Flames ripped through the 17th-century Hotel Lambert in the French capital on Wednesday.
The building was being renovated after its purchase by Prince Abdullah Bin Abdullah al-Thani, brother of the Emir of Qatar, in 2007.
Located on the World Heritage-listed Seine embankment, the mansion was once home to the 18th century philosopher Voltaire.
According to local media, it took six hours for firefighters to put out the blaze, which started in an area below the rooftop.
A large portion of the roof was destroyed along with a section of a central staircase. Some of the brickwork on the front of the building has also collapsed.
Renowned fresco ceiling paintings by Charles Le Brun in the Gallery of Hercules were also “severely damaged by smoke and water”, fire brigade officials were quoted as saying.
The mayor of Paris, Bertrand Delanoe, said the fire had shocked and saddened him.
Renovation plans by the Qatari owner have previously sparked concern from conservationists keen to preserve the historical architecture.
In 2010, the French culture ministry said the prince had signed a deal with a cultural association that had sought to stop his overhaul of the historic building on the Ile Saint-Louis in central Paris.
Under the compromise, the association withdrew its lawsuit and the prince was allowed to start renovation work.