Timber sequestration claims in whole life carbon reporting
Sequestered carbon in timber is reported, not claimed. Where it belongs in the modules, what sourcing evidence a council expects, and the assumptions that get assessments sent back.
Timber sequestration in a carbon assessment is reported, not claimed. The carbon a growing tree took out of the atmosphere is recorded as a negative figure in the product stage, and the same quantity is recorded as a release at end of life unless you can evidence that the material stays in use. Net it off once and your whole life carbon assessment reads as a strong result. Report it properly and a cross laminated timber frame still beats a concrete equivalent, just by a smaller margin and for reasons a reviewer can follow.
Reviewers at the Greater London Authority and at larger authorities have become quick at spotting the difference. The assessments that get sent back are almost always the ones where biogenic carbon has been used to flatter the upfront number.
Why sequestration is reported separately
Whole life carbon assessment follows the modular structure in BS EN 15978, which the RICS professional standard on whole life carbon assessment, second edition, applies to buildings in the UK. Modules A1 to A3 cover product manufacture, A4 and A5 construction, B the in-use period, C the end of life stages and D benefits and loads beyond the system boundary.
Biogenic carbon sits awkwardly in that structure because it moves. It enters the system in A1 to A3 as stored carbon in the material, and it leaves in C3 or C4 when the material is burned, decomposes or is landfilled. The convention is to record both, with the storage shown as a negative value in the product stage and the release as a positive value at end of life. Over the reference study period, usually taken as 60 years, the two cancel unless the timber demonstrably remains in use beyond it.
That is why the standard asks for biogenic carbon to be reported as a separate line rather than blended into a single figure. A reviewer needs to see the stored quantity, the released quantity and the net position, and needs to be able to recalculate each.
Biogenic carbon accounting conventions
Three conventions do most of the work:
- Stored carbon is calculated from the dry mass of the timber and its carbon fraction, converted to carbon dioxide equivalent, and reported as negative in A1 to A3.
- The corresponding release is reported in C3 or C4 according to the end of life scenario you have assumed, with the scenario stated.
- Avoided emissions from energy recovery, reuse or recycling are reported in Module D only, and never deducted from the modules A to C total.
The third is the one that gets abused. Module D is reported separately precisely because the benefit falls outside the building's own life cycle, so an assessment that presents a single headline figure including Module D is not comparable with anything.
The GLA's whole life carbon guidance for referable applications sets out the reporting template, the modules required and the benchmarks against which results are judged, and it changes periodically, so work from the current version rather than from a template inherited from a previous project. A Whole Life Carbon Assessment is a report type councils expect to come from a suitably qualified author, and professional review is strongly advised before it is submitted.
Sourcing evidence a council will accept
A sequestration figure is only defensible if the timber is legally and sustainably sourced, because the accounting convention assumes the forest is regrowing. Reviewers look for chain of custody certification under a recognised scheme, named in the assessment with the certificate reference, and a statement of the species and country of origin.
What gets rejected is a generic sustainability paragraph with no certificate, or a specification that leaves the sourcing to the contractor's discretion at a later stage. If the frame has not been procured at planning stage, say so and secure the standard by condition or in the specification, rather than reporting a sequestration benefit you cannot yet substantiate.
Due diligence obligations on timber placed on the UK market sit outside the planning system but the paperwork overlaps, so the procurement team usually already holds what the assessment needs. Ask for it before the assessment is written, not after a reviewer queries it.
End of life assumptions for timber
The end of life scenario drives the C modules, and it is where honest assessments differ from optimistic ones.
| Assumed end of life | Where it is reported | What a reviewer will ask for |
|---|---|---|
| Landfill | C4, with release of stored carbon and methane factors | Justification of the scenario against current waste routes |
| Energy recovery | C3 release, with avoided grid emissions in Module D only | Evidence the benefit is not double counted |
| Reuse of components | C3 processing, with the benefit in Module D | Demountable connection details and a materials passport |
| Retained in a longer life building | Reduced release within the study period | Design life evidence and the pre-redevelopment position |
Claiming reuse without demonstrating demountability is the most common weakness. Glued and screwed panels with embedded services are not realistically recoverable, and a connection strategy drawn at planning stage is what makes the reuse case credible. The Circular Economy Statement is the natural place to carry those details, and the two documents should agree with one another.
