‘Super Basement’ carbon emissions 30% higher during construction

Eight Associates, the sustainability consultancy commissioned by the Royal Borough of Kensington and Chelsea (RBKC) to be the carbon expert witness in the controversial ‘super-basement’ planning appeal, has released its findings that subterranean extensions in dwellings are generally characterised by a more carbon intense building life cycle.

The Eight Associates report for RBKC drew on 16 different case studies in its lifecycle carbon analysis of above ground extensions, single storey basements and multi-storey basements. In terms of embodied carbon, basements were found to be over 50% more carbon intense than above ground extensions. Multi-storey basements are likely to have carbon intensity for the materials used over 12% higher than single storey basements.

It was a similar case for construction carbon, with single and multi-storey basements likely to have in excess of 50% more carbon emissions than above ground extensions per square metre. Importantly, it was found at this stage of development that multi-storey basements have carbon emissions 28% higher than single storey basements.

The long term implications of the ‘iceberg house’ phenomena were highlighted by the finding that even if multi-storey basements were to utilise advanced retrofit measures, the enhanced operational efficiency, and associated carbon savings, still would not be enough to compensate for the embodied and construction carbon over a 60 year period.

In relation to operational carbon, multi-storey basements have the highest operational carbon emissions, 9% higher than single storey basements. Basements that are exclusively in the garden perform worse than those under or attached to the existing dwelling.

Whilst noting the limited pool of case studies, and hence data, available, the report methodology followed international standards guidelines (BS EN 15643, ISO 14040 and BS ISO 21931-1) and included extensive sensitivity analysis.

The report author, Chris Hocknell, Eight Associates Sustainability Consultant, said:

“Carbon footprint was one among many factors contested during the appeal; our evidence meant the issue received its share of prominence. The robustness of our approach and analysis, and that the results were in line with benchmark studies from peer-reviewed journals, meant the Inspector was minded to agree with our findings.”

“We had to work with the available case studies and information included in the planning submissions for basements – both of which were somewhat limited. It is important the policy is revisited, as planned, in five years. By then, there will be more and more accurate data available” added Chris, who along with Eight Associates’ Rita Margarido, gave evidence at the Examination in Public proceedings.

“As the number of subterranean developments grows, ideally we’d like to see the reinstatement of sustainability assessments for these projects, a requirement for more detailed data in planning submissions and a joined-up, London-wide review of the impact of basements development. The benefits of high quality design go hand in hand with a low carbon future. Our experience shows such measures need not be onerous or expensive to implement, and will safeguard the benefits of sustainable development”.

The 2014 review of RBKC’s Basements Planning Policy has seen the adoption of tighter restrictions on subterranean development to a single storey space (with some exceptions), extending no more than 50% under the garden of a property.

Described by RBKC as “the single greatest cause of concern our residents have expressed in living memory”, the complex issue of the economic, social and environmental impact of basement development was fiercely contested last year, with no technical consensus on the question of carbon footprint impact. The Inspector, David Vickery, concluded that basement developments produce more embodied and construction carbon dioxide emissions during a 60 year lifecycle than similar above-ground extensions. However, he also noted that it was not a robust justification on its own for the proposed size limitations in the policy, with the compounding adverse impacts of noise, vibration, traffic, drainage, appearance and landscape and structural stability justifying RBKC’s policy.