Somebody has a price. It is three years old, or from a price book, or from a neighbour’s job in 2019, and the obvious move is to add some inflation and use it. That move is right in principle and goes wrong in at least four specific places, each of which is worth a few percent and one of which is worth nearly a fifth.

This is the companion to our note on why a published rate is not what you pay: that one is about what a figure excludes, this one is about moving a figure through time.

The Cover Year Is Not the Price Year

A price book’s edition year is a publishing date. Spon’s 2011 is not struck at 2011: its rates sit at a modelled tender price index of 468 on the Outer London base, while the same book’s own table forecasts Outer London at 454 for the second quarter of 2010 and instructs a minus 3% adjustment to reach that market level.

So the book on the shelf marked 2011 is pricing a market roughly a year before its cover, and about 3% above the quarter it was actually modelling. Treat the cover as the price date and you have introduced an error before you start.

The first question about any old figure is therefore not “how old is it” but “when was it true”, and the answer is usually earlier than the label.

Which Index, and Why It Matters So Much

There are two families of construction index and they measure different things. The VOA puts the distinction plainly: the tender price index “measures the trend of contractors’ accepted tenders and therefore reflects movements in the prices paid for completed buildings”, and “should not be confused with the General Building Cost Index which measures changes in costs of labour, materials and plant to contractors”.1

One includes the contractor’s margin. The other does not. Across a market turn they do not merely diverge, they move in opposite directions:

2008 to 2010Movement
BCIS All-in Tender Price Indexdown 11.8%
Construction material price index, all workup 6.6%

That is 18.4 percentage points apart over two years. Escalate a supply-and-fix rate on a materials index across that period and you overstate it by nearly a fifth; escalate a bare material on the tender index and you understate it by about the same.

For a homeowner the practical rule is short. If you are moving a quoted price for finished work, you want a tender price index. If you are moving the cost of a material you are buying yourself, you want a materials index. They are not interchangeable, and in a calm market the error hides.

The Output Price Index Is Not an Independent Check

It is tempting to cross-check one escalation against the ONS construction output price index. It is not an independent check. ONS builds that index from input series, producer price indices for materials, services producer prices for plant hire, earnings or consumer prices for labour, with a profit mark-up added on top. The only thing making it an output index rather than an input index is that mark-up, and ONS states the limitation itself: “the mark-up profit margin is only available two years after the respective reference period”.2

So the most recent two years of that series carry a frozen margin and move almost entirely on input costs. Checking a materials-based escalation against it is checking your working against your own assumptions.

2010 Was Not a Cheap Year, It Was a Low-Margin One

The published tender price series runs 251 in the fourth quarter of 2007 down to 209 in the first quarter of 2010, a fall of 16.7%, and it does not pass 251 again until the second quarter of 2014, six and a half years later.1 Over the same two years material costs rose.

Those tenders were struck at distressed margins. A 2010 rate is therefore not a low cost, it is a low price, and escalating it forward on any tender-based index carries a margin recovery of up to 16.7% dressed as inflation. Anyone whose mental benchmark for “what building work should cost” was formed in that trough is comparing today’s market to a period when contractors were pricing to survive.

Two Things About the Published Series Itself

It changed method. Up to the third quarter of 2018 the BCIS All-in Tender Price Index was computed from project indices taken out of priced bills of quantities. From the fourth quarter of 2018 the quarterly movement is the consensus of a Delphi panel of practising cost consultants.3 Any escalation spanning that date chains a measurement to an opinion survey. The same source notes the pre-2018 basis excluded mechanical, electrical and other specialist trades entirely, and was normalised to projects averaging under £5 million.

Some of it has not happened yet. The tender price table looks like one continuous series, but it carries footnote marks. In the current VOA publication everything from the second quarter of 2024 to the first quarter of 2029 is flagged as forecast, and 2020 to 2023 is still flagged provisional.1 Carrying a cost to 2026 on that table moves it from 391 to 420, which is 7.4%, and every point of that movement is somebody’s projection. A figure carried forward on forecast index values should be reported as a forecast, not as a price.

The Things No Index Can See

An index measures the market. It cannot measure a decision, and several of the decisions are large.

Tax steps. Part of every skip, grab and muckaway rate is Landfill Tax, and it moves when the Chancellor says so:4

Landfill Tax202420252026Change
Lower rate, inert waste£3.30/t£4.05/t£8.65/t+162%
Standard rate£103.70/t£126.15/t£130.75/t+26.1%

The lower rate governs soil, hardcore and rubble, which is most of what leaves a domestic job, and it nearly tripled in two years. Escalate a 2024 disposal rate to 2026 on any construction index and you miss the entire step, then read the gap as inflation when it is legislation. VAT changes behave the same way: the zero rate on approved alterations to protected buildings was withdrawn on 1 October 2012 and those works are now standard-rated,5 and no index carries that either. Our note on VAT on home renovations sets out which rate applies to what today.

Grants. Where a scheme pays, the recorded price is not the market price. Under the Great British Insulation Scheme the energy supplier paid and the householder paid nothing or a part contribution, and the scheme ended on 31 March 2026.6 So any observed price series for loft or cavity wall insulation across those years contains a great many zeroes that are not market prices, and the same measure steps from nothing to full price on a date set by policy rather than by the trade. An average across that boundary describes the scheme, not the work.

