Carbon in the infrastructure a scheme has to build

Most whole life carbon assessments submitted with planning applications stop at the building line. The roads, the drainage, the substation, the retaining walls and the 40,000 cubic metres of material moved to make the site level sit outside the boundary, unreported. On a housing scheme with significant external works, that omission can be a large share of the embodied carbon, and officers are starting to notice.

The fix is not complicated. Set the boundary explicitly, include the infrastructure the development has to build, and report the two parts separately so the figures can be compared with benchmarks that were derived from buildings alone.

What usually falls outside the boundary

Building-level assessment methods were written for buildings. The RICS whole life carbon methodology in its current edition extends to site works and infrastructure, and BS EN 15978 covers buildings, while PAS 2080 is the carbon management standard written for infrastructure. Assessors default to the building because the templates, the benchmarks and the cost plans are all organised that way.

The result is a predictable set of exclusions.

• Adoptable roads, footways and their sub-base
• Section 278 works to the existing highway, including signals and crossings
• Attenuation tanks, oversized pipes, manholes and outfall structures
• Substations, ducting, service trenches and connection works
• Retaining walls, gabions and level platforms
• Car parking areas, hardstanding and boundary treatments
• Remediation, ground improvement and piling mats
• Earthworks, and the haulage that goes with them

None of these is optional. They are conditions of the permission, secured by agreement or by the drainage strategy, and the scheme cannot be built without them.

Roads, drainage and utilities as carbon items

Concrete and asphalt dominate. A road build-up is a deep layer of granular sub-base under bound layers, and the cement in any concrete element carries most of its embodied carbon. Drainage adds concrete in rings, chambers and headwalls, plus plastic or concrete pipe.

Two comparisons are worth running early, because they change the layout rather than the specification.

Getting these right needs the drainage engineer and the carbon assessor in the same conversation at concept stage. By the time the Drainage Strategy is fixed for submission, the choices that mattered have been made.

Setting a boundary an officer will accept

State the boundary in the assessment, in one paragraph, near the front. Say what is included, say what is excluded, and say why.

A defensible position is to include everything within the red line that the development will construct, plus off-site works secured by planning obligation or highway agreement. Exclude the existing highway network beyond the works, the utility network beyond the connection point, and anything the development is not responsible for building.

Where data is thin, say so and use a conservative assumption rather than dropping the item. An estimated figure with a stated basis is worth more than a silent exclusion, and officers respond far better to a transparent assumption than to a gap they find themselves.

Report building carbon and infrastructure carbon separately, then give the total. Published benchmarks were derived from buildings, so a combined figure compared against a building benchmark misleads in the other direction.

Earthworks, cut and fill and haulage

Earthworks are the item most often missed and frequently the largest single line outside the buildings. Moving material is diesel, and importing or exporting it multiplies the distance.

Balance cut and fill on site wherever the layout allows. Every cubic metre that leaves the site is a lorry out and usually a lorry back in with something else. Where the site slopes, a layout that steps with the contours instead of creating a level platform avoids the earthworks entirely, and it usually produces a better scheme visually as well.

Where material must be treated rather than removed, site-based stabilisation or crushing produces both a carbon and a cost saving. Say so in the construction management information, because it also reduces the vehicle movements the highway authority is worried about.

Materials with the largest effect

Once the layout is fixed, specification is what remains, and a short list carries most of the reduction.

Cement replacement is the biggest single lever on any scheme with concrete in it. Ground granulated blast furnace slag and fly ash substitute for a proportion of the Portland cement in a mix, and the structural engineer will know what proportion works for each element. Foundations, attenuation tanks and retaining walls are the places to look first, because they are rarely on the critical path for early strength.

Recycled and secondary aggregate in sub-base is the second. Crushed concrete from demolition on the same site avoids both the extraction and the haulage, and highway authorities generally accept it in sub-base where it meets the specification.

Asphalt comes third. Warm mix and higher recycled content are both available, and the decision belongs with the contractor rather than with the designer, which means it has to be written into the specification before tender or it will not happen.

Boundary treatments are small individually and large in aggregate. A kilometre of close-boarded timber fence against a kilometre of hedgerow is not a close contest.

Reporting it alongside the buildings

A Whole Life Carbon Assessment that covers infrastructure should present four things clearly: the boundary, the quantities, the carbon factors and their source, and the results split by element. Quantities matter most. An assessor can substitute a better factor later, but an assessment with no quantity for the road sub-base cannot be corrected.

Use the same quantities the cost plan uses. Where the two documents disagree, the reviewer will assume the carbon figure is the invented one. Consistency between the appraisal, the drainage design and the carbon assessment is what makes the whole package credible, and the same principle applies to the benchmark figures discussed in embodied carbon benchmarks and how to present them.

State the uncertainty honestly. Infrastructure quantities at outline stage are estimates, and an assessment that pretends otherwise invites a condition requiring a recalculation that nobody has budgeted for.

Conditions and monitoring across the whole site

Where a council conditions carbon performance, the wording usually refers to the approved assessment. If the assessment covered only the buildings, the condition covers only the buildings, and the infrastructure escapes scrutiny entirely.

Applicants sometimes prefer that. It is short-sighted on a phased scheme, because the second phase is assessed against whatever the first phase established, and a narrow boundary early becomes a narrow benchmark later.

Ask for conditions that name the deliverable and the stage. A pre-commencement submission of the infrastructure carbon quantities, and a post-completion report reconciling them against what was built, are both dischargeable. A condition requiring the development to achieve a carbon figure with no method of measurement is not, and the drafting principles are covered in drafting carbon conditions that can actually be discharged.

Start with the quantities you already have. Every scheme has a drainage layout, a highways general arrangement and an earthworks estimate long before it has a carbon assessment. Handing those three documents to the assessor at the right moment is most of the work.

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