Whole life carbon for external works and infrastructure

On a site with heavy external works, the substructure, roads and drainage can account for a larger share of upfront embodied carbon than the buildings standing on them. That is the finding that surprises design teams the first time a whole life carbon external works assessment is done properly, and it is the reason the current RICS methodology brings external works firmly inside the assessment boundary rather than treating them as a footnote. Get the boundary right at the start, because adding the civils later changes the headline figure in a direction nobody enjoys explaining.

What falls inside the assessment boundary

A whole life carbon assessment follows the life cycle modules of the European standard for assessing the sustainability performance of buildings: product and construction stages, the use stage, the end of life stage, and the separate module for benefits and loads beyond the system boundary.

External works belong in those modules on the same basis as the building. That means site clearance and demolition, earthworks and ground improvement, piling, substructure, retaining structures, roads, footways and parking surfaces, drainage and attenuation, services trenches and ducts, boundary treatments, hard landscaping, external lighting and the planting and soils.

Two boundary questions come up repeatedly. Off-site highway works delivered under a section 278 agreement are part of the development's consequences even where another party builds them, and the honest approach is to include them and state the assumption. Infrastructure serving later phases, built now, belongs in the phase that builds it with a note explaining the apportionment.

State the boundary explicitly in the Whole Life Carbon Assessment and list what has been excluded with a reason. An assessor who reads an unexplained exclusion assumes the worst.

Substructure and the ground works

Substructure is where the carbon sits on most sites, and it is decided early by people who are not thinking about carbon.

Piling is the obvious example. A piled solution on a site that could take shallow foundations with modest ground improvement carries a large quantity of reinforced concrete, and the decision is usually taken on programme certainty rather than on material quantity. Basements multiply the effect, since the retaining walls, the slab thickness and the waterproofing all scale with depth.

Three figures to get from the engineer before the planning submission: the concrete volume by grade, the reinforcement tonnage, and the volume of material to be excavated and removed from site. Those three inputs account for the bulk of the substructure result, and they are available at an early design stage with reasonable confidence.

Muck away is the quietly expensive one. Excavated material leaving site carries transport emissions and is replaced by imported fill with its own transport, so a cut and fill balance is a carbon measure as much as a cost measure.

Roads, drainage and hard surfacing

Roads are carbon-dense per square metre because of the sub-base depth and the bituminous layers. Reducing carriageway width, shortening the adopted length, and using shared surfaces where the highway authority permits all have a direct effect.

Drainage choices are less obvious. Concrete pipework and large precast attenuation structures carry considerable embodied carbon, and a surface solution using swales, basins and permeable paving can cut that substantially while also helping the discharge rate. The constraint is land take and adoption, which is why the carbon case has to be made at the same time as the Drainage Strategy rather than afterwards.

Retaining structures and basements

Retaining walls are the most improvable element on a sloping site. A reinforced concrete wall holding a two metre change in level along a long boundary can be replaced by regrading, a reinforced soil slope or a terraced arrangement with planting, at a fraction of the embodied carbon.

The obstacle is land take again. A wall takes 400 millimetres and a slope takes several metres, so the choice is made by the layout rather than by the carbon assessment. That means the carbon question has to be asked during masterplanning, not during technical design.

Basements deserve a specific note. A basement car park on a site where surface parking is possible is one of the largest carbon decisions available, and where a Basement Impact Assessment is required the two assessments should share the same structural assumptions.

Data sources for civils materials

Use product-specific Environmental Product Declarations where the material is specified, and published generic datasets where it is not. State which you used for each element.

Three practical points. Cement replacement rates change the concrete figure more than almost any other single input, so get the actual mix from the engineer rather than assuming a default. Transport distances for aggregate and fill are worth modelling properly on a rural site, because haulage over a long distance can rival the material emissions. And recycled content in sub-base and fill needs evidence at procurement rather than an assumption at design stage.

Where the scheme is in London, the Greater London Authority publishes its own whole life-cycle carbon guidance and a reporting template for referable schemes, and the template's structure is a sensible model even outside London because officers recognise it.

Reducing carbon in external works

The measures that work are unglamorous and they are mostly decided by the layout.

1. Balance cut and fill so excavated material stays on site, with stabilisation where the material is marginal.
2. Reduce the adopted highway length and width to the minimum the highway authority will accept.
3. Replace piped and tanked drainage with surface conveyance and open storage where levels allow.
4. Regrade or use reinforced soil instead of reinforced concrete retaining structures.
5. Specify recycled aggregate for sub-base and fill, with the recycled content stated in the specification.
6. Reduce parking provision and surface area, which cuts the carriageway, the drainage and the attenuation together.
7. Retain existing hard surfaces and structures where they are serviceable rather than clearing by default.

Retention is the one most often dismissed. An existing slab that can be reused as a sub-base, or a boundary wall that can stay, removes both the demolition and the replacement, and the Circular Economy Statement is the document where that argument belongs.

Reporting and conditions

Report the result in absolute terms as well as per square metre, and report external works as a separate line rather than folding them into the building total. An officer comparing your figure with a benchmark needs to know what is inside it.

Give the modules separately, state the reference study period, name the data sources and the version of the methodology, and set out the reduction measures with the saving attributed to each. Where a planning condition requires a post-construction assessment, the as-built data for external works is harder to collect than for the building, so write the data requirements into the civils subcontract at tender stage.

Our note on carbon reporting in an outline planning application covers how to frame commitments where the detailed design does not yet exist. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required.