Setting the scope boundary of a Whole Life Carbon Assessment

The boundary decides the number. Before any material quantity is entered, a whole life carbon assessment scope has to fix which life cycle modules are included, how long the building is assumed to last, which building elements are counted and where the site boundary is drawn. Two competent assessors modelling the same scheme can report figures a third apart, and the difference is almost always scope rather than calculation.

Planning authorities know this. The GLA requires whole life-cycle carbon assessments for referable applications in London and publishes a reporting template precisely so that submissions can be compared, and other authorities increasingly follow the same structure.

The modules and what each covers

The module structure comes from the European standard for assessing the sustainability of construction works, and the recognised professional methodology builds on it.

The product stage covers raw material supply, transport to the factory and manufacturing. It is usually the largest single block for a new building and the one with the best data.

The construction process stage covers transport to site and the site activities themselves, including pre-construction demolition, plant and temporary works, and the treatment of construction waste. Demolition of an existing building on the site sits here, which matters enormously for a redevelopment and is the module most often left out of an early assessment.

The use stage splits into the fabric-related modules covering maintenance, repair, replacement and refurbishment, and the operational modules covering energy and water in use. Whole life carbon includes both. Embodied carbon covers the fabric-related modules and excludes operational energy, which is why the two figures must never be compared.

The end of life stage covers deconstruction, transport, waste processing and disposal at the end of the study period.

Beyond the system boundary sits the module reporting benefits and loads outside the building's life, such as recycling potential and exported energy. It is reported separately and never netted off the total, and an assessment that subtracts it to reach a headline figure will be rejected.

The reference study period

The reference study period is the number of years over which replacement cycles and operational impacts are counted. Sixty years is the convention for most buildings under the recognised methodology, unless a different design life is specified, in which case both the period and the reason are stated.

The period drives the replacement modules directly. A carpet with a ten-year service life is counted six times over sixty years and three times over thirty. Choosing a shorter period lowers the total without changing the building, which is why the period is the first thing a reviewer checks.

Service lives for individual components should come from a recognised source rather than from a manufacturer's warranty period, and where a component is assumed to last the life of the building, say so and justify it.

Substructure, external works and the site boundary

Three areas cause most scope disputes.

Substructure is in, and it is heavy. Piled foundations on a poor ground profile can carry more embodied carbon than several storeys of superstructure, and an assessment that reports only above-ground elements is incomplete. Where the ground investigation is not complete at planning stage, model the assumed solution and flag it.

Building services are in. They are also poorly served by product data, and a services allowance based on a benchmark rate rather than a schedule should be declared as such.

External works within the site boundary are in, including hard landscaping, retaining structures, external lighting and drainage. Works outside the site boundary, such as off-site highway works or a utility diversion, are normally outside the assessment but worth mentioning where they are substantial.

Loose furniture and fittings are usually excluded. Say which convention has been applied rather than leaving the reader to infer it.

Data quality and where the numbers come from

Product-specific environmental declarations, prepared to the relevant European standard and verified, are the strongest data source. Sector-average declarations come next. Generic database figures come last and are appropriate for early-stage work.

State the split. An assessment where 80 per cent of the product-stage carbon comes from generic data is a different document from one built on verified product declarations, and at planning stage the former is normal and acceptable provided it is labelled.

Quantities matter as much as factors. At planning stage the model is usually built from area schedules and assumed build-ups rather than from a measured bill, so record the source of each quantity and the assumed build-up for each element. A reviewer can accept an assumption they can see.

Where a figure is a placeholder, mark it. PlanScope's drafts insert a flag wherever a site-specific fact is missing rather than filling the gap with a plausible number, and a Whole Life Carbon Assessment prepared this way makes those gaps visible to the reviewer.

Retention against redevelopment

The boundary question that matters most in planning is whether the assessment allows a fair comparison between keeping the existing building and replacing it.

A retention option carries the embodied carbon already in the structure at zero cost to the assessment, plus the carbon of the works needed to bring it up to standard, plus a use stage that may be less efficient. A redevelopment option carries demolition, a new substructure, a full new structure and a more efficient use stage. Getting that comparison right depends on both options being assessed over the same study period, with the same modules and the same data sources.

Where an authority's policy asks for the retention option to be tested, present it as a like-for-like table rather than as narrative. And where the comparison favours redevelopment, say which modules drove it, because a conclusion asserted without the module breakdown behind it tends to attract a request for further information.

Reporting so the assessment can be compared

Report by module and by building element, both as totals and as an intensity per square metre of gross internal area, with the area definition stated.

Name the study period, the modules included, the elements included and excluded, the data sources and their proportions, and the assumptions behind service lives. Where a benchmark is cited, cite the publishing organisation and the version, because benchmark figures are updated and an unattributed number tells the reader nothing.

Then say what the assessment led to. The purpose of the exercise in planning terms is to demonstrate that carbon has been reduced through design decisions, so record the options tested and the effect of each, including options rejected. A report that shows a single figure with no iteration reads as compliance rather than design.

The relationship between this document and the operational energy case belongs in the Energy Statement, and the material reuse and disassembly narrative belongs in the Circular Economy Statement. Writing all three from one model, with consistent assumptions, is the practical way to avoid contradictions a case officer will notice.

Omissions reviewers find most often

1. Pre-construction demolition excluded on a redevelopment site.
2. Substructure excluded or modelled without any ground information.
3. Benefits beyond the system boundary netted off the headline figure.
4. A study period shorter than the convention, used without justification.
5. Replacement cycles based on warranty periods rather than service life data.
6. External works and site infrastructure omitted entirely.
7. Gross internal area used for the intensity figure without stating the definition.

Every one of those is a scope failure rather than a modelling error, and every one is avoidable by writing the boundary statement before the model is built.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

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