UK tender price index explained: TPI vs COPI vs building cost index
What each index actually measures, which one to use when, and how to apply it to your cost plan.
Source: ONS Construction Output Price Indices, Open Government Licence v3.0.
"Tender price index" gets used loosely in the UK to mean any of three different things, depending on who's speaking. For a small-practice QS trying to do something practical with the number, getting the terminology right saves time and prevents bad adjustments.
The three things people mean
| Name | What it measures | Published by |
|---|---|---|
| Tender Price Index (TPI) | Forward-looking prices contractors are bidding right now. Captures margin, risk pricing, and market competition. | BCIS (paid); Turner & Townsend / Gleeds in market reports |
| Construction Output Price Index (COPI) | Blended prices clients actually pay for completed work, backwards- looking, covers both new work and repair & maintenance | ONS (free, quarterly) |
| Building Cost Index | Input costs only: what contractors pay for labour, materials and plant. No margin or market effect included. | BCIS (paid); MHCLG materials index is a free proxy |
These three don't move together. In 2022, tender prices rose faster than output prices (margins widened as demand outstripped supply); input costs rose faster still (materials shocks). Knowing which index someone is quoting prevents 20%+ errors in comparisons.
The one most small practices actually use: ONS COPI
If you don't have a BCIS subscription, the ONS Construction Output Price Index (COPI) is the free index you'll rely on. It's:
- Free and Open Government Licence
- Authoritative, used in public procurement
- Quarterly with roughly 4 to 6 weeks of publication lag
- UK-level only, no regional split built in
- Blended, all new work (housing + infrastructure + commercial)
COPI: the long view
From 2015 (base) to N/A: the index stands at N/A, meaning UK construction output prices have risen ~N/A% in total since 2015. The jump is not linear. Quiet 2015 to 2020, sharp rise 2021 to 2022, moderating since.
How to use COPI in real work
Adjusting a historical quote to today's prices
If a project was priced at £X in a past quarter with index I_from, and you want today's equivalent at index I_now:
Adjusted value = £X × (I_now ÷ I_from)
The Cost Tracker does this automatically including regional factors, useful when comparing a client's "my neighbour paid this three years ago" anchor against today.
Projecting a cost plan to a tender date
COPI reflects published prices, not what tenders will return six months from now. For a forward projection, take the current trajectory and extend cautiously. The safer approach: use recent YoY as the central assumption, and test the plan at ±2 percentage points either side.
Common mistakes
- Comparing TPI to COPI values directly. They have different base years and measure different things. Adjust the base years before comparing if you must.
- Using UK-level COPI without a regional factor. A London project adjusted by UK COPI alone understates the number. See regional cost factors .
- Assuming the index applies to your specific project type. COPI is blended. Residential extensions may have moved differently from the blended headline. Use the index as directional, not exact.