this site and you will find listings for government buildings in Paris that can be yours for .....well I won't say a song but they could be yours all the same. Among the most controversial of these is the Hôtel de la Marine on Place de la Concorde, the current headquarters of the French navy and twin to the Hôtel de Coislin, best known as home to the luxurious five star Hôtel de Crillon and the Automobile Club de France. After 220 years in residence, the navy is moving out, headed for what's being dubbed the French Pentagon near metro Balard in the 15th, and they hope to finance the move by selling the current building. But what to do with an 18th century architectural jewel with 215,000 square feet and over 500 rooms? It's certainly not clear. When Le Figaro posed this question to 14 luminaries recently, the answers ranged from "make it into a museum" to "make into anything but a museum." If you don't want to enter the fray, there's also a nice little hotel particulier in the 16th, a spot under the arcades in rue des Pyramides in the 1st, and a chateau or two in the provinces.
You can read more about the history of the Hôtel de la Marine and the contoversy in The New York Times