The British homes shortage is happening not primarily because of population growth - that is low, though immigration is a factor, particularly in London - but because of changes in the way we live. Households are splitting up more than they used to - a family with divorced or separated parents generally means two homes capable of accommodating the children rather than one. Despite punitive house prices, it is still the case that children leave home and set up their own households - renting rather than buying, increasingly - younger than they used to, and stay single longer. There are also more old people, again often single, living much longer. At the same time more and more people are buying weekend homes, which creates a shortage of affordable housing in the countryside. Finally we just don't fill up houses the way we used to. Bedrooms are lost to ever more bathrooms, while downstairs rooms are knocked together to make huge living spaces. We need more rooms, which means the nation as a whole needs more houses.

Put all these factors together and add in the continuing desire of people to move from cities to a suburban version of the countryside, and you get the scary forecasts. Between 2001 and 2021, according to the latest government figures for England, we need to build four million new homes. In the south-east the pressure is most intense and in London - where the people choosing to leave are more than replaced by people choosing to arrive - it is extreme. London's mayor Ken Livingstone is already working on the basis of 30,000 new homes a year, the boroughs are protesting that they cannot possibly find the space for all those, and now the statisticians at the Office of the Deputy Prime Minister say that, actually, the desirable annual figure for London is 46,400. Nobody believes that number can be achieved - there are just too many constraints on building freely in the capital, even given the vast numbers of homes planned for the Thames Estuary, or "Gateway" as it is now called. But it shows the scale of the problem. And why the "Green Belt" areas around our major cities are under threat from politicians and builders as never before.