How to cite an embodied carbon benchmark honestly

An embodied carbon benchmark citation is only useful if the reader can tell what was counted. State the source and its edition, the life cycle modules included, the floor area definition, the building type band and the reference study period, and your number becomes comparable. Leave any of those out and a reviewer is entitled to ignore it.

That is the whole discipline. Most arguments about whether a scheme performs well on upfront carbon turn out to be arguments about scope rather than about design.

Why benchmarks are hard to compare

Published benchmarks come from different bodies, with different sample sets, for different purposes. Some are aspirational design targets. Some are observed values from completed projects. Some cover only the structure and substructure, some add facade and fit-out, and some include external works that others leave out entirely.

The temptation in a planning submission is to pick whichever comparison makes the scheme look best. Reviewers notice. A number quoted against a target that excludes substructure, in a scheme with three basement levels, invites the obvious question.

The honest approach is to name the comparison you are making, say what it covers, and report your own figure on the same basis. Where your scope differs, say so and give the figure both ways.

Scope, boundary and units

Report in kgCO2e per square metre, state which floor area you divided by, and state the modules.

The module structure comes from BS EN 15978 and is applied in the Royal Institution of Chartered Surveyors whole life carbon assessment standard. Modules A1 to A3 cover product manufacture, A4 transport to site and A5 construction. B modules cover use, maintenance and replacement. C modules cover end of life. Module D sits outside the system boundary and records benefits beyond it.

Four rules keep a citation defensible:

1. Upfront carbon means A1 to A5. Quoting A1 to A3 against an upfront carbon benchmark understates the figure.
2. Divide by gross internal area unless the benchmark says otherwise, and state the definition you used.
3. Never net module D off the total. Report it separately or not at all.
4. State the reference study period, normally 60 years, because B and C module figures are meaningless without it.

Sequestered biogenic carbon is the other place scopes diverge. Timber schemes can show a very low A1 to A3 figure if sequestration is counted as a negative in the product stage, and the same scheme looks different if the carbon is accounted at end of life in module C. State the convention you followed.

Building type and structural system effects

Benchmarks are banded by use for a reason. A warehouse with a portal frame, a large ground bearing slab and minimal services is not comparable with an apartment block, and a laboratory with high servicing loads is not comparable with either. Where the benchmark set has no band for your building type, say so rather than borrowing the nearest one silently.

Structural system drives most of the variation in the A1 to A3 figure. Span, grid, floor build-up, basement depth and ground conditions do more to the number than any specification choice made later. A piled foundation on poor ground can account for a substantial share of a low rise scheme's upfront carbon, which is why early comparison between options is worth more than late optimisation of finishes. Our note on energy statements for warehouse and logistics schemes covers the operational side of the same building type.

Ancillary structures carry their own figures and are often left out of the headline. A steel canopy over a car park is a case in point, and the consenting route for one is set out in our note on solar canopies over car parks.

Fit-out is the quiet variable. A shell and core figure and a fully fitted figure for the same building can differ enough to change the band, so state which you are reporting and whether tenant fit-out is excluded.

Naming the source and the edition

Write the citation out properly. The body, the document title, the edition or version, and the year. Benchmarks are revised, and a figure quoted from a superseded edition against current policy will be queried.

For London schemes referable to the Mayor, the Greater London Authority sets out its own whole life carbon assessment requirements and benchmark expectations in its guidance, and the template it publishes is the format officers expect. Outside London, local plan policies increasingly set a numerical target, in which case the local plan policy is the benchmark and the published guidance is supporting method. Design-stage targets published by the Royal Institute of British Architects and the bands published by the Low Energy Transformation Initiative are widely used as comparators, as is the Institution of Structural Engineers guidance on calculating embodied carbon.

Cite the one your authority actually asks for first, then any others as secondary comparison. A Whole Life Carbon Assessment that quotes three benchmarks without saying which policy it is answering reads as evasive.

Where a benchmark does not exist

Some building types have no credible published band. Specialist industrial, data centres, agricultural buildings, highly serviced research space and some retrofit projects fall into this group.

In that situation, build your own comparator and show the working. Model a realistic business as usual version of the same scheme on the same boundary, report both figures, and explain the reduction and where it came from. State plainly that no published benchmark exists for the type, because an assessor who invents a comparison is more exposed than one who declines to make it.

Retrofit deserves its own treatment. Comparing a deep retrofit against a new build benchmark on a per square metre basis flatters the retrofit and tells the reader very little. Report the absolute figure, the retained element percentage by mass or by area, and the avoided new build carbon as a separate calculation.

Reporting against more than one benchmark

A table is the clearest way to do it. One row per benchmark, with columns for the source and edition, the modules covered, the floor area basis, the published value and your scheme's value on that same basis.

Where the scheme passes one comparison and fails another, say so in the text rather than leaving the table to speak. An assessment that acknowledges a near miss against a stretch target and explains the trade-off is more persuasive than one that quietly picks the easier line. The same applies to a Sustainability Statement that pulls the headline figure through without the scope note attached.

What a reviewer should check

Run the citation backwards. Does the quoted value appear in the named edition, on the stated modules and area basis? Is the scheme's figure calculated on that basis, or converted with an undisclosed assumption?

Then look at the data sources behind the scheme figure. Environmental product declarations for the main structural materials carry more weight than generic database averages, and a mixed approach should say which elements used which. Check the quantities against the drawings for the top three or four elements by mass. Check that A4 transport distances are stated rather than defaulted, and that A5 includes site energy and waste rather than a token allowance.

Finally check the conclusion is supported. A statement that the scheme meets a target when the target is only met on a narrower module set is the error that most often survives into a submitted document. An Energy Statement or carbon assessment drafted in PlanScope gives you the structure and flags missing inputs with a clear marker rather than filling them with assumptions, and a competent assessor should still check the quantities and data sources before it is submitted.

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