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Policy D9 Basement development

D9

  1. Boroughs, particularly in inner London, should establish policies to address the negative impacts of large-scale basement development beneath existing buildings.

High residential land values and development constraints have led to increasing levels of basement development beneath existing buildings, particularly within central and inner London boroughs.

Most proposals for the construction of a basement will require planning permission. These proposals need to be managed sensitively through the planning application process to ensure that their potential impact on the local environment and residential amenity is acceptable. Where basement developments cause particular harm, boroughs can consider introducing Article 4 Directions to require smaller-scale proposals to obtain planning permission.

The Mayor considers that smaller-scale basement excavations, where they are appropriately designed and constructed, can contribute to the efficient use of land. They can provide an affordable option for families to provide extra living space without the costs of moving house, although these developments rarely result in the provision of additional residential units to help meet London’s housing need.

The construction of basements can, however, cause significant disturbance and disruption if not managed effectively, especially where there are cumulative impacts from a concentration of subterranean developments. Large-scale basements (i.e. those that are multi-storey and/or those that extend significantly beyond the existing building footprint) can cause particular issues, especially when located in residential or higher density mixed-use areas. Such basement development can impact on land and structural stability as well as causing localised flooding or drainage issues. The extent and duration of construction of large-scale basements can also lead to a large number of HGV trips, as well as noise and vibration issues, causing disturbance to local residents. Measures such as requiring Construction Method and Management Plans can help protect neighbours during construction. Other consents and regulatory regimes may also be involved, such as Environmental Health in regard to noise and contamination, and Highways in relation to licences for skips and temporary structures.

The Mayor supports boroughs in restricting large-scale basement excavations under existing properties where this type of development is likely to cause unacceptable harm. Local authorities are advised to consider the following issues alongside other relevant local circumstances when developing their own policies for basement developments: local ground conditions; flood risk and drainage impacts; land and structural stability; protection of trees, landscape, and biodiversity; archaeology and heritage assets; neighbour amenity; air and light pollution; and the impacts of noise, vibration, dust and site waste. Where there is a known risk of flooding, boroughs may consider restricting the use of basements for non-habitable uses. The Agent of Change Principle (Policy D12 Agent of Change) should be applied to basement development to limit the impact of ground-borne noise and vibration from existing uses and infrastructure. Further guidance will be provided in Supplementary Planning Guidance.