This paper asks when a wealth tax would in principle be a desirable part of the tax system, setting aside the practicalities and politics that would be crucial in reality. The case for a one-off wealth tax is simple. If it were unexpected and credibly one-off – a major challenge in practice – this would be an efficient way to raise revenue and could be used to address existing wealth inequality. Whether such a tax is desirable hinges on whether it is considered fair, about which reasonable people will differ. Making the case for an annual wealth tax, which would affect future wealth accumulation as well as existing wealth, is less straightforward. It requires explaining why it is better to tax the same wealth every year – penalising those who save – rather than raising the same revenue by taxing all sources of wealth once when they are received (and/or when they are spent). Such a case can be made based on subtle arguments for why taxing wealth might help to ease the trade-off between redistribution and work incentives; and a wealth tax might also be justified if holding onto wealth, rather than spending it, benefits the holder or harms others. These theoretical arguments probably justify some taxation of wealth in principle, though we have little basis for judging the appropriate level, so only part of the theoretical benefit could be attained. It is questionable whether the achievable benefits outweigh the costs of an imperfect wealth tax in practice.
There are strong reasons to radically reform how we currently tax the sources/uses of wealth; this includes reforming capital income taxes in order to properly tax high returns. An annual wealth tax would be a poor substitute for doing that. But to the extent that taxes remain imperfect and that responses to a wealth tax would not affect revenue from other taxes (such as on income, expenditure and bequests), there may be a benefit to adding a wealth tax in order to diversify sources of revenue and prevent any one tax getting too high – though that must be weighed against the extra administrative burdens of having another tax.