Embodied carbon in retention against demolition

The retention against demolition argument is won or lost on how the comparison is framed, long before anyone calculates a carbon figure. Compare a deep retrofit of the existing building with a new build of the same floor area on the same site, over the same study period, delivering the same functional performance, and you have an options appraisal a planning committee can use. Compare a modest refurbishment against a larger, better new building and you have two different projects with two different carbon totals, which proves nothing.

Authorities have become quick at spotting the second version. A growing number of adopted plans and supplementary documents expect a retrofit first case, and the ones with teeth ask for the appraisal before they ask for the carbon number.

Framing the comparison so it means something

Fix four things at the start and state them in the assessment.

The functional unit. Floor area, or usable area, or capacity measured some other way. If the new build delivers a third more area, the comparison has to normalise for that or present both absolute and normalised figures.

The study period. Sixty years is the convention in the recognised whole life carbon methodology, and using anything else needs a reason.

The modules assessed. Upfront embodied carbon in modules A1 to A5, in use emissions in the B modules including replacement cycles, end of life in the C modules, and benefits beyond the boundary in module D reported separately rather than netted off.

The options. Three or four, running from light refurbishment through deep retrofit and part retention to full demolition and rebuild. Two options looks like a case being built rather than a question being asked.

What retention typically saves

The savings sit in the substructure and the frame. Foundations, basements, slabs and structural frames are heavy, material intensive and carbon intensive, and they dominate the upfront embodied carbon of most buildings. Keeping them avoids the demolition emissions, the transport and disposal of the arisings, and the manufacture of their replacements.

Facade retention saves less than people assume, because facades are usually replaced or heavily reworked anyway for thermal performance, and the temporary works needed to hold a retained facade carry their own emissions.

The other side of the ledger is that retention rarely means keeping everything. A realistic retrofit involves new services throughout, a new envelope, new cores in many cases, and structural strengthening where loading or stability requires it. The Whole Life Carbon Assessment has to include all of that alongside the elements avoided.

The operational penalty, and why it is shrinking

The standard argument for demolition is that the existing building performs poorly and always will, so the operational emissions over 60 years outweigh the embodied carbon saved by keeping it.

That argument has been weakening, and the reason is grid decarbonisation. As the electricity grid decarbonises, the emissions attributable to operating an electrically heated building fall, which compresses the operational difference between a good retrofit and an excellent new build. Embodied carbon, by contrast, is spent now at today's emissions intensity. An appraisal that applies a flat grid factor across the whole study period will overstate the operational case for demolition, and a reviewer who knows the methodology will say so.

State the grid decarbonisation assumption explicitly, with the source and the trajectory used, and run a sensitivity case. That single paragraph does more for credibility than another decimal place on the embodied figure.

The constraints that do favour rebuild

There are buildings where retention is the wrong answer, and the appraisal is stronger for saying so clearly.

• floor to ceiling heights too low to accommodate modern services and daylight requirements without losing storeys
• structural capacity insufficient for the intended loading, where strengthening approaches the cost and carbon of replacement
• deleterious materials or extensive asbestos requiring removal that compromises the structure
• a grid or core arrangement that cannot deliver accessible, efficient layouts
• a fabric whose condition means a shortened remaining life even after intervention

Evidence each of these with a Structural Survey and a condition survey rather than with assertion. A structural engineer's statement that a frame cannot take the additional load is evidence, and a developer's view that the building is tired is not.

The weakest ground is viability dressed as constraint. If the real reason is that retention delivers less area, say so and argue it on planning terms, because a committee will find the inconsistency between the technical case and the scheme drawings.

Presenting the appraisal

The document that works is short and sequential: the options considered, the assumptions common to all of them, the whole life carbon results by module for each, the operational performance of each, the planning benefits and harms of each, and a reasoned conclusion. Put the detailed calculation in an appendix.

Include a pre-demolition audit of the existing building so the materials that could be reused or recycled are quantified, which is what a Pre-redevelopment Audit covers, and set out the circular economy measures for whichever option is preferred. In London, a Circular Economy Statement is required for referable development and the expectations are set out in Greater London Authority guidance.

Where the scheme also generates significant new infrastructure, the carbon embedded in roads, drainage and utilities belongs in the picture as well, a point covered in our note on carbon in the infrastructure a scheme has to build.

Illustrative example: a 1970s office building with a concrete frame, 2.6 metre floor to ceiling heights and a failing curtain wall. The appraisal compares a deep retrofit retaining frame and substructure against full redevelopment delivering 30 per cent more floor area. Retention shows materially lower upfront embodied carbon, the retrofit achieves operational performance within a few per cent of the new build once the grid trajectory is applied, and the difference in area is argued separately as a planning matter. Illustrative only.

Evidence that survives committee

Three things make the difference when the report reaches a committee.

An independent review of the whole life carbon assessment, commissioned by the applicant or the authority, removes the suspicion that the numbers were chosen. Some authorities now require it for major applications.

Consistency between documents. If the structural report says the frame is sound and the carbon appraisal assumes replacement of the frame in a retention option, someone will notice.

And a conclusion that engages with the answer the analysis produced. An appraisal that runs the numbers, finds retention preferable on carbon, and then recommends demolition for reasons that were never assessed, is the version that gets deferred.

One honest uncertainty. There is no single national policy test that requires retention, and the weight given to embodied carbon still varies between authorities. What has changed is the expectation that the question is asked properly, with a methodology a reviewer can follow.

This is general guidance rather than legal advice. Check the position with your local planning authority or a qualified professional before relying on it, and note that whole life carbon assessment work should be prepared by a suitably qualified assessor.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. Browse the report types to see the documents a redevelopment case usually needs.