Methodology
Every number on this site is computed from a committed dataset of official rate tables and then checked against the three governments' own calculators. Rates shown are for 2026-27 and were verified on 9 August 2026.
One dataset, one calculation
Each authority's rate tables are read from its own published pages and keyed by hand into a dataset that is committed to the site's source code alongside the source name, the source address, the date each rate took effect and the date we last verified it. Nothing is inferred from a summary written by someone else, and no figure is ever typed into prose. Every published number is produced at build time by a single calculation module reading that dataset, and the same module is what runs when you use the calculator on the page. This matters more than it sounds. The common failure on a tax site is a hand-written table that agrees with the calculator on the day it is published and quietly stops agreeing after the next revision. Here there is one implementation, so a page and the calculator cannot disagree; if the dataset changes, both change together. The three main tables are all slice tables: the price is cut into bands and each band is charged only on the portion of the price inside it, so a single high rate never lands on the whole purchase. Scotland's Additional Dwelling Supplement is the exception, and it is handled separately for the reason set out below.
Checked against the governments' own calculators, not against our own arithmetic
A calculator that agrees with its own unit tests has proved nothing except that it is internally consistent. So the accuracy gate here points outwards. A fixture set of 43 cases, spanning all three regimes and every buyer type we publish - standard purchases, first-time buyers, additional dwellings, non-United Kingdom residents, and the combinations of those - is re-driven against HM Revenue & Customs' own Stamp Duty Land Tax calculator, Revenue Scotland's own Land and Buildings Transaction Tax calculator, and the Welsh Revenue Authority's own Land Transaction Tax calculator. Each case is entered into the official tool exactly as a buyer would enter it, the official result is read back, and it is compared with what this site would publish for the same inputs. The build fails if any single case disagrees. Not a warning, not a logged difference: the site does not ship. Each case also runs in a fresh browser session, because reusing one session lets answers from a previous scenario carry over and yields results that look plausible and are wrong, which is precisely the failure an external check exists to catch.
Four rules this check proved, which arithmetic alone would have got wrong
The gate is not decorative. It settled four rules that are easy to state incorrectly and that a sensible-looking implementation gets wrong. First, first-time buyer relief in England and Northern Ireland is a cliff, not a taper. Up to the cap at £500,000 the relief applies; one pound over it, the entire relief is lost and the purchase is taxed as an ordinary one, so a single pound of price can cost thousands in tax. Second, the minimum consideration threshold at £40,000 for the additional-property surcharge is a floor on whether the surcharge applies at all, not a nil-rate band underneath it. At the threshold the surcharge applies from the first pound of the price, not merely to the part above the threshold. Third, the non-United Kingdom resident surcharge stacks on every band, including bands charged at zero and including first-time buyer bands. A buyer who pays nothing on the lowest slice as a resident does not pay nothing on it as a non-resident. Fourth, Scotland's Additional Dwelling Supplement at 8% is charged on the entire purchase price, not banded like the English and Welsh surcharges. In the dataset it is deliberately stored in a different shape from a band table so the engine cannot treat it as banded by accident. Scotland's first-time buyer relief is likewise a flat deduction capped at £600, not an extra band, and Wales has no first-time buyer relief at all, so the site does not offer that option there.
Rounding
All arithmetic is done in integer pence. Rounding to the whole pound happens once, at the very last step, after every band, surcharge and relief has been applied. Rounding at intermediate steps is a common source of results that are out by a pound or two against the official tools, and because the accuracy gate compares against those tools exactly, a difference of a pound fails the build in the same way a difference of thousands does.
Rates change, and old pages have to stay honest
These three taxes are set by three different governments on three different timetables, so rates can move at up to three separate budgets in a year: the United Kingdom Autumn Budget for Stamp Duty Land Tax, the Scottish Budget for Land and Buildings Transaction Tax, and the Welsh Budget for Land Transaction Tax. A change to one has no effect on the other two. Every page therefore carries the tax year it describes, the date the rates took effect, and the date the figures were last verified against the source, so a reader can see the age of what they are reading rather than guess. When an authority revises a rate, a new dated record is added to the dataset and the old record is never overwritten. Pages describing an earlier period keep the rates that actually applied then, and the external check is re-run before anything is republished.
What the calculator does not model
The engine covers standard residential freehold purchases in the three regimes, for the buyer types named on the page. It does not model multiple dwellings relief, linked transactions, mixed-use property, leasehold premium and net present value calculations, corporate purchases and the flat higher rate that can apply to them, or any relief not named on the page. It also does not decide questions of fact that change the answer, such as whether a buyer counts as a first-time buyer, whether a purchase leaves someone owning more than one dwelling, or whether a buyer meets the residence test. Those cases are decided on the specific facts of a transaction, and the correct sources are the relevant authority's guidance and your conveyancer, who files the return and settles the tax actually due. If a figure here disagrees with the relevant authority's own calculator, write to support@inventum.com.au with the price and buyer type and it will be checked against the source.
Written by StampBand Editorial, published by Inventum. Rates verified 9 August 2026 against HM Revenue & Customs, Revenue Scotland and the Welsh Revenue Authority.