Embodied carbon in a retrofit against a new build

Authorities asking whether a building should be retained rather than replaced want a carbon comparison, and the comparison is only meaningful if both options are assessed over the same study period with the same boundaries. The modules that differ are the product and construction stages for new materials, the end of life stage for whatever is demolished, the replacement cycles over the study period, and operational energy. Everything else tends to cancel out. A Whole Life Carbon Assessment that sets those four out side by side answers the question. One that reports a single total for each option does not.

The retained structure usually wins on embodied carbon and loses on operational carbon. How large each effect is depends on the building, and on how quickly you think the electricity grid decarbonises.

Why the comparison is now asked for

Two things drive the request. The London Plan requires whole life carbon assessment for referable applications and circular economy statements that address retention and reuse, and a number of authorities outside London have adopted retrofit first policies or supplementary guidance to similar effect. Separately, demolition of a serviceable building has become a material consideration in its own right, argued on the basis that national policy supports the reuse of existing resources and the mitigation of climate change.

What has not happened is national standardisation. There is no single prescribed method for the comparison, no national benchmark for the result, and no threshold above which demolition is refused. That makes the method you choose part of the argument, which is why stating it clearly matters more than the number it produces.

Expect the question on any scheme involving the loss of a structurally sound building, and expect it earlier than the planning application. By the time a demolition scheme is drawn up, the comparison reads as justification rather than analysis.

Modules that differ between the options

Use the RICS whole life carbon assessment methodology, second edition, and report by module. Four groups do the work.

The product and construction stages, modules A1 to A5, are where new build carries its load. A retrofit also has A stage emissions for new fabric, services, internal works and strengthening, and on a deep retrofit those can be substantial. Reporting a retrofit as near zero in the A modules is the most common error in these comparisons.

The end of life of the existing building, modules C1 to C4, falls on the new build option. Demolition energy, transport and disposal all count, and the treatment of material sent for recycling has to follow the method's rules rather than being credited optimistically.

The use stage replacement cycles, module B4, can go either way. A retained 1960s frame with original windows, roof covering and services will need all three replaced within a 60 year study period, and if the retrofit does not replace them now it has to account for replacing them later.

Operational energy, module B6, usually favours the new build. A new envelope to current standards with heat pumps will use less energy than a retrofitted one with constrained floor to ceiling heights and limited insulation depth, and over 60 years that difference accumulates.

Module D, benefits beyond the system boundary, should be reported separately and never netted off the total. Authorities have become alert to comparisons that rely on module D credits to reach a conclusion.

Structural reuse and its limits

The engineering case decides how much reuse is actually available, and it has to be evidenced rather than asserted.

Load capacity is the first test. An existing frame designed for office loading may not take residential loading plus a new topping, and additional storeys are often where the case breaks. Testing tells you where you stand: concrete coring and carbonation testing, reinforcement scanning, steel sampling for weldability, and a search for the original design records.

Geometry is the second test. Floor to floor heights set by an earlier generation of services limit what can be achieved within them, and a 2.6 m slab to slab office floor will not deliver a compliant residential ceiling height with a new raised floor and ceiling void.

Deleterious materials are the third. High alumina cement, woodwool shuttering, asbestos in structure and cladding fixings, and reinforcement corrosion all push a scheme towards replacement, and all of them need to be found before the options are compared rather than after.

A pre-redevelopment audit records what is in the building and what could be reused or recovered, and it should be done early enough to inform the options appraisal. Done late, it becomes a demolition inventory.

Operational carbon against embodied savings

The crossover point is the question that decides most of these comparisons, and it is sensitive to one assumption: the carbon intensity of grid electricity over the study period.

Use a declining grid factor rather than a flat one, state the source of the trajectory, and run the comparison with at least one alternative trajectory as a sensitivity test. A retrofit that looks better under a rapidly decarbonising grid and worse under a static one is telling you something useful, and saying so is more persuasive than presenting one answer.

Two further points affect the balance. Retrofit avoids the emissions now, in the part of the carbon budget that matters most for near term targets, while operational savings accrue slowly over decades. And a retrofit that fails on overheating or ventilation will be rectified later at a carbon cost nobody modelled, so the overheating assessment and the ventilation strategy belong in the comparison. The modelling outputs that support all of this are the same ones covered in SAP and SBEM outputs a planning officer will ask for, and the electrical capacity question behind an all-electric retrofit is the one set out in peak electrical demand and all electric heating.

Evidence a retrofit first policy requires

Where a policy requires retention to be considered first, the authority is looking for an options appraisal rather than a conclusion. A defensible one contains the following.

1. At least three options: light touch retrofit, deep retrofit including extension or additional storeys, and demolition with new build, each described in enough detail to be costed and modelled.
2. The same study period, functional unit and system boundary for each, with the floor areas stated per option.
3. A structural appraisal with testing results, not a desktop opinion, covering capacity, geometry and deleterious materials.
4. Whole life carbon results by module for each option, with module D reported separately.
5. Operational energy modelling for each option on a consistent set of assumptions.
6. A statement of what the scheme delivers in planning terms under each option, including floorspace, housing numbers, accessibility and daylight to existing occupiers.

That last item is where the honest argument sits. If demolition delivers 40 more homes and retrofit does not, that is a planning judgement for the authority to make against the carbon difference, and presenting it openly is better than burying it.

Where demolition is still justified

Demolition remains the right answer in several situations, and a comparison that never reaches that conclusion for any building is not credible.

A structure at the end of its service life, or one with widespread reinforcement corrosion or deleterious materials, is a replacement case. A building that cannot be made safe or accessible without effectively rebuilding it is a replacement case. A site whose allocated capacity cannot be delivered within the existing envelope is a planning judgement rather than a carbon one, and should be argued as such.

What does not work is an assertion that retention is unviable without the figures, or a structural letter saying the frame is unsuitable without the testing behind it. Both invite a holding objection and both are easy to fix at the right stage.

Presenting the comparison fairly

Four habits make the comparison credible to an officer.

Report module by module in a table, by option, per square metre and in absolute terms, so a reviewer can see where the difference arises. State every assumption that could reverse the conclusion, including the grid trajectory, the study period, the replacement cycles and the treatment of recycling. Keep the floor areas consistent with the accommodation schedule and the Circular Economy Statement. And date the assessment, naming the version of the methodology used.

One uncertainty worth admitting. Embodied carbon factors for the same product vary between data sources by more than the margin between many retrofit and new build options, which means a comparison built on generic data cannot be treated as decisive. Say which data source you used, prefer product specific declarations where they exist, and present the result as a range rather than a point.

This is general guidance, not legal advice. Whole life carbon and structural appraisal work are specialist disciplines and councils usually expect suitably qualified authors, so professional review is strongly advised before submission.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.