I thought it might be interesting to put the latest Nationwide house price data and the recent land write downs made by Taylor Wimpey into a different context. I’d say for fun, but the real consequences are far from funny to a lot of people.

So here is a thought to conjure with: The value of all housing in the UK has dropped (assuming the Nationwide measure is accurate) by about £500 billion in less than a year. To put that figure into perspective, that is equivalent to about four months of the national output wiped off the nation’s asset register.

This is the flip side of when, during the boom, people said: “I earned more money from the rise in my house price than I did from getting out of bed and going to work.”

Anyway, there is a couple of ways of doing the maths. The figures are pretty crude but in “fag packet calculation” terms close enough.Britain’s housing stock in 2006 was measured at more than 26 million homes of which about 21.5 million were in private ownership. That figure will have gone up a bit.

Nationwide figures show that from peak in October 2007 to the latest marker at August the average price of a home in the UK dropped (using unsmoothed data) by 11.5%. The numbers they give are £186,044 and £164,654. That is a cash drop of about £21,400.

Making the perhaps rash assumption that the average is a true mean (this I hasten to add is no reflection on the work of Nationwide’s statisticians, I just don’t know their assumptions and definitions) and taking just the private homes globally that amounts to a total cash drop of about £460 billion. You then need to take account of the value of housing association and local authority stock.

You can do a bit of a cross check on this by looking at the Nationals Accounts Blue Book, which ascribes an asset value to the residential property in the UK.

The combined value of residential building for households and (oddly combined perhaps) housing associations the figure for the end of 2006 was put at just under £3,696 billion (page 233)  and the total figure for the value of residential buildings is put at £3,915.3 billion (page 229). Scale up for the rise to October 2007 and you get a figure of about £4.2 Tn.

So an 11.5% drop on this data comes in at about £480 billion.

So there you have it – in rough and ready terms we are half a trillion quid worse off.

Now that rather puts the trifling matter of £10 billion or so for the 2012 Olympics into perspective.