Karl Denninger looks at recently-failed US banks and by comparing their asset valuations with losses charged to the Federal Deposit Insurance Corporation discovers that they overvalued their properties - at the time of failure.
If you add up the nominal assets of the three banks - $903 million - and downgrade them to their real value as implied by the losses borne by FDIC - $602.3 million - you will find the collective assets were overvalued by 49.9%. In other words, current estimated real estate values should be cut by 33.3%.
These were banks operating in (now) economically distressed states - Florida, Illinois (both with official unemployment rates at or exceeding 11%), Maryland (over 7 % unemployment), so you can't necessarily apply that regrading to the whole of the USA.
Nevertheless, those states are relatively heavily populated, and so are the others now showing high rates of unemployment - see this US population map. So it may well be that the US housing market in general may need to be reassessed. If, as Denninger says, he is "generous" in estimating houses to be overvalued by 25%, that means we need to cut nominal prices by 20%.
This is borne out to some extent by reported house sale prices - see this real estate website - though the Northwest has shown a rise (why?).
And then we have to consider properties repossessed by lenders but not sold, and owners who are sitting on their properties and refusing to sell.
The UK, with its much more densely-populated land, maybe somewhat different; but I think that when all the recent financial stimuli stop and we get past the next General Election, we may see clearer evidence of declining valuations here, too.
ADDENDUM (10 March):
A counter-argument would be that the FDIC has applied a "forced-sale" valuation, as with individual or company insolvency. On the other hand, the FDIC must be in no hurry to overstate its obligations/losses - its own finances are already very shaky - and there are already many forced residential property sales actually ongoing, so the regrading of assets may to some extent reflect actual market conditions.