Substructure and basements in whole life carbon

Substructure is the element most often missing from a whole life carbon assessment submitted with a planning application, and on a constrained urban site with a basement it can be one of the largest single contributors to the upfront figure. Piles, pile caps, ground improvement, retaining walls, thicker slabs, waterproofing and the haulage of excavated material are all concrete, steel and diesel, and none of them appear in a report that stops at the superstructure.

The RICS methodology for whole life carbon assessment expects reporting by element and by life cycle module. So when a report arrives with a tidy figure in kilograms of carbon dioxide equivalent per square metre and no substructure line, the correct first question is not whether the number is good. It is what the number excludes.

Why substructure is left out so often

Four reasons, in descending order of honesty.

The design really is incomplete at planning stage. Pile layouts are frequently developed by a specialist contractor after permission, so the structural engineer has indicative loads rather than a pile schedule, and the assessor is working from an allowance.

Data for groundworks is harder to find than data for a facade system. Environmental product declarations exist for concrete mixes and reinforcement, but the quantity side depends on volumes that nobody has yet calculated, and spoil tonnage depends on a cut and fill exercise that has not been done.

Normalisation hides the gap. Reporting in carbon per square metre of gross internal area, where a basement adds area as well as carbon, can flatter a scheme and make the omission harder to spot.

And sometimes the exclusion is simply convenient, because including substructure moves a scheme above a benchmark it was presented as meeting. That last case is the one a reviewer is looking for.

Piling, ground improvement and retaining structures

Three groups of items belong in the substructure line, and each has its own data problem.

Cement replacement is the lever that moves these numbers most. A mix with a high proportion of ground granulated blast furnace slag or fly ash carries materially lower embodied carbon than a plain Portland cement mix, and the assumption made at planning stage should be stated and later secured. Where the assessment assumes a replacement level that the piling contractor has not agreed to, the saving is an aspiration rather than a commitment.

Temporary works count too. Secant pile walls and sheet piling installed to allow excavation and then abandoned are part of the scheme's carbon, and they rarely appear.

Basement excavation and spoil movement

Excavation carbon has two parts: the plant that digs and the lorries that take the material away. Both are reported within the construction modules, and the second edition of the RICS methodology separates demolition and site clearance, construction waste and construction activities so that they can be seen individually rather than hidden inside a single construction figure.

Get the spoil quantity from the engineer's cut and fill calculation rather than estimating from the basement volume, because over-dig, working space and the removal of existing foundations all add to it. Then state the haulage distance and the destination, and distinguish between material disposed of as waste, material taken to a treatment facility and material reused on site.

Contaminated ground changes the arithmetic. Material requiring treatment or disposal to a specialist facility travels further and attracts different handling, and the assessment should say so rather than applying a single haulage assumption across the whole tonnage.

How the omission distorts a comparison

Three comparisons go wrong when substructure is missing, and all three turn up in committee reports.

A basement option compared with a no-basement option looks almost identical if neither includes substructure, when in reality the basement is where the difference sits. A retrofit option compared with a new build looks worse than it should, because the retained existing substructure is the retrofit's biggest advantage and excluding substructure deletes it. And a scheme compared with a published benchmark may be measured against a benchmark that includes substructure, which makes the comparison meaningless in the scheme's favour.

The retrofit case is the one worth protecting, because it is the comparison that most often determines whether a building is demolished. A Pre-redevelopment Audit records what exists and can be retained, and its findings should feed straight into the substructure line of the carbon assessment. Councils usually expect a suitably qualified author for this work and professional review is strongly advised.

Reporting element by element

Present the results as a matrix: elements down the side, life cycle modules across the top. Substructure gets its own row, subdivided where the detail exists, and every cell is either a number or a stated exclusion with a reason.

Add an explicit exclusions list. Reviewers read it first. The common entries are below-ground elements at end of life, which are usually assumed to remain in place, temporary works, and site establishment. Each of those may be a legitimate exclusion under the methodology, and saying so openly is entirely different from leaving a blank.

State the assumptions that carry the most weight: concrete mix and cement replacement level, reinforcement rates, pile quantities, spoil tonnage and haulage distance, and the data sources used for each. A Whole Life Carbon Assessment that lists those five things can be checked. One that does not cannot be relied on, whatever the headline figure says.

Data sources for groundworks

Build the quantities from the design team rather than from benchmarks wherever possible.

1. Ask the structural engineer for a concrete and reinforcement schedule by element, even if it is indicative, with the basis of the estimate stated.
2. Ask for the pile schedule assumption, including diameter, depth and number, and whether a raft alternative was considered.
3. Use product specific environmental product declarations for the concrete mixes actually proposed, and published datasets only where no declaration exists.
4. Take spoil tonnage from the cut and fill calculation, with bulking factors applied.
5. Confirm haulage distances from the proposed disposal and treatment destinations, not from a default.
6. Record which figures are measured, which are estimated and which are allowances.

That last step is the one that makes a report usable later. When the design develops and the assessment is updated to discharge a condition, the distinction between measured and estimated values tells you what changed and why.

What a reviewer asks for first

Working in this order finds the problems quickly.

Does the elemental breakdown include a substructure row with a number in it. If not, the assessment is incomplete and the headline figure cannot be compared with anything. Then: what concrete volume and mix sit behind that row, what cement replacement level is assumed, and is that level secured by anything. Then the spoil position: tonnage, haulage distance, reuse proportion, and whether contaminated material is handled separately. Then the exclusions list, read against the methodology rather than against expectation. And finally the normalisation, because a figure per square metre including basement area is not comparable with one that excludes it.

In London, the authority's whole life-cycle carbon reporting template for referable schemes requires the elemental and modular breakdown to be completed, and an incomplete substructure line is usually picked up at stage one. Where the scheme also needs a Circular Economy Statement, the retention and reuse claims in that document have to match the substructure assumptions in the carbon assessment, and the two are frequently inconsistent.

Expect a verification condition on anything you commit to. Where a cement replacement level or a retention assumption carries the result, the authority will want as-built evidence, and the wording matters as much as the commitment, which we cover in post-construction verification conditions. Claims about future improvement with no mechanism behind them have the same weakness, discussed in future proofing claims in an energy statement.

Enter the postcode on PlanScope to see which carbon and sustainability reports your scheme is likely to need, then send the draft for professional review.

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.