Basement Impact Assessment — Residential Conversion with Basement Extension, Peckham
We prepared a Basement Impact Assessment for the conversion of a detached dwelling into five flats at 39 Consort Road, Peckham — lowering and extending the existing basement, underpinning perimeter walls, and addressing the implications of working adjacent to a Network Rail retaining wall.
Basement Impact Assessment — Residential Conversion with Basement Extension, Peckham
Location: 39 Consort Road, London, SE15 2PH, London Borough of Southwark | Client: Matra Group Ltd | Services: Basement Impact Assessment
More Than Just Digging Down
Basement extensions in London boroughs carry a level of scrutiny that goes well beyond standard structural design. At 39 Consort Road, the proposal involved reworking and extending an existing single-storey basement beneath a detached Victorian dwelling, lowering the slab to create habitable accommodation and extending the footprint beneath a new rear ground-floor extension. With a Network Rail retaining structure on the southern boundary, a shared boundary wall with the neighbouring property, and groundwater expected at or near formation level, the BIA needed to demonstrate that the works could proceed without adverse impact.
We were appointed by Matra Group Ltd to prepare the BIA for conversion of the existing house into five self-contained flats, with rear and side extensions at ground and first-floor levels, a new loft storey, and a fully reworked basement forming Flat 01.
Ground Conditions and Groundwater
The site is underlain by Interglacial Lacustrine Deposits — clay and silt — over the Lambeth Group. A Groundsure "Railway Sidings, 1920–1938" polygon overlapping the site was confirmed as a map-boundary artefact from adjoining railway land. BGS records indicated groundwater at approximately 2.1m below ground level, with the proposed basement formation expected at or slightly below this level.
Structural and Neighbour Impact
Two boundaries required particular attention: the wall shared with 37 Consort Road, engaging Section 6 of the Party Wall etc. Act 1996, and the Network Rail retaining structure to the south. A Phase 1 PRA identified nine source-pathway-receptor linkages with risk classifications from Very Low to Moderate/Low, with the adjacent railway corridor and High UXO rating as main drivers.
This BIA formed part of a comprehensive environmental package including FRA and SuDS Strategy delivering 58% peak runoff betterment, and a Phase 1 Contamination Risk Assessment.
Outcome
The BIA demonstrated that the proposed basement works could proceed safely with appropriate design controls, giving Southwark Council the confidence to approve the application.