THE problem with today’s housing crisis, politically, is that it is just not all that visible. At the end of the Second World War, families with kids crammed into shared houses. In 1946, more than 46,000 families took over military camps. Later, they took over empty hotels and flats—culminating in a mass occupation of empty flats in places like Kensington and Pimlico. That was a proper housing crisis–one that politicians couldn't deny existed. Today's does not come close: most people still have somewhere to live. It is nonetheless real, and as we arguein this week's print edition, worthy of political attention. But the question of exactly why our expensive homes constitute a crisis is more nuanced than it can appear. Take the graph published alongside this piece. Looking at it, it would be easy to conclude that there is no housing shortage at all. Since 1973 at least, the total number of “dwellings” has climbed far faster than the population. How is this possible, given that building rates have collapsed? And doesn’t it mean that there isn’t really a housing crisis?

First, in the 1960s and 1970s, while councils happily knocked up new estates, they were also busy demolishing lots of old “slum” houses too. Social housing helped people move from crumbling old cramped houses into shiny new flats or houses, typically with much more space. Meanwhile, new homes were built on green fields. Average household sizes fell dramatically, even as the average house got bigger. Though the housing stock increased by less than the rate of building, everybody got a lot more space in which to live.

In recent decades, by contrast, everyone has got a lot less space. We have had relatively little new building, but somehow we have magically created lots of new housing. Go almost anywhere in London–especially in the outer-suburbs–and this conundrum is quickly explained: houses everywhere have been turned into flats. A really remarkable part of the increase in the dwelling stock has come not from building but rather from breaking up existing homes into more “dwellings”. Essentially, so far, the housing crisis has been solved by subdividing our homes to make way for the extra population growth. Meanwhile, what new homes we do build are the smallest in the developed world.

One interesting side effect of this has been that unoccupied or otherwise vacant houses have been brought back into use. The map below shows the change in the vacancy rate of homes between 2004 and 2012. In London, it is clear, the number of homes vacant at any one time has dramatically fallen. Houses and flats do not stay empty for long, because of the high cost of keeping them without tenants. In large parts of the north, by contrast, the rate has increased sharply—more than doubling in places.

All this is evidence of a growing squeeze on space, at least in London and the south east. And it is accelerating. Between 2001 and 2011, the average household size in London leapt, for the first time in a century. Landlords convert living rooms into bedrooms to house more graduates; adults live with their parents long after they should have left. Thus London’s population can grow by 80,000 each year, though just 20,000 new homes–mostly small flats–are built. People are squeezing into every available space, like relatives visiting at Christmas. In the poorest places–forgotten corners of Brent and Newham–new slums are being created as garden sheds and derelict buildings are (illegally) turned into new homes.

Some argue that the answer to this is regulation. The Greater London Authority has introduced space standards to increase the size of new homes, for example. Newham Council has tried to ban what it calls "Beds in Sheds". But these measures will be futile unless the cost of housing can fundamentally be reduced. As long as housing costs as much as it does, only the rich will be able to buy spacious new homes. The middle classes will squeeze in ever tighter, and far more of the poor will have to claim benefits to pay their rent.

So when architects such as Richard Rogers (who, incidentally, is wealthy enough to live in two houses in Chelsea knocked together) defend the green belt by arguing that London’s density is its great strength, they only consider one half of the ledger. Density makes London’s transport systems work better; it ensures that there is always somewhere good to eat within a five minute walk. But it comes at a cost: that people are squashed in ever more tightly. For all of their faults, the planners of the 1960s and 1970s understood at least that whatever the benefits of a busy city, above all what people want is space.

Note: This piece was edited to include the map of vacant homes.