Three times as many homes. That is the difference between a mansion tax £1.5m threshold and the £2m level ministers have already confirmed for April 2028. According to Knight Frank analysis reported by Property Industry Eye on 28 September 2026, lowering the entry point to £1.5m would pull 222,800 properties into the valuation "grey area", up from 73,600 under the current £2m plan. For owners across Notting Hill, Kensington, and the wider West London property belt, that is not an abstract statistic. It is a direct question about how their home will be valued, banded, and taxed within the next two years.
The confirmed policy, known formally as the High Value Council Tax Surcharge and informally as the "mansion tax," introduces annual charges of £2,500 to £7,500 across four bands on homes worth £2m or more, starting in April 2028. Speculation ahead of the 28 October 2026 Budget suggests the Treasury could push the threshold down to £1.5m. Nothing has been confirmed at that lower figure. But the possibility alone is already shaping how buyers and sellers are behaving in high-value pockets of the capital.
Key Takeaways
- The confirmed mansion tax starts in April 2028 at a £2m threshold, with charges from £2,500 to £7,500 a year across four bands.
- Speculation, not confirmed policy, points to a possible mansion tax £1.5m threshold ahead of the 28 October 2026 Budget.
- Knight Frank estimates 222,800 homes would sit in the valuation grey area if the threshold fell to £1.5m, versus 73,600 at £2m.
- Outer London boroughs and parts of the South East are expected to feel the biggest impact, though West London's high-value streets remain exposed too.
- Owners near either threshold should consider an independent RICS Red Book valuation to prepare for Valuation Office Agency banding decisions.
What the Confirmed Mansion Tax Actually Says
Before addressing the speculation, it is worth being precise about what is settled. The High Value Council Tax Surcharge will apply from April 2028 to homes in England valued at £2m or above. The Valuation Office Agency (VOA) will assess properties and place them into one of four bands, each carrying a different annual surcharge ranging from £2,500 to £7,500. The government's consultation also includes deferment proposals, aimed at asset-rich but cash-poor owners, think retirees sitting in homes that have appreciated sharply but who have limited income to cover a new annual bill.
This structure alone creates uncertainty. Property values are not static, and the VOA's assessment will not always match an owner's own expectation of worth. That mismatch becomes far more consequential if the threshold moves lower.
Why a Mansion Tax £1.5m Threshold Would Change the Picture
Knight Frank's analysis shows just how much a single number can reshape the scale of a policy. At £2m, roughly 73,600 properties fall into the £1.8m-£2.2m band, the zone where valuation judgment calls matter most, because owners sit close enough to the line that a modest revaluation could push them over it or keep them under it.
Drop the threshold to £1.5m, and that grey area expands to 222,800 homes. That is not a marginal increase. It is a near-tripling of the population affected by borderline valuation decisions.
Tom Bill, head of UK residential research at Knight Frank, put it plainly:
"The proposal to drop the lower threshold for the HVCT will cause concern in outer London boroughs and parts of south-east England in particular."
He added that a lower threshold "will inevitably lead to more pointed negotiations between buyers and sellers and bunching below price thresholds."
That second point deserves attention from anyone active in the West London market right now.
Price Bunching: The Hidden Risk for Sellers and Buyers
"Bunching" describes what happens when sellers and buyers deliberately negotiate a sale price just under a tax threshold to avoid triggering a higher band or an entirely new charge. This is a well-documented pattern in property markets with stamp duty cliffs, and it would likely repeat itself around any mansion tax threshold, confirmed or speculative.
For a mansion tax £1.5m threshold specifically, the risk is that homes genuinely worth £1.55m or £1.6m get priced, marketed, or negotiated down toward £1.49m to sidestep the surcharge altogether. This creates several knock-on effects:
- Distorted comparables. Estate agents and buyers relying on recent sales data may see artificially clustered pricing just below the threshold, making true market value harder to judge.
- Awkward negotiations. Sellers may resist any survey or valuation that pushes a property over the line, while buyers may push back hard on price to stay under it.
- Delayed or renegotiated deals. If a VOA valuation later contradicts the agreed sale price, both parties may need to revisit terms mid-transaction.
Notting Hill's stock of period conversions, garden flats, and family houses spans a wide price range, much of it sitting close to both the £2m confirmed line and the speculative £1.5m mark. That makes bunching a realistic near-term concern for this specific market, not a theoretical one.
