Let's clear up one myth about the mansion tax straight away: it is not going to work the way Tim Montgomerie suggests in his Times column today. Montgomerie writes:
Last week Ed Miliband joined Nick Clegg in proposing a mansion tax so that the people who live in London’s parallel universe — many of whom come from overseas and pay little in the way of income taxes — might make a greater contribution. It was a perfectly reasonable intervention but, in a sign that the Conservative Party still hasn’t understood why it can’t win elections, many Tory MPs reacted with fury. Such a tax was, they complained, unfair on the person in a £2 million home who didn’t have the necessary £20,000 to spare.
A person with a £2m home would pay nothing in tax under any version of the mansion tax previously suggested. A £20,000 tax bill implies a house worth £4m. That's because the tax Miliband eventually produces is near certain to follow the same lines as the Lib Dems' desired tax, and be set at 1 per cent of the value of the home above £2m.
The reason why is obvious: if the tax was set at 1 per cent of the total value of any home above £2m, then there would be a huge incentive to depress, either artificially or actually, the value of the home. If your house was worth £2,010,000, it would be worth paying someone up to £10,000 to come round and do £10k worth of damage to it.
More practically, the lack of a cliff-edge at which the tax comes in is also likely to prevent it doing too much to property values. It will have a depressive effect, getting stronger as the house gets more valuable, and will likely knock quite a bit off the price of a £4m house. But the changes will be about pricing in the expected future cost of the tax to the sale price, not about avoidance. For much the same reason that no-body ever says "no thanks, I'd rather earn just £8,104 and not pay tax on my income", houses aren't going to start being sold at £1.99m in any real numbers.
But that example does reveal one of the bigger problems with the fairness of the tax. No, it's not the ridiculous example given by Toby Young of someone who finds themselves living in a £4m house without the money to pay the tax bill. If you can sell your house, buy a £2m one, and pocket the lifetime earnings of someone on the median wage as the difference, you do not really get to plead poverty.
Instead, it's that the vast majority of the incidence of the tax will be on the people who own the houses today. The depressive effect it will have on house prices will be pretty much instantaneous, and will then sit there forever. The tax will also slightly dampen the rate at which house prices above £2m increase — because every £100 increase in price imposes a £1-a-year increase in tax liability — but that is small fry compared to the initial hit.
That quirk explains why the suggestion of a tax provokes such vociferous outrage amongst those owning £2m+ houses. It really is unfair on them; but it's not a matter of unfairness against the rich, so much as unfairness against this generation of the rich. And really, for a government which has done so much to harm the cause of intergenerational fairness, that's a small hit in return.