sections of the roof they were stripping are very perilous.”
Police confirmed that inquiries were continuing.
Mr Cummings said: “We’re now in the process of installing CCTV cameras around the property, along with other security measures,” he said.
“But this will set back our remedial works substantially, and will cost a lot of money to repair.”
Mr Cummings’ firm is developing four new homes in the mansion’s two-acre property.
However, he is also keen on transforming Seaview House itself back into a liveable property.
Built in 1860, the grand mansion was constructed by James Fairweather Low, whose wealth stemmed from a number of industrial ventures including Monifieth’s first iron foundry.
In 1928, Low’s family donated Seaview House to Angus Council for use as a primary school.
Mr Cummings has asked all homeowners in the area to be wary of similar thefts.