Flood Risk Assessment — Lower Ground Floor Extension, The Dene, Sevenoaks
We prepared a site-specific Flood Risk Assessment for a lower ground floor study extension with a glazed link, rooflights and landscaping at 38 The Dene, Sevenoaks, Kent, TN13 1PB — a Flood Zone 1 site — on behalf of James R Brown & Company Ltd. The assessment worked through all six flood sources and found tidal, fluvial, surface water, groundwater, sewer and artificial-source risk all low, with no historic flood events recorded at the address. Groundwater matters most, because the extension creates habitable accommodation below existing ground level and a low rating is what makes that arrangement straightforward. Mitigation was limited accordingly to threshold levels and external ground profiling to shed water away from the building, with no waterproofing, pumped drainage or raised floor level required. Sevenoaks District Council granted permission on 29 May 2026 under 26/00417/HOUSE.
Flood Risk Assessment for a Lower Ground Floor Study Extension, The Dene, Sevenoaks
Location: 38 The Dene, Sevenoaks, Kent, TN13 1PB | Client: James R Brown & Company Ltd | Services: Flood Risk Assessment
Below-Ground Accommodation Is Why the Assessment Was Needed
A Flood Zone 1 site with low ratings across every source might look like a site that needs no flood risk assessment at all. Two things make one worthwhile here.
The first is that the extension creates accommodation at lower ground floor level. Any proposal that places habitable space below existing ground level invites questions about groundwater, about surface water ingress at thresholds, and about what happens if water reaches the building, regardless of the flood zone the site sits in. A study at lower ground floor is a room people occupy, not a store.
The second is that the finding of low risk is itself the deliverable. Establishing on evidence that no source presents a material risk is what allows the design to proceed without waterproofing specifications, raised thresholds or pumped drainage that the site does not warrant. A properly evidenced low rating saves money at construction stage.
We were appointed by James R Brown & Company Ltd to prepare the assessment in support of the planning application for a lower ground floor study extension with a glazed link, rooflights, fenestration alterations and landscaping at 38 The Dene, Sevenoaks.
Site Characteristics and Flood Risk Profile
The site occupies an established residential plot in Sevenoaks, in a setting of detached dwellings with mature gardens, on ground that falls gently across the plot. That local topography is what makes a lower ground floor arrangement natural here.
We drew on the Environment Agency Flood Map for Planning, the Risk of Flooding from Surface Water and Risk of Flooding from Rivers and the Sea datasets, BGS geological and hydrogeological mapping, LiDAR topographic data, sewer flooding records for the address and the Sevenoaks District Council Strategic Flood Risk Assessment. The site lies within Flood Zone 1.
The assessment worked through all six sources in turn.
Tidal flooding: low. The site is well inland with no tidal influence.
Fluvial flooding: low. The site lies outside the 1-in-100-year plus climate change extent, with no main river or ordinary watercourse in the immediate vicinity.
Surface water (pluvial) flooding: low. Environment Agency mapping shows no significant modelled ponding across the plot and no overland flow route crossing the site.
Groundwater flooding: low. This is the source that matters most given the below-ground accommodation, and the low rating is what makes the lower ground floor arrangement straightforward to build and to occupy.
Sewer flooding: low, with no incidents recorded at the development address.
Flooding from artificial sources: low. The site sits outside any reservoir inundation extent.
No historic flood events are recorded at the address, and none is recorded in the immediate surrounding streets, which is consistent with the mapped position of the site relative to the local drainage pattern.
Proportionate Mitigation
Because no source presents a material risk, the recommendations are limited to good practice rather than set as conditions of safety. They comprise threshold levels and external ground profiling around the lower ground floor to shed water away from the building, together with standard measures to keep surface water clear of the new opening. No waterproofing system, pumped drainage or raised floor level was necessary, and none was specified.
Outcome
The FRA demonstrated that the extension is at a low level of flood risk and that flood risk at the site and at surrounding sites is not increased. Sevenoaks District Council granted planning permission on 29 May 2026 under reference 26/00417/HOUSE. It is a useful reminder that a flood risk assessment is not only a document for difficult sites: on a straightforward one its value lies in closing the question quickly and keeping unnecessary specification out of the build.