Indexing comparables with the UK House Price Index: old sales in today's money

A house that sold for £340,000 in mid-2021 did not sell for £340,000 in today's money. Markets have moved — and using the raw figure in an appraisal silently imports three or four years of price movement as error. Indexation corrects each historic sale to present-day terms before you compare anything.

How it works

The UK House Price Index (Land Registry/ONS) tracks price levels monthly. Divide today's index by the index at the sale date, multiply the sold price by that factor, and you have the sale restated in current money. Do it to £/sqft rather than the headline price and the evidence becomes directly comparable across years.

Why it matters more than radius

Developers agonise over the right search radius and then average 2021 sales against 2025 ones — a bigger distortion than a half-mile of geography in most markets. Indexation lets you use a longer time window (more comps, better statistics) without importing stale pricing.

Limits to respect

• The HPI is a broad index — regional variants exist, but no index captures one street's micro-movement. Weight recent sales more heavily even after indexing.
• Indexation preserves relative quality: it will not fix a comp that was a poor match to your product in the first place.
• In fast-turning markets, the last 6–12 months of raw evidence should anchor the range; indexed older sales widen and stabilise it.

Threshold indexes every one of its 4.6 million matched sales automatically and shows both raw and indexed £/sqft, so the adjustment is visible rather than hidden.

Data referenced in Threshold is displayed under the Open Government Licence (HM Land Registry Price Paid Data, EPC Register, ONS, UK HPI). All figures produced by Threshold are statistical estimates for site appraisal, not RICS valuations.