Rennes, France | The ad on the French finance ministry website is deadpan and succinct: "For sale: late 19th century prison in the centre of Grasse, 1,277.42 square metres (13,750 square feet). No garage or parking space. Needs renovating." 

Prospective buyers of the site in southeast France are required to lodge a deposit of 50,000 euros ($56,000).

There are around 10 such notices for defunct prisons on the website, testament to a growing trend that has seen many urban jails turned into social housing or concert halls.

Since many are protected buildings they cannot be demolished. But this means those that find no buyer are left to rot.

Reprimanded at the European level for its overpopulated and dilapidated jails, over the past 15 years France has been busy modernising its prison stock.

This process has been given added urgency by growing fears about the spread of radical Islam among the prison population after a string of attacks in the last two years.

In early September, Prime Minister Manuel Valls said France needed room for 10,000 more prisoners over the next 10 years.

"Generally, it's cheaper to build a new prison" than to renovate one, explained Laurent Vilbert, who is in charge of urban planning in the western city of Nantes.

The city recently bought the local downtown jail -- which fell short of European standards -- for 4.9 million euros ($5.5 million). By 2018, the complex will become social housing, complete with a creche and underground parking.

"Local authorities have the right of first refusal for property on their land. In such cases sales take place by mutual consent," according to the finance ministry's website.

A prison in the Parisian suburb of Coulommiers, a stone building built in 1851, was bought by the municipal government for 2.3 million euros ($2.6 million) and turned into a library. Its cells are now lined with books.

Guingamp, a commune in Brittany, bought its prison back in 1992 and this month began turning the building into a visual arts centre with its central courtyard transformed into a 160-seat performance space. 

France is not the only country to sell off its disused prisons: one in Slovenia has been turned into a youth hostel; another in Oxford is now a luxury hotel.

And not all French towns can afford to develop old sites. In such cases, the private sector sometimes steps in. 

This happened in Fontainebleau, another Paris suburb, where a prison was sold at auction for 480,000 euros ($538,000) to a buyer who plans to turn it into housing. 

"We are really watching the developer. A whole section of the building is protected," said Pierre Tsiakkaros, chief of staff to the Fontainebleau mayor.

The ministry was quick to point out that the law is strict when it comes to protected buildings, noting that buyers of old prisons have to promise to respect rules about national heritage sites.

- From nick to nightclub -Although two 19th century prisons in the central city of Lyons have been converted and modernised into a vast university campus and housing, key features such as facades and a watchtower rotunda have been preserved.

Some prisons find no buyers at all, for reasons including contamination, low real estate prices and the sheer complexity renovation would entail.

One in the southeastern city of Avignon, covering 11,000 square metres (120,000 square feet) next to the medieval Palais des Papes and valued at 2.5 million euros ($2.8 million) for example, has been on the market since 2015.

The Marriott group once thought about building a luxury hotel there but abandoned the project when it realised how much work would be involved.

Avignon town hall, which owns the building, has put out a call for proposals with the idea of turning it into a space for artists, housing and shops.

Another Avignon prison, which dates back to the Middle Ages, is now a nightclub that is popular with students.