How VOA Banding Is Expected to Work
Under the confirmed framework, the VOA will value properties and assign them to one of four bands, each with its own annual charge. While exact banding cut-offs beyond the initial £2m entry point have not been fully detailed in public policy documents, the model mirrors council tax banding in structure: a valuation exercise followed by a banding decision, with an appeals or query mechanism expected to follow standard VOA practice.
The consultation's inclusion of deferment options signals that the government recognises the surcharge could create cash-flow strain for some owners, particularly those on fixed incomes in homes that have simply risen in value over decades of ownership. Deferment would allow the charge to accrue rather than be paid immediately, likely settled on sale or transfer of the property, though final mechanics remain subject to consultation outcomes.
| Detail | Confirmed Policy | Speculated Change |
|---|---|---|
| Threshold | £2m | £1.5m (unconfirmed) |
| Start date | April 2028 | Not stated |
| Annual charge range | £2,500 to £7,500 | Not stated |
| Properties in valuation grey area | 73,600 | 222,800 |
Why Owners Near a Mansion Tax £1.5m Threshold Should Get a RICS Red Book Valuation
Given the scale of uncertainty, owners of homes valued anywhere between roughly £1.4m and £2.2m have good reason to seek clarity now rather than wait for the Budget outcome or the VOA's own assessment.
An independent RICS Red Book valuation offers several practical advantages:
- An evidence-based benchmark. A Red Book valuation follows strict RICS professional standards, giving owners a defensible, well-documented figure to compare against any future VOA banding decision.
- Negotiation leverage. Sellers and buyers alike can use a professional valuation to anchor discussions, reducing the likelihood of disputes driven by threshold anxiety.
- Appeal preparation. If a VOA valuation later seems too high, having an existing independent assessment strengthens any query or appeal.
- Clarity for financial planning. Understanding where a property sits relative to £1.5m or £2m helps owners plan for potential surcharge costs, including whether deferment might be relevant.
For West London owners specifically, local market knowledge matters. A valuer familiar with Notting Hill's garden squares, communal gardens premiums, and conversion quirks will produce a far more reliable figure than a generic desktop estimate.
What Buyers Negotiating in Notting Hill Should Consider
Buyers should treat threshold uncertainty as a negotiating factor, not a reason to walk away from good properties. Practical steps include:
- Ask for valuation history. Request any existing RICS valuations or recent survey figures on a target property.
- Model both scenarios. Calculate likely annual costs under both the £2m and £1.5m threshold scenarios before making an offer.
- Watch for bunched pricing. Be sceptical of properties priced suspiciously close to £1.49m or £1.99m, and factor in that comparables nearby may be similarly distorted.
- Build in contingency. Where a purchase price sits close to a threshold, consider negotiating price flexibility tied to any future banding outcome.
FAQ: Mansion Tax £1.5m Threshold
Is the £1.5m threshold confirmed?
No. The confirmed policy sets the threshold at £2m from April 2028. The £1.5m figure is speculation ahead of the 28 October 2026 Budget.
How many homes would be affected at £1.5m versus £2m?
Knight Frank estimates 222,800 properties would sit in the valuation grey area at £1.5m, compared with 73,600 at £2m.
Who values properties for the surcharge?
The Valuation Office Agency will assess and band properties under the confirmed framework.
Can owners defer payment if cash flow is tight?
The government's consultation includes deferment proposals aimed at asset-rich, cash-poor owners, though final terms are still being developed.
Which areas are expected to be hit hardest by a lower threshold?
Knight Frank points to outer London boroughs and parts of the South East as most exposed, though inner West London markets like Notting Hill remain relevant given local price levels.
Should sellers reprice now based on speculation?
Not without professional advice. An independent RICS Red Book valuation offers a more reliable basis for decisions than reacting to unconfirmed threshold speculation.
Conclusion
The confirmed mansion tax begins in April 2028 at a £2m threshold, but the possibility of a mansion tax £1.5m threshold has already introduced real uncertainty into West London's high-value property market. Knight Frank's figures make the stakes clear: nearly triple the number of homes would enter the valuation grey area if the lower figure is adopted. Owners and buyers near either threshold should not wait for the 28 October 2026 Budget to act. Commissioning an independent RICS Red Book valuation now provides a defensible benchmark, supports stronger negotiating positions, and prepares owners for whatever the Valuation Office Agency eventually decides. For anyone buying or selling in Notting Hill this autumn, professional valuation advice is the most practical safeguard against a policy still very much in flux.