Government officials are to snoop on people's extensions and home improvements in a bid to drag more people into Labour's mansion tax.

New documents reveal the Valuation Office will 'record changes to properties' such as extensions and renovations to monitor whether they hit the threshold for paying the controversial levy.

The Conservatives branded the plans a 'tax on aspiration'.

Ministers also revealed plans to claw back the tax from grieving families in cases where homeowners could not afford to pay it while they were alive.

Official estimates suggest the mansion tax will apply to around 165,000 properties worth more than £2 million when it is introduced in 2028.

But a new consultation document reveals that ministers plan to drag in more properties over time by recording improvements and revaluing homes every five years.

Tory frontbencher Gareth Bacon said: 'Labour's new family homes tax is a tax on aspiration. It's a double whammy on top of inheritance tax. Many pensioners will be pressed into deferring the new tax surcharge in a cruel 'pay as you die' policy.

'And Labour politicians will no doubt drag more and more families into the net.'

The mansion tax was announced by Rachel Reeves at the last Budget in a move criticis have branded an act of 'class war'.

Under her proposals, people in homes worth more than £2 million will pay an extra £2,500 per year; those in homes valued at more than £2.5 million will pay £3,500; properties worth more than £3.5 million will face a £5,000 annual bill; while properties valued at over £5 million will be charged £7,500.

The Office for Budget Responsibility (OBR) warned this year that the 'narrow' bands would distort the property market and result in property prices 'bunching' below them. And it suggested that 20 per cent of owners could appeal against their valuation, with 40 per cent likely to be successful.

The watchdog also suggested that housebuilders are likely to build fewer high end homes because of the increased cost of owning them. Taxpayers will also have the right to challenge any new assessment.

But Tory MP David Simmonds said the proposals would 'punish those who have invested well. And it will put people off improving their homes for fear of being dragged into the net'.

The new proposals suggest that people will be able to defer the annual payment if they have an income of less than £35,000 or savings below £16,000.

In these circumstances, the money would be clawed back from their estate after their death. The bill would come on top of any inheritance tax due.

An analysis by the OBR suggests that up to 50,000 households could struggle to pay the charge from their annual income. It estimates that around 5,300 families could be forced to sell up and move somewhere cheaper.

The analysis also predicts that tens of thousands of homeowners are likely to be caught up in a bureaucratic appeals system.

Ministers are also considering levying a 'premium' on mansions left empty, which could see the annual charge rise by as much as 300 per cent.

Introducing the changes, the communities secretary Steve Reed said it was 'unfair' that a mansion in Mayfair could end up paying less council tax than a Band D home in Blackpool under the current system.

He said the new High Value Council Tax Surcharge would ensure 'those who own the most valuable properties in the country will pay their fair share'.