The stamp duty surcharge on a second home or buy-to-let purchase works out to almost exactly the same number of months' mortgage payments whether the property costs £200,000 or £1,000,000. On an interest-only mortgage it's 18.5 months. On a 25-year repayment mortgage it's 12.2 months. Both figures hold flat across the entire price ladder — because the surcharge and the monthly payment it's being measured against grow at exactly the same rate as price rises, so price itself cancels out of the comparison.
The numbers behind it
The additional-property surcharge adds a flat 5 percentage points on top of the standard stamp duty rate, applied to the whole price with no nil-rate band of its own (HMRC, higher rates for additional dwellings). Homecost's stamp duty calculator confirms it comes out to exactly 5% of price at every level tested, from £10,000 on a £200,000 purchase to £50,000 on a £1,000,000 one (figures fetched 17 September 2026):
| Price | Surcharge (5% flat) | Loan at 75% LTV | Interest-only £/month | Months of surcharge (IO) | Repayment £/month (25yr) | Months of surcharge (repayment) |
|---|---|---|---|---|---|---|
| £200,000 | £10,000 | £150,000 | £540 | 18.5 | £818.50 | 12.2 |
| £400,000 | £20,000 | £300,000 | £1,080 | 18.5 | £1,637.00 | 12.2 |
| £600,000 | £30,000 | £450,000 | £1,620 | 18.5 | £2,455.49 | 12.2 |
| £800,000 | £40,000 | £600,000 | £2,160 | 18.5 | £3,273.99 | 12.2 |
| £1,000,000 | £50,000 | £750,000 | £2,700 | 18.5 | £4,092.49 | 12.2 |
Mortgage figures assume a 75% loan-to-value mortgage at the Bank of England's quoted 5-year fixed rate of 4.32% (April 2026 print, the most recent published at the time of writing), modelled both as interest-only and as a 25-year capital-repayment loan. Actual rates depend on the lender, deposit and credit profile and will differ from this quoted average.
The "months" column doesn't drift as the price rises — 18.5 months at £200,000 is the same 18.5 months at £1,000,000, and it would hold at £2,000,000 too. That's a different pattern from first-time buyer relief measured the same way, where the equivalent figure shrinks steadily as price climbs, because that relief is capped at a flat £5,000 while the surcharge is not.
Why the number stays flat
Both halves of the comparison are proportional to price, at a fixed deposit and mortgage rate. The surcharge is 5% of the purchase price, full stop. The monthly payment on a 75%-LTV loan is also a fixed fraction of price, because the loan itself is 75% of price and the interest rate is the same 4.32% at every level. Divide one price-proportional number by another price-proportional number and the price cancels out algebraically, leaving a constant — the same mechanism behind Homecost's earlier finding that a capped relief and an uncapped surcharge move in opposite directions as price rises. Here, because neither side of the fraction is capped, neither direction wins — the ratio just sits still.
This only holds while the underlying assumptions stay fixed. Change the deposit size, the mortgage rate or the loan term, and the constant shifts to a different (still flat) number — it's the LTV and rate that set the level, not the price of the property.
Why interest-only and repayment give different numbers
The 6.3-month gap between the two columns comes from the structure of a repayment mortgage rather than anything about the surcharge. A capital-repayment loan's monthly payment covers both the interest and a slice of the principal, so it is always higher than the interest-only payment on the same loan, rate and term — in this case roughly 51% higher throughout the table (£818.50 vs £540 at £200,000, the same ratio at £1,000,000). A bigger monthly payment is a bigger denominator, so it takes fewer of those payments to add up to the same £-surcharge — hence 12.2 months on repayment against 18.5 on interest-only, a ratio that itself stays constant across the price ladder for the same reason the two "months" columns individually do.
What this does and doesn't tell a buyer
This is a way of expressing a tax charge in payment-sized units, not a claim about what any individual buyer will pay or should do. Actual mortgage rates vary by lender, deposit size, credit profile and product term, and the 75%-LTV, 25-year, interest-rate assumptions used here won't match every purchase. It also doesn't include the standard stamp duty a buyer would owe regardless of the surcharge, legal and survey costs, or the ongoing running costs of a second property. Speak to a qualified mortgage adviser or conveyancer about your own figures before acting.
To see the maths on a specific loan, try Homecost's mortgage comparison calculator with the interest-only and repayment options side by side, or run a full stamp duty breakdown on the stamp duty calculator. To see the same worked example on a real street, try a postcode like M1 1AE in Manchester on Homecost's true-cost tool. More stamp duty and mortgage maths is in our cost intelligence section.