On these numbers the surcharge is a Barnes problem rather than a Mortlake one.
The Budget is on 28 October, a date the Chancellor set out in a letter to MPs at the end of July. Richmond Council wants residents worried about it, and in Barnes there is arithmetic to back the worry up.
The council says 4,300 more homes in the borough would be caught by the High Value Council Tax Surcharge — the mansion tax — if the threshold drops from £2m to £1.5m, and 9,200 if it falls further still. The surcharge itself is settled: announced at last November's Budget, it will charge £2,500 to £7,500 a year on homes valued above £2m from April 2028. The cut to £1.5m has been announced by nobody in government. It is speculation the councils are pre-empting.
Where it would land is the local question. Over the past year the average home sold in SW13 went for about £1.33m — but the average semi-detached house in Barnes sold for £2.23m, above the current threshold, never mind a lower one. Barnes terraced houses averaged about £1.43m, roughly £70,000 short of a £1.5m line. In SW14, covering Mortlake and East Sheen, the average sale was under £1m and even semi-detached houses averaged £1.39m.
So on these numbers the surcharge is a Barnes problem rather than a Mortlake one. A cut to £1.5m is what turns it from a tax on the borough's largest houses into one that reaches an ordinary Barnes terrace.
Council leader Gareth Roberts says a lower threshold "would drag thousands more Richmond homes into an already unfair tax", and puts the borough's wider funding hit at £102m over four years. Richmond has signed a joint letter to the Chancellor with Wandsworth, Westminster and Kensington & Chelsea arguing that their four boroughs could end up paying more than half of everything the tax raises nationally, about £275m a year. That estimate is the councils' own. It does not come from the Treasury or an independent forecaster.
The council has not published a ward-by-ward breakdown of the 4,300 homes, or the valuations behind the number.
That matters because Barnes and Mortlake sit in the same borough and nowhere near the same housing market, and a warning aimed at the whole borough works better if a household on Castelnau or White Hart Lane can see which side of the line its own street falls on. There are 26 days until the Budget. The question for Gareth Roberts is whether that breakdown appears before the Chancellor stands up.
- Richmond Council claims 4,300 more homes, and 9,200 at a further threshold cut, would face the mansion tax if it drops from £2m to £1.5m
- The council's own webpage and press release give no methodology or data source for these figures
- The Chancellor confirmed the Budget will be held on Wednesday 28 October 2026, in a letter to the Treasury Select Committee dated 31 July
- SW13 (Barnes) semi-detached homes average £2,227,911 in sold prices over the past year, already above the current £2m threshold
- SW14 (Mortlake/East Sheen) homes average £972,565 overall, and even semi-detached houses average £1,394,128, below a £1.5m threshold
- No published source gives the proportion of SW13 or SW14 sales above £1.5m or £2m
- Richmond, Wandsworth, Westminster and Kensington & Chelsea councils jointly estimate their four boroughs could pay over half the national mansion tax revenue, a £275m-a-year figure that is the councils' own estimate, not independent analysis
- The High Value Council Tax Surcharge was announced at the November 2025 Budget, charging £2,500–£7,500 a year on homes above £2m from April 2028
Every factual claim in this story is linked to the evidence behind it. Where the newsroom has added judgement or framing, it is listed separately. Sources accessed 29 September 2026.
- New 'mansion tax' fears for 4,300 more Richmond homes (media)
- Richmond Council mansion tax warning (council)
- Autumn Budget 2026 date confirmed for 28th October (media)
- HM Treasury statement on Budget date (direct_statement)
- Four London councils warn Mansion Tax could cost residents £275m (media)
- SW13 house prices - Rightmove (dataset)
- SW14 house prices - Rightmove (dataset)
- ONS House Price Statistics for Small Areas (dataset)
Claim by claim (8 facts, 10 editorial judgements)
- Richmond Council leader Gareth Roberts said lowering the threshold from £2m to £1.5m would drag thousands more Richmond homes into an already unfair tax, and cited a £102 million funding hit over four years — media, 2026-09-22“Councillor Gareth Roberts, Leader of Richmond Council, said: 'Lowering the threshold from £2 million to £1.5 million would drag thousands more Richmond homes...'”
- Richmond Council's own webpage restates the 4,300 and 9,200 figures but provides no methodology or data source — council, 2026-09-22“No supporting methodology or data source documentation is provided”
- The Chancellor confirmed on 31 July 2026 that the Budget will be held on Wednesday 28 October 2026, in a letter to the Treasury Select Committee — media, 2026-07-31“confirmed on Friday 31 July 2026 that the Budget will be held on Wednesday 28 October 2026, writing to the Treasury Select Committee to set the date”
- HM Treasury's own account states the 2026 Budget will take place on Wednesday 28 October — direct_statement, 2026-07-31“The 2026 Budget will take place on Wednesday 28 October”
- Wandsworth, Richmond, Westminster and Kensington & Chelsea sent a joint letter to the Chancellor estimating the four boroughs could pay more than half of national mansion tax revenue, with an associated £275m annual cost; the figure is the councils' own estimate — media, 2026-09-01“households across the four boroughs could end up paying more than half of the revenue expected to be raised across the entire UK”
- SW13 average sold price over the past year is £1,333,500 overall; semi-detached homes average £2,227,911; terraced £1,428,714; flats £647,326, based on Land Registry Price Paid Data via Rightmove — dataset, 2026-09-01“Overall average £1,333,500; terraced £1,428,714; flats £647,326; semi-detached £2,227,911”
- SW14 average sold price over the past year is £972,565 overall; semi-detached £1,394,128; terraced £1,112,245; flats £520,878, based on Land Registry Price Paid Data via Rightmove — dataset, 2026-09-01“Overall average £972,565; flats £520,878; terraced £1,112,245; semi-detached £1,394,128”
- The High Value Council Tax Surcharge was announced at the November 2025 Budget, charging £2,500–£7,500 a year on homes above £2m from April 2028 — media, 2026-09-22
- Judgement: No source found publishes the proportion of SW13 or SW14 sales above £1.5m or £2m
- Judgement: Richmond Council wants residents to be worried about the Budget, and in Barnes there is arithmetic to back the worry up.
- Judgement: The cut to £1.5m is speculation the councils are pre-empting rather than an announced policy.
- Judgement: Where a lower threshold would land is the local question.
- Judgement: On these numbers the surcharge is a Barnes problem rather than a Mortlake one.
- Judgement: A cut to £1.5m is what turns the surcharge from a tax on the borough's largest houses into one that reaches an ordinary Barnes terrace.
- Judgement: Barnes and Mortlake sit in the same borough and nowhere near the same housing market.
- Judgement: A warning aimed at the whole borough works better if a household on Castelnau or White Hart Lane can see which side of the line its own street falls.
- Judgement: The council could publish a ward-level breakdown before the Budget, and should.
- Judgement: The Barnes terraced average sits roughly £70,000 under a £1.5m threshold (arithmetic from the verified SW13 terraced average).