Basement Impact Assessment, Replacement House with Basement, Brent
A two-storey detached house in Aylestone Avenue, NW6, in the London Borough of Brent, is to be replaced by a five-bedroom house over basement, ground, first-floor and loft levels. The Air & Flood Consultants Basement Impact Assessment, prepared against Brent Local Plan Policy BD3, the Brent Basement SPD and London Plan Policy D10, places the single-storey basement (about 113 m² external footprint, 92.11 m² GIA) wholly within the new footprint, with formation about 2.94 m below ground in the London Clay Formation, no groundwater recorded above formation and ground movement assessed as Burland Category 0 (negligible) at neighbouring property. The application is yet to be registered.
The site is a 650.40 m² residential plot in Aylestone Avenue, NW6, in the London Borough of Brent, occupied by a two-storey detached house with no basement. The proposal is to demolish the house and build a four-storey, five-bedroom replacement, including a single-storey basement for a cinema, games room, laundry and lift lobby. The basement has an external footprint of about 113 m² and a gross internal area of 92.11 m², and lies wholly within the footprint of the new house. Its floor is set at +60.38 mAOD against a mean existing ground level of 62.97 mAOD, with a 2.50 m clear height and formation about 2.94 m below ground. It is reached by internal stair and lift only, with no lightwell, rooflight or external opening.
Air & Flood Consultants prepared the desk-based Basement Impact Assessment against Policy BD3 of the Brent Local Plan 2019 to 2041, the Brent Basement Supplementary Planning Document (June 2017) and Policy D10 of the London Plan. BGS mapping shows the London Clay Formation directly beneath the site, with no superficial deposits mapped. No groundwater is recorded above formation level, and the West London SFRA maps the site as 0% susceptible to groundwater flooding. The site lies in Flood Zone 1 at very low surface water risk. Sewer flood risk is assessed as low to medium, with 19 incidents recorded in postcode sector NW6 7 on the Thames Water register.
The nearest building in separate ownership stands about 12 m from the basement, outside the zone of influence of the excavation, and ground movement is assessed as Burland Category 0 (negligible). Because the house is demolished first, the basement is built in open cut with no underpinning. To meet Policy BD3(g), basement drainage is to run through a pumped connection with a non-return valve, twin pumps on separate circuits and a high-level alarm. The box is to be waterproofed to BS 8102:2022 for full hydrostatic pressure. The assessment recommends a ground investigation, a quantitative ground movement assessment by the structural engineer, an arboricultural survey to BS 5837:2012 and ground gas monitoring.
The assessment was reissued on 15 September 2026. The application is not yet on Brent's planning register, and its reference is to be added when it appears.
More on these services:
Basement impact assessments: https://www.airandfloodconsultants.co.uk/basement-impact-assessment
Basement development under the NPPF 2026: https://www.airandfloodconsultants.co.uk/post/basement-development-nppf-2026