A Short Procedure

  1. Establish when the price was true, not when it was published.
  2. Pick the index family that matches what you are moving: tender prices for quoted work, material indices for materials.
  3. Check whether your period crosses a method change or a forecast boundary, and say so if it does.
  4. Ask separately what changed in tax, VAT or grants over the same period, because the index cannot tell you.
  5. Report a band, not a figure. An escalated price is an estimate of an estimate.

And where the money matters, prefer a current quotation to an inflated old one. Escalation is a way of sanity-checking a number, not a way of producing one; our note on getting and comparing building quotes is the better route to a figure you can rely on.

References (6)

Figures in this note were checked against the sources below on 14 September 2026. Superscript numbers in the text point to them. Every source the site cites, by topic.

  1. 1technicalvery strongValuation Office Agency, guidance.RN-L37ZLPGuidance Note edition 9 to the Valuation Office Agency's Rating Cost Guide 2026. Explains the methodology behind the estimated replacement costs in the guide and, critically, the adjustments applied to them: the location factor, the contract size adjustment and how the unit rates are meant to be used. Read this before quoting any figure out of the cost guide itself, because a raw ERC without its adjustments is the wrong number.
  2. 2technicalvery strongOffice for National Statistics, guidance.RN-4E929VThe ONS quality and methodology information for the Construction Output Price Indices: how the indices are built, what they cover, and their stated strengths and limitations. Worth reading before using an index, because it says plainly what the OPI does and does not measure, and an index applied to something it was never built to measure is the commonest way a cost estimate goes quietly wrong.
  3. 3technicalstrongBCIS, a data sheet.RN-XL6LAKBCIS's quarterly estimate of tender price inflation from the All-In Tender Price Index, the index the whole industry uses to move construction prices between dates. The published note gives the quarter-on-quarter and annual change with commentary. This is the series behind every escalated figure on the site: an old price book rate brought to 2026 is brought on this index.
  4. 4technicalvery strongHM Revenue & Customs, guidance.RN-FG9RYDHMRC's rate table for Landfill Tax, which applies to all waste disposed of by way of landfill at a licensed landfill site on or after 1 October 1996 unless exempt. The tax is charged by weight at two rates, with inert or inactive waste at the lower rate. Standard rate is 130.75 pounds per tonne from 1 April 2026, 126.15 pounds from 1 April 2025 and 103.70 pounds from 1 April 2024; the lower rate is 8.65 pounds per tonne from 1 April 2026, 4.05 pounds from 1 April 2025 and 3.30 pounds from 1 April 2024. This is the single largest fixed input in a skip hire price and explains why mixed skips cost far more than clean inert loads.
  5. 5technicalvery strongHM Revenue & Customs, guidance.RN-KBNHMBHMRC's main VAT notice for building work, last updated 26 August 2026. Sets out that construction of a new building and work to an existing building is normally standard rated at 20%, with exceptions: 0% for new qualifying dwellings and for housing association conversions of non-residential buildings, and 5% for a 'changed number of dwellings' conversion (section 7) and for renovation or alteration of a dwelling not lived in during the 2 years immediately before work starts (section 8, conditions at 8.1.2 and 8.3). Cites the law: VAT Act 1994 section 30 and Schedule 8 Group 5 for zero rating, section 29A and Schedule 7A Groups 6 and 7 for the reduced rate. Also explains the developer input tax block on goods that are not building materials (VAT (Input Tax) Order 1992 articles 2 and 6) and points self builders to the DIY housebuilders refund scheme.
  6. 6technicalvery strongDepartment for Energy Security and Net Zero (GOV.UK), a government page.RN-VTMJ4ZStatus page for the Great British Insulation Scheme (formerly ECO+), the obligation on larger energy suppliers to fund single insulation measures. Two facts matter for costing: the scheme runs to 31 March 2026, and the central GOV.UK eligibility checker has closed, so applications now go direct to energy suppliers, some of which are still taking them. The page also states that whether a household pays anything depends on a property assessment rather than a fixed rate, which is why no per-measure figure is published. It points to Ofgem for the detailed general group and low income group criteria (council tax band, EPC band and benefits tests). Cite this for the scheme's existence, closing date and the fact that any customer contribution is assessment-dependent, not for the eligibility bands themselves.

  1. Rating Cost Guide 2026: Guidance Notes, Valuation Office Agency, accessed 11 September 2026. assets.publishing.service.gov.uk

  2. Construction output price indices QMI, Office for National Statistics, accessed 11 September 2026. ons.gov.uk

  3. BCIS tender price index and estimate of tender price inflation, BCIS, accessed 11 September 2026. bcis.co.uk

  4. Landfill Tax rates from 1 April 2013, HM Revenue and Customs, accessed 11 September 2026. gov.uk

  5. Buildings and construction (VAT Notice 708), HM Revenue and Customs, accessed 11 September 2026. gov.uk

  6. Great British Insulation Scheme, GOV.UK, accessed 11 September 2026. gov.uk