Embodied carbon benchmarks and how to present them

An embodied carbon benchmark is only useful if the reader knows what it covers, and most reported figures do not say. A number in kilogrammes of carbon dioxide equivalent per square metre means nothing on its own, because the same building can produce very different figures depending on which life cycle stages were counted and which floor area was used as the denominator. Getting the embodied carbon benchmark planning comparison right is mostly a matter of declaring those two things clearly and then choosing a comparator that resembles the building.

Officers are not going to audit your quantities. They will check whether your figure sits above or below a recognised benchmark, and whether the comparison is fair. That is the whole of the assessment as far as the decision is concerned.

Which benchmarks are in circulation

Four kinds of published figure get used in planning submissions, and they are not interchangeable.

• The professional standard published by RICS, which sets out the assessment methodology and carries default and reference data for use where product specific information is unavailable
• Local authority benchmarks, most prominently the Greater London Authority's whole life carbon guidance, which publishes benchmark and aspirational figures by building type for schemes referable to the Mayor
• Industry target figures from bodies such as LETI and the RIBA climate challenge, which set out trajectories rather than compliance thresholds
• Structural engineering guidance, including the Institution of Structural Engineers material on calculating embodied carbon, which is where frame comparisons usually come from

Name the source and its edition when you quote a figure. Benchmarks have been revised, the underlying carbon factors for electricity and steel have changed, and a target quoted without a date invites a reviewer to ask which version you meant. Verify the current published values at the point of writing rather than copying a number from an older report.

Normalising by area, and the definition traps

The denominator does more damage than the numerator. Three traps recur.

The first is the area definition. Gross internal area is the usual basis, and gross external area or net internal area produce different results for the same building. Using the wrong one can move a figure by a tenth or more on a building with thick walls or a high wall to floor ratio.

The second is what the area includes. Basement car parking, plant rooms, undercrofts and unheated ancillary space all add area, which lowers the figure per square metre while adding carbon in absolute terms. A scheme with two basement levels of parking can look better per square metre than the same scheme without them, which is the wrong answer.

The third is per unit reporting. Housing schemes are sometimes reported per dwelling, hotels per bedroom and student accommodation per bed space. Those bases can be defensible, and they are not comparable with a per square metre benchmark.

An illustrative example shows how the arithmetic works. Take a scheme with a modelled total of 4,800 tonnes of carbon dioxide equivalent for the product and construction stages, and a gross internal area of 8,000 m². The figure is 600 kgCO2e per m². Add a 2,000 m² basement car park that itself contributes 900 tonnes, and the total becomes 5,700 tonnes over 10,000 m², which reports as 570 kgCO2e per m². The building got worse and the benchmark comparison improved. Those numbers are illustrative only, and the effect is the point rather than the values.

State the area basis, state what is included, and give the absolute tonnage alongside the normalised figure. The absolute number is the one that cannot be gamed.

Upfront carbon against whole life figures

Upfront carbon covers the emissions released before the building is handed over, from raw material supply, manufacture, transport and the construction process. Whole life figures add the use stage, including replacement of components, and the end of life stages.

Both are legitimate and they answer different questions. Upfront carbon is what the design decision controls now, and it is the figure most benchmarks are expressed against. Whole life figures matter where a specification trades a low upfront number for short replacement cycles, which a facade or a services strategy can easily do.

Report them separately and say which modules each covers, using the module naming from the standard. The note on whole life carbon modules A to C sets out what each group contains. Benefits beyond the system boundary, covering reuse and recovery potential, are reported on their own and must not be subtracted from the headline.

Comparing like with like

A benchmark for an office is not a benchmark for a school. Beyond building type, five factors move the number enough to make a comparison unfair if they differ.

Say which of these your figure includes. A reviewer comparing a shell and core figure with a fully fitted benchmark will reach the wrong conclusion, and correcting that later costs you credibility on everything else in the report.

Presenting the figure so an officer can use it

One table near the front, then the detail behind it. The table should give the absolute tonnage, the normalised figure, the area basis, the modules covered, the benchmark used with its source and edition, and a plain statement of whether the scheme is above or below it.

Add two sentences of honesty. Say what the biggest single contributor is, which is usually the substructure or the frame. And say where the assessment is uncertain, typically because product data is not yet fixed at this design stage, with a note on what will change by technical design.

A Whole Life Carbon Assessment written that way survives scrutiny. One that presents a single number against an unnamed benchmark does not, and it gives a consultee an easy reason to ask for the work again.

Design decisions that move the number most

The order is fairly consistent across building types.

1. Retaining an existing structure instead of demolishing, which avoids the substructure and frame entirely and is normally the largest single saving available. A Pre-redevelopment Audit establishes what can be kept before anything is stripped.
2. Avoiding or reducing basements, since excavation, retaining walls and piling carry high carbon for little floor area.
3. Structural efficiency, meaning a sensible grid, shallower spans and fewer transfer structures, which usually beats material substitution.
4. Cement replacement and steel with high recycled content, with product specific data rather than generic factors.
5. Facade strategy, where glazing ratio, secondary framing and replacement cycles interact.
6. Reducing fit-out and services quantities, and designing for a longer replacement interval.

Circular economy thinking overlaps with all of these, and in London it is a separate document. A Circular Economy Statement records the reuse and recovery commitments that sit behind the numbers.

Reporting at later design stages

A planning stage figure is an estimate built on outline quantities. Say so, and set out when it will be updated.

The normal sequence is an assessment at planning, an update at technical design when the structural scheme and the main products are fixed, and a post-construction assessment using as-built quantities and the actual products procured. Where a condition secures that sequence, it has to fix the methodology, the module scope and the area basis, or the later figures will not be comparable with the approved one.

PlanScope drafts whole life carbon and circular economy documents structured to UK planning policy, and flags the quantities and product data it cannot verify rather than inventing them. Councils usually expect a suitably qualified author for carbon assessment work and professional review is strongly advised.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.