There are problems with wealth taxes but avoidance, for once, isn't one of them

Taxing illiquid assets is, well, taxing.

This mansion, in Kensington, London, was once the most expensive in the world. What'd the tax be? Photograph: Getty Images

Fraser Nelson, editor of the Spectator, has a column in today's Telegraph arguing that George Osborne needs to learn the lesson that "wealth taxes simply don't work". He writes:

The problems involved were fairly basic. Do you tax people’s worldwide wealth? If so, those much-needed businessmen will stay away. Do you tax only British wealth? Then people will move their investments abroad. The Treasury warned Healey that the proposed wealth tax was “political dynamite” – and not in a good way. At a time when Britain was in a desperate economic state, it risked dragging the country down further still. Healey gave up, saying he could not find any wealth tax that would be worth the political hassle.

This is disappointing, because Nelson is largely duelling with straw men here. It is indeed true that a tax on overall wealth could well lead to tax avoidance by the relatively simple tax planning strategy known as "keeping your cash in a Swiss bank account". The thing is, that isn't actually that much of a problem. After all, people avoid income tax as well, yet somehow we struggle on.

The measure for a tax is never "will everyone pay it rather than put their energy into avoiding it?", but "will enough people pay it to make it worth our while?" That's a different calculus, and one which Nelson doesn't address.

But the real disappointment is that, for all Nelson talks about the rising support for a wealth tax, he neglects to mention that most of that support is for taxing a very specific type of wealth: property. It's a shame, because that sort of tax – a "mansion tax", a "land value tax", or whatever form it takes – has its own set of problems which are under-discussed.

The important thing to note about property taxes do is that they completely fix the problems Nelson is concerned about when it comes to wealth taxes in general. You can keep money overseas, but a house in Britain is rather stuck where it is. To the extent that such a tax it increases the cost of living in Britain, it may keep wealthy foreigners away – but only those who haven't already been put off by the extraordinary cost of the sort of houses wealthy foreigners buy.

The thing is, land and property taxes aren't the golden bullet that many on the left like to think, for the key reason that houses aren't particularly liquid assets.

We've all heard the sob-story of the elderly pensioner who could be forced to sell the house he's lived in all his life to pay the land value tax (although we rarely hear it alongside its counterpart, the elderly pensioners who are being forced to moved out of houses they've lived in all their lives because of benefit cuts), but the key concern with such a tax is related to that problem.

If you are taxed, say, 20 per cent of your cash holdings, you pay that tax by handing 20 per cent of your cash to HMRC. If you are taxed on 20 per cent of the value of your land, you can't just hand a wing of your house over to the taxman. You either have to have the cash equivalent available, or sell your house.

Taken individually, that's not the end of the world – few will cry too hard if the odd landowner has to sell a few acres to pay the bill. The problem comes if the tax is set high enough that that sort of sale becomes commonplace.

If too many people end up trying to sell their mansions or land at the same time, then you're stuck with a sadly inevitable collapse in the price that land can go for. That's not just unfair – it means you would be taxing people on assets which are no longer worth what they were when you assessed them – it's also staggeringly inefficient. A well-designed tax should not encourage a fire-sale of assets.

These problems aren't insurmountable, by any means. But they do give pause for thought when considering the truely radical proposals like Peter Tatchell's plan to set the wealth tax rate at 20 per cent. Although back-of-the-envelope calculations suggest such a tax would generate truly staggering revenue, a more modest rate would be a better idea – at least at the start.