This is the second in a series of stories on global property that examines the shifts and trends in the housing market on the international stage.

When the fictional Granthams of television's Downton Abbey travel to London, they stay at their palatial town house in Belgravia. For centuries Britain's actual landed gentry have made their city homes in the Georgian mansions of Mayfair.

When they weren't there, the properties stood empty, often for more than half the year.

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Today, though, the growing problem of absentee owners has swelled far beyond Knightsbridge, Mayfair and Chelsea into London's far-flung corners. The difference now is that billionaires with countryseats in China, Russia or the Middle East are the ones buying up London homes.

They see the city as a safe place to park money. On the other hand, Londoners who are dealing with an acute housing crisis see buying homes as an investment and leaving them to stand empty as an obscene luxury. London needs close to 50,000 new homes annually to meet this demand and its growing population. Roughly 26,000 are actually being built.

The buy-to-leave trend is fuelling debate ahead of Britain's May 7 general election, as some politicians propose to tack the gusts of wealth that by last March had pumped up house prices 24.8 per cent above their peak before the downturn.

It's "a bubble that's fuelled by an almost infinite supply of desperate global capital at the moment," said Peter Rees, who was the City of London's city planning officer for nearly 30 years before becoming a professor of city planning at University College London in 2014. Working in London's financial hub after the economic crisis, he witnessed international investors looking for refuge by buying up property in the stable world capital.

"Because of the inability to meet the demand, that money is spreading out further and further," he continued. This investment wave is tracking across prime to mainstream property in London, Mr. Rees said.

People tend to think about the absentee owners of £50-million ($91-million Canadian) flats at One Hyde Park, he said. "That almost doesn't matter," he continued. "It's what that represents. In large areas throughout the South Bank and along the river in Aldgate, or up toward Dalston," he said, "we're seeing developments going up which are heavily underoccupied."

Mr. Rees has seen it in his own building, The Heron – a 36-storey condo complex on the edge of the City of London where penthouses go for £18-million. He and other residents "tried to form a residents association," he said, but couldn't get in touch with enough other owners.

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"After 12 months, 26 of them hadn't even collected their keys," he said, pointing out, "it's very difficult to see" how these properties will be managed long-term.

Demand in prime London has amplified prices and rippled outward as investors see opportunity further and further afield. Will Martindale, a Labour candidate in south London, is an eyewitness to the buy-to-leave phenomenon in his community. "Rents are up 15 to 20 per cent in the area. And the average house price is rising £100,000 a year," he said.

"The riverfront at times can be eerily quiet. If you look at some of these buildings and the number of lights on, there's no one home," he said. "It creates perverse incentive on the market," he went on, stating the government needs "to be a little more imaginative about the levels of council tax paid by those that leave their properties empty."

London's supply and demand housing dilemma has become a major campaign issue taken up by Ed Miliband, leader of the Labour Party, who promises a new mansion tax on properties worth more than £2-million if he's elected prime minister. It's a way, he says, to help boroughs pay for more affordable homes, which are now in short supply.

He wouldn't be the first who has tried to put the brakes on the market. Under the Conservatives, Britain's 2014 budget activated a 15-per-cent stamp duty on those using a company name to buy properties worth more than £500,000. A new tax raised by Chancellor of the Exchequer, George Osborne, which went into effect on April 6 will charge foreign investors 28 per cent of the value their property gains from that date forward to the time they sell.

In Camden, north London, a surcharge of 50 per cent on local council tax on homes left empty for more than two years has led to a 40-per-cent reduction in the number of empty properties there. The surcharge is a tool available to boroughs across the city, but few have taken it up.

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Tessa Jowell, a former MP who aims to be Labour's candidate in London's 2016 mayoral race, and who led the capital's bid to stage the 2012 Olympics, believes local authorities should be able to charge unlimited penalties to homeowners who leave properties empty for more than six months. London Mayor Boris Johnson has alternately said it's "utterly nuts" to deter foreign investors, and that they should be charged taxes 10 times the current rates.

Other politicians have proposed that foreign investors who leave their properties empty for more than six months should have them seized by town halls and rented out.

But targeting wealthy foreign buyers is an ineffective solution, charges Adam Challis, a residential property analyst with consultants Jones Lang LaSalle. "I think what they're going to find," he said of these politicians, "is they set up a fairly expensive program, which would be incredibly difficult to make work in practice, and find that they raise very little revenue because there is very little of the activity that is genuinely taking place."

When it comes to international investors, "the simple reality is the vast majority, our surveys show between 85 to 92 per cent of them, over the last couple of years, have been investors seeking a renter," Mr. Challis said.

To which Prof. Rees says, "That's exactly the sort of dumb answer you would expect from an estate agent. They just asked them: 'Are you thinking of it?' Of course they're thinking of it! It doesn't necessarily mean they're going to do it!"

When it comes to talking about empty properties in real numbers, however, it's hard to get a handle on something solid. Savills estate agents peg the amount of international money spent on high-end London residences at £7-billion in 2012, with two-thirds representing investors rather than owner-occupiers. Beyond numbers like these no one has exact figures.

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"Estate agents aren't going to want to come up with the figures because they're making a very good living on it," Prof. Rees said, but agrees that politicians' plans are ineffective.

"None of those people have mentioned planning," he said. "I think the only way to control this is through strengthening our planning system," he continued, suggesting that property zoning and tighter regulations are essential to putting the brakes on the London market.

"We're not talking about a normal market and normal people," Prof. Rees added, stating that marginal charges that double or increase council tax tenfold are neither here nor there to investors.

"These are people who stand to lose 100 per cent of their capital if they leave it at home," he said. "All they're interested in is that at least half their capital is safe."