Labour’s proposed “mansion tax” could hinge on surprisingly granular details, from box rooms and home offices to garages, architectural style and the age of refurbishments, according to official documents outlining how properties may be valued.
Modelling by the Valuation Office Agency (VOA), used in the recent revaluation of homes in Wales, is now expected to inform how officials identify which properties in England fall within scope of Labour’s planned high-value council tax surcharge.
Under the policy, homes worth more than £2 million would face an additional annual charge of up to £7,500 on top of existing council tax bills. The tax is not due to come into force until April 2028, but preparatory work is already under way.
The document, seen by The Telegraph, details how the VOA reassessed 1.5 million homes in Wales in 2024, the first full revaluation there since 2003. It reveals the criteria officials believe materially affect property values, and therefore potential tax liability.
Central to the process is the VOA’s “automated valuation model”, which uses large datasets to estimate values at scale. The system draws on property characteristics, historic sales data, neighbourhood trends and national house price indices.
Among the features taken into account:
• Bedrooms – including box rooms and rooms used as studies, provided they are large enough to fit a standard single bed
• Bathrooms – only counted if they include at least three fittings (for example, a toilet, basin and shower)
• Garages and parking – both garages and open parking spaces are factored in
• Property type – whether a home is detached, terraced or a flat
• Architectural style and age – including whether the home has period features
• Upgrades and refurbishments – quality and recency of improvements
• Plot size and outdoor space – sourced from Land Registry and Ordnance Survey data
Lucian Cook, head of residential research at Savills, said such details could prove decisive.
“Once you get down to individual attributes of a property, it can start to make big differences in value,” he said. “Given the bands are quite tight, they will be heavily scrutinised by anyone close to the threshold.”
Data gaps and disputes
The VOA also relies on historic transaction data and local sales evidence, cross-referenced with the Office for National Statistics house price index. However, this poses challenges for homes that have not changed hands for many years.
“Where properties have been in long-term ownership, there’s less sales data to work from,” Cook said. “That makes valuations more complex and potentially more contentious.”
In such cases, officials may need to dispatch a human valuer rather than rely solely on automated models, a process critics say risks becoming slow and bureaucratic.
John Muellbauer, professor of economics at the University of Oxford, said the system was generally robust but not foolproof.
“The valuation office can give reasonably accurate values for perhaps 80 per cent of homes using automated systems,” he said. “But where confidence is lower, you may need an on-site valuation.”
Who will be affected?
Jonathan Russell, chief executive of the VOA, has told MPs that between 150,000 and 200,000 homes in England are likely to fall within scope of the new surcharge.
The proposed annual charges would be banded as follows:
• £2m–£2.5m: £2,500
• £2.5m–£3.5m: £3,500
• £3.5m–£5m: £5,000
• £5m+: £7,500
A VOA spokesperson said: “To support the high-value council tax surcharge, the VOA will employ professional valuers experienced in valuing domestic properties, using industry-standard techniques. We will also recruit additional staff to deliver valuations accurately and efficiently.”
For homeowners near the £2 million threshold, however, the prospect of box rooms, garages or period features tipping them into a higher tax band is already fuelling anxiety, and setting the stage for disputes long before the tax comes into force.
