A lower carbon figure does not necessarily mean a lower-carbon building. It may mean the assessment counted different things.
That is the practical challenge raised by a new RICS analysis published on 14 October. Whole life carbon assessments are already used to direct investment, inform building design and report emissions. Yet differences in how they are conducted can undermine the comparisons those decisions depend on.
For property investors, developers and procurement teams, this creates a question that belongs alongside every headline result: what, exactly, are we comparing?
Why the same project can produce different answers
RICS identifies two broad sources of inconsistency: differences in methodology and differences in input values.
Methodological differences can change the assessment’s scope, calculation rules and reporting structure. One approach might include external works while another excludes them. Different methods may apply uncertainty factors differently or report biogenic carbon separately. Even when total emissions are similar, the way results are presented can make them appear different.
Using the same methodology does not remove every source of variation.
Assessors may still use different assumptions about material replacement, transport distances or future energy consumption.
They may also select different carbon factors—the values used to translate quantities of materials, energy or activity into emissions.
The implication is straightforward: comparing headline totals without examining their foundations can reward differences in accounting rather than differences in environmental performance.
Project stage changes the evidence
An early design assessment and a completed-building assessment do not have access to the same information.
RICS explains that early assessments may rely on estimated material quantities and generic carbon factors. As design progresses, more precise quantities and project-specific information should become available. After completion and during operation, measured data should increasingly inform the assessment.
This creates an important distinction between design improvement and information improvement.
If a project’s reported footprint changes, decision-makers should ask whether the building changed, the evidence improved, or both. Otherwise, a revised estimate could be mistaken for an emissions reduction.
A better brief for carbon assessments
The response should not be to abandon whole life carbon assessment. RICS argues for correctly applied methodologies, established conventions and greater transparency about inputs and professional judgement. It also identifies software validation and collaboration between standard-setting organisations as ways to improve consistency.
For organisations commissioning assessments, a practical starting point is a common assessment brief.
Before comparing projects or design options, ask for:
The methodology and version used.
The assessment boundary, including exclusions.
The project stage and source of quantity data.
Significant assumptions about service life, replacement and energy use.
The carbon-factor sources and reasons for their selection.
An explanation of material changes since the previous assessment.
This checklist is BSustainable Today’s suggested procurement response to the inconsistencies RICS describes—not a substitute for the applicable professional standard.
What investors should do with the results
Carbon assessments are more useful when decision-makers can distinguish a genuine design advantage from a methodological difference.
For an investment committee, that means requesting an explanation of comparability before accepting a ranking of assets. For a developer, it means maintaining a record of changes as estimates become more detailed. For procurement teams, it means aligning assessment requirements before bids arrive.
RICS also stresses that whole life carbon assessment remains a technical skill requiring professional judgement. Greater consistency does not eliminate the need to interpret requirements, identify anomalies and understand context.
The next step for construction decarbonisation is therefore not simply more carbon calculations. It is better-controlled comparison.
A credible carbon number should explain both the building and the choices made to measure it.