Reporting modules C and D in a carbon assessment

Modules C and D in a carbon assessment are the two most commonly mishandled parts of the calculation, and the mistakes pull in opposite directions. Module C, the end of life stage, is often left out or filled with default assumptions nobody has examined. Module D, benefits and loads beyond the system boundary, is often added into the total, which it must never be. Report module D inside the headline figure and the assessment overstates the scheme's performance by crediting it for something that happens outside the building's life.

Both modules reward a reviewer's attention precisely because they are where the assumptions live.

What the end of life modules cover

Module C is split four ways. C1 covers deconstruction and demolition of the assessed building at the end of the study period. C2 covers transport of the resulting materials. C3 covers waste processing for reuse, recovery or recycling. C4 covers disposal of whatever is left.

The distinction that catches teams out is between C1 and the demolition you are doing now. Demolition of an existing building to make way for the proposed one belongs in the construction stage of the new building's assessment, in the pre-construction part of module A5 under the current professional methodology, rather than in module C. C1 is a future event: the eventual removal of the building you are proposing.

That matters for comparisons. A retrofit option against a demolish-and-rebuild option differs mainly in upfront carbon, including the demolition of what is there, and treating today's demolition as an end of life figure moves it to the wrong end of the assessment.

Why module D sits outside the totals

Module D accounts for benefits and loads beyond the system boundary: the avoided impacts of materials recovered from the building at end of life, and exported energy from on-site generation over the study period. It is reported separately from the whole life total under the standard and under the professional methodology that implements it.

The logic is a boundary question rather than an accounting preference. The benefit in module D accrues to someone else's building or someone else's grid, in a future with assumptions nobody can verify, so including it would let a scheme claim a reduction it has not delivered. Report it, label it, keep it out of the total.

Recycling and recovery assumptions

Every module C figure rests on a disposal scenario: what proportion of each material is reused, recycled, recovered or landfilled at end of life, and how far it travels. Those proportions are assumptions about the construction industry sixty years from now.

State them. Three rules keep this defensible. Use current recovery rates rather than aspirational future ones, because current rates are evidenced and future ones are not. Apply the same scenario across options being compared, so the comparison is not decided by a scenario change. And where a material has a demonstrably different route, such as structural steel sections designed for reuse, say what makes it different and what secures that outcome.

Steel and aluminium carry the largest module D credits, which is the reason the module is so often pulled into the total. Concrete is usually crushed for aggregate, which is recovery rather than recycling and attracts a much smaller credit. Timber scenarios vary with the recovery route assumed, and an assumption of energy recovery produces a very different answer from an assumption of reuse.

Demolition and deconstruction carbon

C1 is small on most buildings and nobody argues about it. Where it does matter is on heavily serviced or structurally unusual buildings, and on anything where deconstruction rather than demolition is being claimed as the end of life route.

A deconstruction claim needs something behind it. Bolted rather than welded connections, accessible fixings, a materials passport, and a design that allows elements to come apart in the order they went together. Without those, a deconstruction scenario is a sentence in a report rather than a property of the building.

For the demolition you are carrying out now, the evidence sits in the existing building survey. A Pre-redevelopment Audit records what is there, its condition and what can be reused, which is the document that supports both the upfront demolition figure and any reuse claim.

Avoiding double counting with reuse claims

Reuse is reported in two places for two different things, and conflating them is the most common error in this part of an assessment.

Material reused from the existing building in the new building reduces the product stage figure, because less new material is bought. The transport and processing needed to prepare it still has to be counted. That is a module A effect.

Material leaving the building at end of life and going into someone else's project is a module D effect. It is a credit outside the total.

What cannot happen is claiming both for the same material, or claiming a recycled content reduction in the product stage while also claiming an end of life recycling credit computed as though the material were virgin. Where a Circular Economy Statement also quantifies reuse and recycled content, the two documents have to use the same numbers and the same boundaries, because a reviewer will read them together.

Presenting the modules so they are comparable

1. Give a table of all modules, with figures in kilograms of carbon dioxide equivalent per square metre of gross internal area and in absolute tonnes.
2. State the study period, the floor area basis, the methodology edition and the date of the calculation at the top of the table rather than in an appendix.
3. Subtotal upfront carbon, the use stage and the end of life stage separately, then give the whole life total.
4. Report module D on its own line below the total, with a label saying it is excluded from it.
5. Set out the end of life scenario in a short table of recovery proportions by material.
6. Where options are compared, use one page per option with identical row headings.

That layout takes an afternoon and saves a round of questions. Officers and reviewers are looking for the upfront figure against whatever benchmark applies, and anything that makes them hunt for it invites a request for clarification.

Common reporting errors

Module D added into the whole life total, usually by a spreadsheet that sums a row too many. Module C omitted entirely, with a note saying it is not material, which is a conclusion rather than a calculation. Today's demolition reported in C1 instead of in the construction stage. Recovery rates set at optimistic future levels with no statement that they are assumptions. Different scenarios applied to the options in a comparison. And a reuse claim counted twice, once as reduced product carbon and once as an end of life credit.

The last one is worth a specific check before submission, because it is the error that most often survives into a committee report. Where a draft of a Whole Life Carbon Assessment is generated from structured inputs, the honest discipline is to leave the scenario assumptions visibly flagged rather than populated with plausible defaults, so the assessor filling them in knows what they are signing.

Councils usually expect a suitably qualified author for this work and professional review is strongly advised. Reports should be reviewed before submission, and PlanScope does not guarantee planning approval.

To see which supporting documents a scheme is likely to need alongside the carbon work, enter your postcode on PlanScope.