Policy information sourced from the Westminster City Plan
45. Basement developments
A: Basement developments should:
- incorporate measures recommended in the structural statement or flood risk assessment to safeguard structural stability, and address surface water and sewerage flooding;
- be designed and constructed to minimise the impact at construction and occupation stages on the surrounding area;
- protect heritage assets, and in the case of listed buildings, not unbalance the building’s original hierarchy of spaces where this contributes to its significance; and
- conserve the character and appearance of the existing building, garden setting and the surrounding area, ensuring lightwells, plant, vents, skylights and means of escape are sensitively designed and discreetly located.
EXTENT AND DEPTH OF BASEMENTS
B: Basement developments will be supported where they:
- do not extend beneath more than 50% of the garden land – on small sites where the garden is 8m or less at its longest point, basements may extend up to 4m from the original building in that direction;
- leave a margin of undeveloped garden land proportionate to the scale of the development and the size of the garden around the entire site boundary;
- not comprise more than one storey beneath the lowest original floor level – exceptions may be made on large sites with high levels of accessibility for construction;
- provide a minimum of one metre of soil depth (plus minimum 200mm drainage layer) and adequate overall soil volume above the top cover of the basement; and
- not encroach more than 1.8m under any part of the adjacent highway and retain a minimum vertical depth below the footway or carriageway of 900mm between the highway surface and vault structure.
For more information please see the City Plan