Andy Burnham will send “mansion tax police” to homes to decide whether the owners should pay thousands of pounds more under the new levy, The Telegraph can disclose.
HMRC valuation agents will be able to demand entry to properties to assess whether they are worth more than £2m and therefore liable for the levy.
Homeowners who refuse an inspector entry would be committing a criminal offence, punishable by a fine of up to £200.
The move has been described as a “sinister assault on civil liberties” and an invasion of privacy.
The “mansion tax”, unveiled by Rachel Reeves, the former chancellor, at last year’s Budget, will hit properties worth more than £2m with an extra levy of between £2,500 and £7,500 a year, depending on how much they are worth.
London and the South East will be the areas hit hardest by the policy, although many homes in upmarket rural areas will also be affected.
To identify which homes will be subject to the levy, HMRC agents will use third-party data and publicly available information to value properties.
But ministers have now admitted – through answers to a series of parliamentary questions posed by the Conservatives – that Government inspectors will also carry out “internal inspections”.
Mel Stride, the shadow chancellor, said: “We may have changed prime minister, but it’s the same old story with Labour – how can we raise taxes to pay for more benefits? Now they have chosen a sinister assault on civil liberties as their latest method.
“Burnham and Angela Rayner are sending their council tax police to people’s homes so they can drag them into paying their new family homes tax. This is a tax on aspiration, on working hard and investing well, and Labour will stop at nothing to make as many people as possible liable for it.”
Officially known as the high-value council tax surcharge, the “mansion tax” will require a huge revaluation exercise in which officials examine houses in England’s top council tax bands.
The Valuation Office, part of HMRC, is responsible for tracking council tax bands for homes in England and Wales.
Earlier this year, Jonathan Russell, the Valuation Office chief executive, told MPs that houses previously valued at more than £1.5m would be reassessed to determine whether they now exceeded the £2m threshold.
Mr Russell said that although plans had not been finalised, he wanted to “make sure we’re not missing anything”.
According to the Valuation Office, property inspections will be necessary where “attributes can only be confirmed internally or a re-measurement is required”.
Home inspectors will gather information on the size and architectural style of a house as well as the number of floors, rooms, bedrooms and bathrooms.
Property experts have advised ministers that “regular” home inspections would be required to ensure valuation data are up to date, and that this would require a qualified surveyor to come and carry out a full physical survey.
Sir James Cleverly, the shadow housing secretary, said: “Hard-working families and pensioners face the prospect of HMRC tax inspectors snooping around their gardens and inside their homes, to justify hiking taxes on them.
“Even homes which are below the new tax thresholds face these intrusive checks, with the threat of criminal prosecutions and fines if people refuse.
“Labour will force more and more people to pay their new family homes tax. The Conservatives are the only party that will cut taxes, cut wasteful spending and get Britain working again.”
The Conservatives used the series of parliamentary questions to find out about the Government’s plan to send inspectors to people’s homes and impose penalties if they refused entry.
Dan Tomlinson, a junior Treasury minister, confirmed that homeowners faced the prospect of the Valuation Office contacting them to “arrange a visit” to their homes.
He pointed to guidance from the Valuation Office, which states that failing to provide the “required information” to agents assessing properties for council tax is “a criminal matter that could result in a fine or, potentially, imprisonment if someone makes a false statement”.
A housing minister disclosed that a £200 fine could be imposed on anyone who “intentionally delays or obstructs” a valuation officer carrying out their duties. Failing to provide information requested without a “reasonable excuse” could result in a fine of up to £500.
A Government spokesman said: “The public understands that we are addressing a longstanding unfairness, where a Band D home in Darlington pays more in council tax than a £10m mansion in Mayfair. The Valuation Office has extensive experience valuing domestic property, and will use a wide range of evidence to determine bandings.”
The spokesman added that any visits would take place by prior agreement with the property owner and in line with the Government’s code of practice.