ONS WAS, is Rubbish.

As a follow on from the FT article on the Mansion Tax below, one of the other things it also brought to light is a dispute about the ONS methodology. 
"The results were based on the ONS Wealth and Assets Survey, which has come in for criticism recently by US and French academics who allege that it does not sufficiently record the assets of the most wealthy households. Professor Thomas Piketty, the author of the best-selling book Capital in the 21st century, said the survey was “particularly bad at measuring the top part of the wealth distribution of the UK”.

The ONS plans to respond to these criticisms on July 18, but said in a statement yesterday that its results came from “a large sample survey specifically designed to collect information on all aspects of individual and household wealth for all private households across Great Britain”. The wealth survey retains the official quality-assurance Kitemark of a “national statistic”." Here is what the IPPR had to say in it's 2013 paper "Property and Wealth Taxes in the UK"

"The IPPR wealth tax model allows us to consider the impact of a property tax, with some important limitations. The Wealth and Assets Survey (WAS) data that underpins the model does not capture the value of residential properties that are owned by entities other than private individuals and households.

 It is difficult to ascertain the proportion of residential properties owned by companies and trusts, but HMRC stamp duty statistics show that the share of properties whose buyers were not private individuals tends to rise with property value. 
For example, two-thirds of properties bought for more than £2 million in 2011 in the UK were purchased by companies or trusts, compared to around 10 per cent of properties worth less than £500,000.
 It is also likely that a relatively large share of high-value properties are owned by foreign nationals who are not permanently resident in the UK and are therefore unlikely to be captured in household surveys. In the ‘prime central London’ market, where the majority of high-value properties are located, foreign buyers accounted for more than half of property transactions in 2010. "
Furthermore, the ONS puts total(net) UK property wealth at £3.4trn. Even allowing for £1.3trn in mortgage debt, this still puts them some way off the £6.3trn residential property is now reckoned to be worth. While it's easy to have some sympathy with the ONS, it's figures are relied on in the debate surrounding inequality and tax policy. But, it seems as though they may have grossly under reported the true levels of wealth disparity in the UK.