By Jonny Dymond
Europe correspondent, BBC News
Protests about the housing situation are becoming common in Paris
It took six months for Liberation journalist Ondine Millot to get to the truth about the most sordid side of France's housing crisis.
Look through some property websites and you can see the advertisements: the phrase you are looking for is contre services - when a room in an apartment is offered, sometimes "free", in exchange for services.
Sometimes the service is perfectly innocent - cleaning the apartment or washing clothes, to defray some of the high cost of renting property.
But sometimes it is not: instead the requests are sexual, demeaning, bordering on the perverse. "Sex twice a month," is one blunt demand. Another asks for someone "open in spirit and elsewhere".
"Flat in exchange for libertine services," goes another.
Ondine Millot - who in her day job is the court correspondent for the French daily paper - spent six months researching these advertisements and the people who place them, for an article which exposed the trade in property-for-sex.
"I was very surprised to find out that this kind of thing was going on," she says.
"We called a lot of men, I made something like 50 phone calls. Most of the ads that were 'against services', where no amount was specified for the rent, were men that were looking for sex in exchange for housing."
But the problem is not just hidden away on websites.
Take a quick look through the bookshelves of any decent-sized newsagent, and tucked between the biographies of the former French First Lady and the former American First Lady is the extraordinary account of Laura D.
Her book, My Dear Studies (Mes Cheres Etudes), details the anonymous young woman's slide from being a fresh-faced undergraduate, to a poverty-stricken student, to a 19-year-old selling her body to pay the rent.
"Laura", when I meet her at her publisher's, is charming, if desperately concerned to keep her identity secret, to spare her parents the horror of knowing how their daughter fell.
Laura D's book has exposed the situation facing the young
But she is also angry. It was, she says, the astronomical cost of property that sent her on to the streets.
"Rent was over 70% of my budget," she says.
"Looking at friends, people I know, they live in places that are unhealthy, squalid. Or they negotiate with landlords who rent them rooms and who sometimes abuse them."
Sex for rent is the extreme end of an extreme problem which is catching swathes of France's most vulnerable people - the young and the poor - in its grip.
France, the government admits, is in the grip of its worst housing crisis since the end of World War II.
Of course, sky-high property prices are hardly exclusive to France.
But some combination of circumstances has left the French - and especially the Parisian - rental market horribly stretched between supply and demand.
And too many people caught between homelessness and bankruptcy as they struggle to put a roof over their heads.
There are a far higher proportion of properties empty in Paris than in London. As the number sleeping rough has declined in the British capital, it has climbed in the French capital.
In Britain the norm is owner-occupation. While in France rental is seen as the way to live.
The French Republic was built on street protest. So, unsurprisingly, a new generation of housing protest groups have sprung up to campaign in innovative ways.
Groups such as Jeudi Noir, Droit au Logement and Les Enfants de Don Quichote have grabbed publicity - and, they say, won political concessions - through their protests.
Jeudi Noir is the guerrilla wing of the movement, conducting sting operations on landlords looking for something extra on the side. They also view and film tiny apartments being advertised for extortionate rents.
The Ministry of Housing Crisis... a former bank in the centre of Paris
Les Enfants de Don Quichote left the French government twisting with embarrassment with a mass sleep out in Paris last year, as hundreds of activists and homeless people slept in tents by the Seine.
Droit au Logement, alongside Jeudi Noir, has occupied a former bank right opposite the old Paris stock exchange, just down the road from the Bank of France. It has rechristened the building The Ministry of Housing Crisis, and uses it as both an operational base and a squat.
The groups' actions keep the issue in the public eye.
They know that great stunts are not going to get the million homes that France needs built.
But they also believe that unless the pressure is kept up on politicians local and national, then change will never come.
Christine Boutin, the French Housing Minister, has acknowledged the efforts of the campaign groups and has begun looking at ways to improve the conditions of the rental market.
Crossing Continents on BBC Radio 4 reports on the housing situation in France on Thursday, 13 March at 1100 GMT.