At the house in the Brittany town of Pont-Aven the workers found a lead container that they initially took for a World War II artillery shell.
But then "they shook it and heard the sound of coins," the head of Bat'isol construction company, Laurent Le Bihan, told AFP at the weekend.
Inside, they found 600 Belgian gold coins dating to 1870 and stamped with the effigy of King Leopold II, who reigned from 1865 to 1909.
The value of the bounty, which the workers handed over to the police, is not yet known.
Under French law, the proceeds should be divided 50-50 between the finders and those who own the land where it was found.
Le Bihan said the owner of the house "was not surprised" by the find as his grandfather was a coin collector.
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