Policy information sourced from Lambeth Local Plan 2020-2035

Q27 Basement development

  • The council will support basement and associated development (light wells, basement area excavation, access ramps etc.) where applicants can demonstrate that no unacceptable impacts will result to:
    • subterranean ground water flow (ground water);
    • slope stability (land stability);
    • surface flow and flooding (see also Local Plan policy EN5 and Annex 4);
    • the ability of trees and soft landscaping (existing and proposed) to thrive without irrigation;
    • cumulative effects of basement development in the locality;
    • waste to land fill and carbon emissions; and
    • designated and non-designated heritage assets (including archaeology).
  • Proposed basement accommodation will generally be expected to:
    • have external features and details that respond appropriately to the character and materials of the host building and cause no harm to the visual amenity of the wider context;
    • include a positive pumped device (or equivalent) to mitigate against the risk of sewer flooding;
    • fully integrate plant and machinery in order to minimise visual and noise impacts;
    • incorporate sustainable urban drainage measures or any other mitigation measures where required; and
    • where possible be naturally ventilated.
  • Beneath existing residential buildings and in new-build residential schemes basement level accommodation should:
    • not result in any more than one storey of basement accommodation below ground level;
    • not create basement accommodation beneath existing basements or semi-basements; and
    • meet the necessary standards for habitable accommodation in relation to room size, outlook, daylight and sunlight etc.
  • Full (as opposed to semi) basement extensions beyond the ground floor footprint of a residential building, and any associated basement level outdoor space and steps, will be supported where the proposal:
    • is limited to the rear;
    • has a roof treatment level with ground level which allows it to continue to provide amenity space for the host building;
    • the basement footprint does not exceed the existing footprint of the house to the side or at the front; and
    • generally retains no less than 70 per cent of the rear garden area free of subterranean development and at its original ground level.
  • In front and side gardens basement lightwells (on existing buildings or with new build) should:
    • only be provided where required for outlook and daylight for the accommodation they serve;
    • not entail the inappropriate alteration of existing basement areas and enclosures;
    • not result in the excavation or loss of front or side garden space which would harm the integrity of the host building or the character of the locality (especially on heritage assets);
    • minimise the visual impact through good design (in many cases, especially conservation areas, this is likely to mean lightwells with pavement grilles rather than open basement areas enclosed with balustrades); and
    • ensure existing parking bays are not shortened to below the minimum standard (where this occurs the council will seek the removal of the parking bay);
  • In the case of wholly non-residential buildings, it may be possible to have a non-residential basement greater than one storey in depth if it can be robustly demonstrated that no unacceptable impacts will result in accordance with part (a) of this policy. In addition, the proposed scale and quantum of development must be appropriate to the site and its context and all other planning policy requirements should be addressed to the satisfaction of the Council.
  • Applicants for basement proposals should submit a Stage 1 (Screening) Basement Impact Assessment, undertaken by a qualified professional, which captures all issues relevant to the proposal including:
    • subterranean ground water flow (ground water);
    • slope stability (land stability);
    • surface flow and flooding (see also Local Plan policy EN5 and Annex 4);
    • cumulative effects of basement development in area;
    • waste to land fill and carbon emissions; and
    • designated and non-designated heritage assets.
  • The council may, upon review of the Stage 1 (Screening) require further, more detailed Basement Impact Assessment (stages 2 – 4). Applicants will be expected to carry the cost of any independent assessment on the council’s behalf of this further work.

For more information please see the Local Plan