Basement Impact Assessment — Garden Outbuilding with Basement on River Terrace Gravels, Hillingdon
We prepared a Basement Impact Assessment for a detached single-storey garden outbuilding with a basement level at 121 Lowdell Close, Yiewsley, UB7 8BA — a Flood Zone 1 site over a highly vulnerable Principal aquifer, on behalf of Adam Pisar. Each floor has a gross internal area of approximately 33m², with the basement formation level assumed at approximately 3.0m below existing ground level. The superficial deposits are the Lynch Hill Gravel Member over the London Clay Formation, with Worked and Made Ground recorded on and adjacent to the site. Two BGS boreholes 250-325m to the south recorded no groundwater to 2.0m and to approximately 6.0m, indicating that groundwater lies below the formation level. Groundwater flood risk is very low, the site is outside any Source Protection Zone, and the assessment delivered the screening and scoping outcome required by Hillingdon Local Plan Policy DMHD 3.
Basement Impact Assessment for a Garden Outbuilding on the West Drayton River Terrace Gravels
Location: 121 Lowdell Close, Yiewsley, Hillingdon, UB7 8BA | Client: Adam Pisar | Services: Basement Impact Assessment
Why a Garden Outbuilding Needs a Basement Impact Assessment
Subterranean development is scrutinised in London regardless of scale, and a 33m² basement at the end of a garden is assessed on the same principles as a substantial residential basement: what the ground is, where the water is, what the excavation does to neighbouring land, and what it does to the site's drainage behaviour. The scale of the works changes the depth of investigation warranted, not the questions asked.
We were appointed by Adam Pisar to prepare a Basement Impact Assessment addressing Policy DMHD 3 (Basement Development) of the Hillingdon Local Plan Part 2, for the erection of a detached single-storey garden outbuilding ancillary to the existing dwelling, at the far eastern end of the rear garden at 121 Lowdell Close, Yiewsley, Hillingdon, UB7 8BA, with a single basement level of matching footprint beneath. Each floor has a gross internal area of approximately 33m². The ground floor accommodates a home bar and store; the basement accommodates a gym and office, reached by an internal stair. The basement formation level is assumed at approximately 3.0m below existing ground level.
The Ground Model
The site is a two-storey detached dwelling on a wedge-shaped plot of approximately 270m², with a long rear garden extending east, centred on National Grid reference 506518, 181184.
Superficial deposits comprise the Lynch Hill Gravel Member — sand and gravel, a river terrace deposit — overlying the London Clay Formation of clay, silt and sand. BGS mapping records Worked and Made Ground on and immediately adjacent to the site, consistent with the historical gravel-bearing character of the West Drayton river terrace. That combination is the defining feature of the site: a granular, permeable superficial deposit of variable thickness over a low-permeability bedrock, with a history of extraction and backfill in the vicinity.
We built the model from BGS 1:50,000 mapping and the borehole records held for the surrounding area, Environment Agency aquifer designation and Source Protection Zone data, historical mapping for the extraction and backfill history, and LiDAR topographic data for the plot and its garden.
Hydrogeology and Groundwater
The superficial gravels form a Principal aquifer, classified by the Environment Agency as a highly vulnerable Principal superficial aquifer in river terrace gravels with only a thin low-permeability cover. The London Clay bedrock is Unproductive. The site is not within a Source Protection Zone, and there are no groundwater abstractions within 500m.
Two BGS boreholes approximately 250-325m to the south recorded no groundwater to 2.0m and to approximately 6.0m respectively, indicating that groundwater is likely to lie below the proposed basement formation level of approximately 3.0m. That is the finding that governs the assessment: a basement founded above the water table in granular deposits presents a materially different, and lower, risk profile than one founded below it, both for construction dewatering and for long-term groundwater flow. The permeability of the gravels means that any excavation extending below the water table would have required a dewatering and flow-path assessment of a different order.
Flood Risk and Screening
The site lies within Environment Agency Flood Zone 1, and groundwater flood risk at the site is assessed as very low, consistent with the borehole evidence. We worked through the standard screening and scoping matrices for hydrogeology, hydrology and land stability, together with a contamination conceptual site model reflecting the recorded Made Ground and the gravel extraction and backfill history of the area. Aquifer vulnerability, rather than groundwater flooding, is the constraint the design has to respect: excavation into a highly vulnerable Principal aquifer places a premium on controlling what enters the ground during construction.
Outcome
The BIA provided the London Borough of Hillingdon with a documented ground model, an evidenced groundwater position, and a screening outcome tied to Policy DMHD 3 — the assessment a subterranean development in a vulnerable aquifer setting requires, at a depth of investigation proportionate to a 33m² ancillary structure.