More than 11 million homes sit empty across Europe, enough to shelter the entire continent's homeless population twice over.
More than two million homes are empty in each of France and Italy, 1.8 million in Germany and more than 700,000 in Britain. Most of Europe's empty homes, 3.4 million, lie in Spain, which experienced a big construction boom in the mid-2000s, fed largely by Britons and Germans buying homes in the sun.
There are also a large numbers of vacant homes in Ireland, Greece, Portugal and several other countries, according to information collected by The Guardian.
Many of the homes were built in holiday resorts constructed during the housing boom in the run up to the 2007 financial crisis and have never been occupied.
On top of the 11 million empty homes, many of which were bought as investments by people who never intended to live in them, hundreds of thousands of half-built homes have been bulldozed to shore up the prices of existing properties.
The European Union says there are 4.1 million homeless people across Europe.
"It's incredible. It's a massive number," says David Ireland, chief executive of the Empty Homes charity, which campaigns for vacant homes to be made available for those who need housing. "It will be shocking to ordinary people. Homes are built for people to live in. If they're not being lived in then something has gone seriously wrong with the housing market."
Ireland says that policymakers need to tackle the issue of wealthy buyers using houses as investments instead of homes.
Europe's 11 million empty homes might not be in the right places, "but there is enough [vacant housing] to meet the problem of homelessness", he says. Freek Spinnewijn, director of Feantsa, the European Federation of Organisations working with the Homeless, says it was a scandal that so many homes have been allowed to remain vacant. "Governments should do as much as possible to put empty homes on the market," he says. "The problem of homelessness is getting worse across the whole of the European Union."
Last month, members of the European Parliament passed a resolution demanding that the European Commission "develop an EU homelessness strategy without any further delay".
Gavin Smart, director of policy at the Chartered Institute of Housing, a British professional organisation for those working in the housing industry, says that many of the empty homes had probably fallen into disrepair in regions that lack jobs. But many structures could be easily brought back to the market.
He says Europe suffers from a growing problem of rich investors "buying to leave" buildings empty, hoping to profit from rising property prices.
The prices of prime London property, defined as homes that cost more than £1,000 (HK$12,910) per square foot, are now 27 per cent above their 2007 peak, according to estate agent Savills. Smart says there is growing evidence of the practice in "rich parts of London, other areas of the country ... probably all over Europe".
In Spain, the results from the latest census, published last year, indicated that more than 14 per cent of all properties were vacant. The number of empty homes had risen by more than 10 per cent over the past decade.
The Spanish government estimates that an additional 500,000 partially built homes have been abandoned by construction companies across the country. During the housing boom, which saw prices rise by 44 per cent between 2004 and 2008, Spanish builders raised new homes at a rate of more than 800,000 a year.
In some tourist resorts, more than a third of homes are still empty five years after the peak of the financial crisis.
The Spanish census suggests that more than 7,000 of the 20,000 homes in Torre-Pacheco, a holiday region between Murcia and the coast, are empty.
The area has undergone a holiday home construction boom with several new golf holiday resorts, including a 2,648-apartment complex called Polaris World, which opened as the crisis struck. Apartment owners in the Polaris World resort, with a golf course designed by Jack Nicklaus, are struggling to sell homes for half the €200,000 (HK$2.1 million) they paid before the crisis.
More than 18 per cent of homes in Galicia, on Spain's northwest coast, and La Rioja, near Pamplona, are vacant.
Many of the empty Spanish properties were repossessed by banks after owners defaulted on mortgages. "Spain is suffering from high numbers of repossessions and evictions, so we have reached a point where we have too many people without a home and many homes without people," says Maria Jose Aldanas of Spanish housing and homelessness association Provivienda.
Some city councils in Catalonia have threatened banks with fines of up to €100,000 if homes they repossess remain empty for more than two years. The city council of Terrassa, to the north of Barcelona, has reportedly written to banks holding more than 5,000 homes demanding they take "all possible actions to find tenants" or relinquish the homes to the council to use for social housing.
France's National Institute for Statistics and Economic Studies says 2.4 million homes were empty in 2012, up from two million in 2009.
Italy will release figures for the number of empty properties in the country's census, published this summer. A survey by the Italian statistics institute estimated there were 2.7 million in 2011, and a 2012 report by the Italian General Confederation of Labour, a national trade union, estimated two million.
In Britain, more than 700,000 homes are empty, according to data collated by the Empty Homes campaign. Campbell Robb of Shelter, Britain's biggest homelessness charity, says the government needs to come up with ideas to tackle the lack of available, affordable homes.
In Portugal, there are 735,000 vacant properties, a 35 per cent increase since 2001, according to the 2011 census. An estimated 300,000 lie empty in Greece. Ireland has 400,000 empty homes.
The Irish government has begun demolishing 40 empty housing estates built during the boom. It is studying what to do with another 1,300 unfinished developments. Deutsche Bank said it would take 43 years to fill the oversupply of empty homes in Ireland at the current low population growth rate.