Methodology and sources

Where the numbers on these guides come from, and how they're derived.

Trails Research·Updated 2026-07-22

Every figure on these guides is traceable to a named, public source. This page sets out what those sources are, how we turn them into the numbers you see, how often they refresh, and — just as important — what the data does not cover.

The sources

ONS Construction Output Price Indices (COPI)

The backbone of the cost figures. Published quarterly by the Office for National Statistics, referenced to 2015 = 100, covering blended output prices plus separate labour and materials series. Free, under the Open Government Licence. We use COPI for inflation indexing and for the labour-versus-materials split.

DBT Monthly Statistics of Building Materials and Components

Monthly material-level price indices across roughly 60 categories, on a three-to-four week lag. Free, under the Open Government Licence. We use it for material-specific movement — steel, timber, cement and the other commodities that move faster than the blended series.

Regional and elemental adjustment factors

Regional premiums and elemental context are derived from the freely published quarterly and annual market reports of Turner & Townsend, Gleeds and Costmodelling, reconciled against the ONS series so the levels stay consistent. Where a factor is attributed, it is named on the guide that uses it.

How the figures are derived

Inflation indexing: historical rates are re-indexed to the latest ONS release by the ratio of the current index to the index at the reference period.
Regional adjustment: UK-average figures are scaled by the published regional factor for the selected region.
Labour / materials split: where a page separates the two, the ONS labour and materials sub-indices are applied to the respective portions rather than a single blended multiplier.

How often it refreshes

Figures track their source cadence — ONS COPI quarterly, DBT monthly. Numbers are fetched live when you open a page, and each guide shows the release it currently reflects plus a "last updated" date. Nothing here is a static number frozen at publication.

What the data does not cover

These figures are for indexing, inflation adjustment and directional benchmarking. They are not a substitute for elemental unit rates at NRM2 detail (for which BCIS remains the market reference), they exclude VAT and professional fees unless a page states otherwise, and they are strongest for residential work. Every guide states its own caveats; treat the numbers as a defensible starting point, not a black-box quote.

Corrections

If a figure looks wrong or a source has moved, tell us — accuracy and traceability are the whole point of publishing this.

Frequently asked questions

Where does Trails construction cost data come from?

The figures are built from official UK open data: the ONS Construction Output Price Indices (blended, labour and materials series, base 2015 = 100) and the DBT Monthly Statistics of Building Materials and Components, both published under the Open Government Licence. Regional and elemental adjustment factors are derived from the free published market reports of Turner & Townsend, Gleeds and Costmodelling, reconciled against the ONS series.

How often is the data updated?

Figures refresh on each source release — ONS Construction Output Price Indices quarterly, DBT building materials monthly. Every page shows the release it currently reflects, and the "last updated" date in its header. Nothing on the page is a static number frozen at publication.

Is the data a substitute for a paid service like BCIS?

For indexing, inflation adjustment and directional benchmarking, the open-data stack covers most small-practice needs. It is not a substitute for BCIS elemental unit rates at NRM2 detail. We are explicit about this on the relevant guides — the aim is a defensible, transparent starting point, not a black-box quote engine.