Comparing frames on a consistent basis
Frame comparisons are useful and frequently misleading. To compare a timber option against steel or concrete, hold five things constant: the functional unit and floor area measurement basis, the reference study period, the module scope reported, the data sources used for product data, and the treatment of substructure. Change any one of them and the comparison breaks.
An illustrative example. A four storey residential block is tested as a cross laminated timber frame and as a reinforced concrete frame. Reported on modules A1 to A5 with biogenic carbon excluded from the headline and shown separately, the timber option is materially lower on upfront carbon, mainly from reduced concrete volume rather than from sequestration. Add the C modules and the gap narrows, because the stored carbon is released. Add Module D and the timber option looks stronger again, which is exactly why Module D is quarantined. This example is illustrative and no figures should be carried across to a real scheme.
Substructure deserves a specific mention. A lighter frame often allows a smaller foundation, and that saving is real, reportable and frequently left out because the structural engineer priced the superstructure first. Ask for the comparison to include it.
Claims that will not survive review
A short list of statements that reviewers routinely strike out:
- A negative upfront carbon figure produced by including biogenic storage in A1 to A3 without the matching C module release.
- A net zero embodied carbon claim resting on Module D.
- Sequestration reported for timber with no named certification scheme or country of origin.
- An assumed 100 year design life used to push the end of life release beyond a 60 year study period, with no evidence for the longer life.
- A sequestration figure derived from volume without stating moisture content and density.
- Carbon storage described as permanent.
None of these are fatal to a timber scheme. They are fatal to the assessment, which then has to be redone, usually at the point in the programme where there is least time.
Presenting the figures honestly
Lead the assessment with modules A1 to A5 excluding biogenic carbon, give the modules A to C total, then report biogenic storage and release as their own clearly labelled lines, then Module D separately. State every assumption in a table a third party can rerun: densities, moisture content, carbon fraction, transport distances, end of life routes and the data source for each material.
And say what you do not know. An assessment that flags the frame supplier as unconfirmed, with the sourcing standard secured by specification, is stronger than one that quietly assumes a certificate. PlanScope can draft the narrative, assumptions register and policy sections of a whole life carbon report to that structure, inserting flags where site or product data is missing, and the draft is then for your qualified assessor to complete and sign. The same applies to the Sustainability Statement that usually accompanies it. For how the energy and carbon documents divide up on a large shed, see our note on energy statements for warehouse and logistics schemes.
This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. For specialist technical reports, qualified professional input may be required.
PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
Can I report a negative upfront carbon figure for a timber building?
Only if you also report the matching release in the end of life modules and show biogenic carbon as a separate line. Presenting a negative A1 to A3 figure that includes stored carbon, without the corresponding C3 or C4 release, is the single most common reason a whole life carbon assessment is sent back. Report the modules separately and let the reviewer see both halves.
Where does Module D fit in a timber assessment?
Module D covers benefits and loads beyond the system boundary, such as avoided emissions from energy recovery or component reuse. It is reported separately and is never deducted from the modules A to C total. An assessment that presents one headline figure including Module D cannot be compared with any other scheme, which is why reviewers quarantine it.
What sourcing evidence do councils want for sequestration claims?
A named chain of custody certification scheme with the certificate reference, plus species and country of origin stated in the assessment. A general sustainability paragraph is not enough. If the frame has not been procured at application stage, say so and secure the standard through the specification or a condition rather than reporting a benefit you cannot yet substantiate.
How long a study period should I use?
Sixty years is the usual reference study period in UK practice, and using a longer design life to push the end of life release outside the period needs real evidence rather than an assertion. If you do assume a longer life, state the basis and show the sensitivity of the result to that assumption so a reviewer can test it.
Does a timber frame still perform better once biogenic carbon is reported properly?
Usually yes on upfront carbon, but for a different reason than people assume. The saving comes mainly from using less concrete and from a lighter substructure, not from sequestration. Once the end of life release is included the gap narrows, so make the structural and foundation savings explicit rather than relying on the stored carbon line.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.