Comparing refurbishment and rebuild on whole life carbon

A retrofit versus demolition carbon comparison usually turns on one number: the embodied carbon locked into the existing structure and substructure. Keep a sound frame and foundations and you avoid the largest single slice of a new building's upfront emissions, which is why a refurbishment option tends to win on modules A1 to A5 even where the new building performs better in use. Whether it wins overall depends on the reference study period, the operational performance gap between the two options, and what each option actually delivers.

Councils increasingly ask for the comparison before they will accept demolition. Getting it wrong in either direction costs an applicant months.

Why the comparison is now asked for

Two things drove this. Policy caught up with the recognition that upfront emissions are spent at the point of construction and cannot be recovered later, and the methodology matured enough to be assessed by an officer.

The RICS Whole Life Carbon Assessment for the Built Environment, second edition, is the standard reference in England, built on the modular structure of BS EN 15978. In London, policy SI 2 of the London Plan requires whole life carbon assessments for referable applications, with the Greater London Authority's own guidance setting the reporting format and benchmarks. Outside London, requirements vary by authority and are often expressed through a local plan policy on sustainable design rather than a dedicated carbon policy, so read the adopted plan rather than assuming the London approach applies.

The National Planning Policy Framework in its August 2026 edition supports the transition to a low carbon future and expects plans and decisions to help shape places that reduce greenhouse gas emissions. It does not set a carbon budget for a building. The quantum comes from local policy, so cite the policy you are actually being assessed against.

Setting the two options on a common basis

Most weak comparisons fail here rather than in the arithmetic. The two options have to be comparable in scope before the numbers mean anything.

The awkward part is that the two options rarely deliver the same thing. A refurbishment option usually yields less floorspace, a different unit mix and a shorter remaining structural life. Reporting only per square metre flatters the larger new build, and reporting only in absolute terms flatters the smaller refurbishment. Report both and explain the difference in what is delivered, because an officer who spots the normalisation trick stops trusting the rest.

Where the structure dominates the result

Substructure, frame and upper floors are typically the largest contributors to upfront embodied carbon in a new building, with concrete and steel doing most of that work. Retaining them is the move that changes the answer.

Partial retention is where the interesting engineering sits. Keeping the foundations and the core while replacing the facade, or keeping the frame and adding a lightweight extension, captures much of the saving without accepting the constraints of the existing envelope. That needs a structural appraisal early, because the question is not whether the frame stands up today, it is whether it can carry a changed loading pattern and achieve the required floor-to-floor dimensions.

A Structural Survey at feasibility stage costs less than discovering at technical design that the slab cannot take the new plant. And where the existing building is to come down in part, a pre-demolition audit records what can be reused or recycled, which the Pre-redevelopment Audit page describes.

Operational carbon over the study period

The counter-argument for a new building is operational performance: better airtightness, continuous insulation, mechanical ventilation with heat recovery, and no cold bridging at junctions that a retrofit cannot reach. Over 60 years that gap accumulates.

Two things temper it. The grid has decarbonised substantially and is projected to continue, which shrinks the carbon value of each saved kilowatt hour over the study period, so an assessment that holds grid intensity constant overstates the new build case. And a deep retrofit can close much of the fabric gap, particularly where external insulation is possible and the existing fabric is solid rather than framed.

The honest position is that the result is sensitive to assumptions. Run the sensitivity analysis, show the result under a decarbonising grid and under a flat one, and state which assumption carries the conclusion. Our note on thermal bridging detail at planning stage covers the detail that most often separates predicted from delivered performance in a retrofit.

Illustrative example: an office building from the 1980s, structurally sound, being compared against a replacement with 30 per cent more floorspace. The refurbishment avoids most of the substructure and frame emissions. The new building performs better in use, but under a decarbonising grid the operational saving over 60 years recovers only part of the upfront difference. Per square metre the two options converge, because the new scheme spreads its emissions over more area. The decision then rests on whether the extra floorspace is needed, which is a planning judgement rather than a carbon one. These figures are illustrative.

What the carbon answer does not decide

Carbon is one material consideration among several, and a strong carbon case does not carry a weak scheme.

• Housing or employment floorspace delivered, and whether the site is being used efficiently.
• Design quality, heritage significance and townscape effects, where the existing building contributes positively.
• Daylight and sunlight to neighbours and to the proposed units.
• Biodiversity net gain, which is calculated from the site's habitats rather than from the building.
• Viability, where retention carries abnormal costs or reduces the developable area.
• Occupational suitability, including floor-to-ceiling heights, servicing and accessibility.

Where retention would deliver materially fewer homes on an allocated site, expect that to weigh against it. Where the existing building is a non-designated heritage asset, expect it to weigh in favour. These pull against each other, and the officer's report has to balance them rather than apply a carbon test.

Evidence that an option was properly tested

An options appraisal that reads as a justification written after the decision will be treated as one. What holds up is a structured appraisal prepared before the scheme was fixed.

1. A condition survey and structural appraisal of the existing building, with the retention capacity stated.
2. A stated set of options, including at least one substantial retention option and one partial retention option.
3. Whole life carbon results for each option on the common basis set out above, module by module.
4. A sensitivity analysis on the grid intensity, the study period and the services replacement cycles.
5. The floorspace, unit numbers and mix delivered by each option.
6. Cost and viability implications where retention is being rejected on those grounds.
7. A clear statement of which non-carbon considerations the applicant says outweigh the carbon result.

The carbon work belongs in a Whole Life Carbon Assessment, with the material reuse and recycling strategy in a Circular Economy Statement where local policy requires one.

Presenting the conclusion without overclaiming

Write the conclusion as a comparison with stated assumptions, not as a verdict. Give the absolute totals and the per square metre figures side by side, name the study period, and say in one sentence which assumption would reverse the result. Officers trust an assessment that identifies its own weak point.

Avoid single headline percentages. A claim that one option saves a given percentage of carbon invites a challenge on the boundary conditions, and the challenge usually succeeds because the boundary was chosen by the applicant.

You can see which supporting documents a scheme is likely to need, including the energy and carbon set, by entering the postcode on PlanScope.

This is general guidance, not legal advice. Assessment methodologies and local policy requirements change, so confirm the current RICS edition and your authority's adopted policy before relying on this. For specialist technical reports, qualified professional input may be required.

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