When a Normandy homeowner told the workmen extending her property to "let me know if you find any treasure", she was only joking.

The three builders, however, had a secret. Unbeknown to their employer, who lives in the Eure department, they had indeed found a cache of gold ingots and coins buried in her garden … and allegedly sold the treasure trove for €900,000 (£720,000).

Police have now charged the workmen with theft and a specialist collector of money with receiving stolen goods.

"It's an unusual and rare story," a police source told Paris Normandie newspaper. "While they were working in the grounds of the house building an extension, the bucket of their small digger hit something hard."

On closer inspection, the men found 16 golden ingots weighting 1kg each and 600 gold $20 coins weighing 33g each, dating from 1924-27, that appeared to have lain hidden in glass jars buried in the ground since the second world war.

Police claim that the builders, aged 20, 33 and 40, decided to sell the treasure and split the proceeds. Detectives were alerted when one of the men, according to reports, made two large deposits to his bank account of €270,000 and €30,000, ringing alarm bells with the finance ministry's Tracfin, which investigates money laundering.

Questioned by detectives, the man and his two colleagues admitted finding and selling the gold. The collector, however, denies receiving stolen goods. Police seized a number of vehicles, including a Land Rover, and several hundred thousand euros.

The owner of the property, who was not named, told French journalists: "I remember saying to them as a joke 'If you find treasure, let me know!', but I had no idea …"

The defendants will appear in court in Évreux in December.