A Conservative council in London says it is to be the first local authority in the UK to levy a voluntary mansion tax on high-value homes - and it is lobbying the government to be able to make the charge mandatory.
Westminster council says the optional charge would be open to owners of properties valued at £10m or more.
The tax would be £1,376 a year: if all 2,000 owners of the £10m-plus properties paid up, the council would recoup some £2.75m a year, although this would have to be offset from the collection cost. This would be minimal, the council says, because the supplement would be collected alongside the normal Band H council tax.
"This scheme might have its cynics, but I have spoken to very wealthy people who want to help the borough more" says council leader Nickie Aiken.
Westminster is believed to have discussed the principle with another Conservative administration, Surrey county council, which also has a large number of high-value homes.
In recent years mansion taxes similar to that proposed by Westminster have been suggested by the Labour and Liberal Democrat parties.