Is that a big number? Campaign to Protect Rural England edition

Paul Miner, a campaigner for the Campaign to Protect Rural England, writes to the FT letters page to defend planning restrictions with an unwitting demonstration of how to mislead with a (presumably) truthful statistic:

Green belt designations are hardly a "chafing collar" as more than 3 square miles of green belt have been lost to development each year in England since 1995.

Now as my predecessor on More or Less, Andrew Dilnot, would ask: is that a big number?
Well, there are a couple of ways to do the maths. One is to ask how big the London Green Belt is; Wikipedia says 2000 square miles. 3 square miles a year is not nothing, of course, but it would take another 66 years of development to use up 10 per cent of green belt at that rate.
Or you can ask whether 3 square miles fits a lot of houses. Well, up to a point. The government wants 3 million homes to be built in the next 12-13 years. If the collar continues to chafe at 3 square miles of new development a year, that would be less than 40 square miles of green belt land. How many of the 3 million homes would fit on 40 square miles? About a quarter, at current housing densities of 25 per hectare or 20,000 per square mile.
I find these calculations rather hopeful, actually. There is some slack in the system, as Mr Miner wishes to point out. But there could be a lot more slack if we wanted, without concreting over the green belt.
Here is the original FT editorial that attracted Mr Miner’s ire. Oh, and hate mail in the comments below, please.

Tim Harford’s blog

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Tim, also known as the Undercover Economist, writes about the economics of everyday life.