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CASE STUDY · APPROVED

Basement lowering and internal alterations to a high street building

Balham High Road, Balham · London Borough of Wandsworth

A full planning application to lower the floor of part of an existing basement to create a lounge area, brick up storage doors in the side elevation and make internal alterations at basement and ground floor. We prepared the design and access statement, basement impact assessment and construction management plan alongside the drawings.

Existing and proposed front elevations, unchanged by the works (front elevation drawing)
Existing and proposed front elevations, unchanged by the works (front elevation drawing)

The site

The site is a four-storey end-of-terrace building on Balham High Road with Class E commercial use at basement and ground floor and flats above, part of a parade of late Victorian and Edwardian mixed-use buildings. It is not listed or in a conservation area, lies in Flood Zone 1 and has a PTAL of 3. A change of use of the rear basement and ground floor to three flats was approved in 2021 (2021/1280) and a mansard and rear extension scheme was not approved in 2020 (2020/2652).

The challenge

Wandsworth's Local Plan Policy LP6 requires basement works to protect neighbouring structures, the character of the building and flood risk. The excavation would extend to the shared boundary and about 4 m from the public pavement, and construction access had to be managed on a busy high street.

What was submitted

  1. Design and Access StatementShowed that the excavation would be confined to the footprint of the existing basement store, turning it into a lounge linked by steps to the existing lobby, with no change to the front elevation or upper floors and the side door openings infilled in matching materials.
  2. Basement Impact AssessmentAssessed a partial excavation of about 0.42 m within the existing basement. Ground investigation found stiff silty clay to about 8 m over dense sand, with groundwater expected at 3.5-4.0 m below ground and none met in the borehole; it recommended underpinning, vertical shoring at the boundary and construction to BS 8102, with damage to neighbours limited to Category 1.
  3. Construction Management PlanSet out an 8-12 month programme, works carried out in line with the council's Code of Construction Practice (8am-6pm weekdays, 8am-1pm Saturdays), and all deliveries and site access via a 2.87 m wide side alley with 4.0 m clearance, with banksmen, pre-booked deliveries and a holding point to keep vehicles off the main road.
  4. Existing and proposed section A-AShowed basement floor-to-ceiling height increased from 2.08 m to 2.5 m beneath an unchanged 3.13 m ground floor.
  5. Existing and proposed front elevationConfirmed no change to the street elevation.

Outcome

Planning permission was approved by the London Borough of Wandsworth on 10 July 2025.

From the reports

Existing and proposed section showing the basement floor lowered to give 2.5 m headroom (section A-A drawing)
Existing and proposed section showing the basement floor lowered to give 2.5 m headroom (section A-A drawing)

Images and findings are taken from the documents submitted with the planning application. The full application is available on the council's planning register.

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