Basement
conversions
in Cobham, KT11
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A Cobham basement starts with the flood map and the ground, not the floor plan. Close to the Mole some plots rule a basement out; across much of the town the questions are groundwater and structure.
Cobham sits low beside the River Mole, at around 20m above Ordnance Datum with the river's floodplain at 15 to 20m, on sand and gravel or alluvium over Bagshot sand and the Claygate Member. Most of the settlement area is Flood Zone 1, but whether a basement is possible at a given address depends on the floodplain and the groundwater along it.
So the first stage of any Cobham basement is investigation. It is better to learn early that a plot is unsuitable than to design a basement the council's flood guidance will not allow.
Where the flood assessment rules a basement out
The council's 2024 flood assessment treats Cobham, Oxshott, Stoke d'Abernon and Downside as a single settlement area: 84% Flood Zone 1, 5% Flood Zone 2, 1% Flood Zone 3 and 11% Flood Zone 3b, the last comprising the rural floodplain of the Middle Mole and some developed areas. The town itself lies low, at around 20m above Ordnance Datum, with the floodplain at 15 to 20m.
Near the river its requirements leave little room. Its table of floor-level requirements, under Flood Zone 3, says new basements, basement extensions, self-contained units and conversions of basements to a more vulnerable use are not to be permitted in Flood Zone 3b, the functional floodplain, nor in areas prone to groundwater flooding. On developed land inside Flood Zone 3b it repeats that no basement, basement extension or higher-vulnerability basement conversion should be permitted.
In Flood Zone 3a the same table allows neither a self-contained basement dwelling nor a bedroom at basement level, and other basements in Flood Zone 3 must have access from inside the house to a higher floor 300mm above the 1 in 100 year flood level, with an allowance for climate change. The table, whose columns cover Flood Zones 3 and 2, also says all basements, basement extensions and conversions must have that internal access. Basement dwellings appear on the assessment's list of highly vulnerable uses.
On groundwater, it finds limited potential across most of the settlement area, some potential at and below the surface in parts along the Mole floodplain, and, at borough level, greater potential in Stoke d'Abernon. Water entering basements is one of the chief harms it describes.
Sources: Elmbridge Borough Council: Level 1 Strategic Flood Risk Assessment, April 2024 (AECOM)
The groundwater assessment for a Cobham basement
The council's flood assessment says a preliminary hydrogeological risk assessment should be undertaken for every proposed development, recording how deep and how widely the works reach below ground, basements and piled foundations among them. Because a basement is below-ground work by definition, the full hydrogeological assessment then follows; the guidance notes it is sometimes known as a Basement Impact Assessment.
That study should establish whether the new structure will reach the water table, considering its highest level, and where the ground is uncertain the guidance points to trial pits and boreholes on the site.
Beneath the settlement area lie Taplow Gravel or alluvium, on Bagshot sand and Claygate Member beds. Almost the entire Cobham Conservation Area lies within an area of high archaeological potential, which we allow for in planning any dig below a house there.
Sources: Elmbridge Borough Council: Level 1 Strategic Flood Risk Assessment, April 2024 (AECOM); Elmbridge Borough Council: Cobham Conservation Area Character Appraisal and Management Plan (endorsed 11 June 2019)
Permissions for a basement in Cobham
Adding space underground is not one of the internal works the planning act leaves outside development: its exclusion does not cover works begun after 5 December 1968 that provide additional space underground. We therefore treat a new basement as needing a planning application.
Where the house is in the Green Belt, DM18 of the Development Management Plan allows a basement that lies entirely below ground, brings no significant extra activity to the site, takes up no more ground than the existing house's footprint (or that of the house as extended or replaced), and shows itself only through discreet light wells, vents or escape routes. Anything else is added to the volume and footprint test, which our Elmbridge page quotes in full.
Digging below and near a neighbour's foundation level needs notice to them under the party wall rules, which for a basement beside an attached neighbour in Cobham's semi-detached pairs and terraces is very likely.
If the basement will hold habitable rooms, Approved Document B wants a way out of it: an emergency escape window, an outside door, or a protected stair that runs to a final exit. Building control is Elmbridge Building Control Services (EBCS), for Elmbridge Borough Council, until 31 March 2027, and from 1 April 2027 the provider named by East Surrey Council, or a private registered approver.
Sources: legislation.gov.uk (The National Archives): Town and Country Planning Act 1990, section 55 (meaning of development); Elmbridge Borough Council: Development Management Plan (April 2015); GOV.UK: Party walls and building work (printable guide); GOV.UK (MHCLG): Approved Document B (fire safety) volume 1: Dwellings, 2019 edition incorporating 2020, 2022, 2025 and 2026 amendments; Elmbridge Borough Council: Building control; GOV.UK: Building regulations approval
Taking a Cobham basement from idea to site
We check the flood zone and groundwater position for the address first, then commission the site investigation. Only once the ground is known do we start drawing.
Our structural engineer designs the retaining walls, the underpinning sequence and the waterproofing together, and the drawings show how any water that gets in is collected and pumped out.
Excavation, propping and spoil removal are planned around the street: where lorries stand, how spoil leaves the site, and how the neighbours' walls are monitored while the work goes on.
A cellar already under the house is a different starting point from digging new space, so at the first visit we record its depth, its walls and any signs of water before talking about use.
Outside Cobham, and in London rather than Surrey: Holland Avenue, Wimbledon SW20. That completed project of ours combined a loft conversion, a double-storey extension to the rear and side, and a whole-home refurbishment, as its case study describes.
Sources: Hampstead Renovations: Holland Avenue, Wimbledon SW20 case study
Basement Conversions in Cobham: Questions
Can a Cobham house have a basement, given the flood zones?
Away from the river, it can be: the council's assessment places 84% of the settlement area in Flood Zone 1 and finds limited groundwater potential across most of it. Near the Mole the answer may be no, because the assessment's Flood Zone 3 requirements say basements are not to be permitted in the functional floodplain (Zone 3b) or in areas prone to groundwater flooding. We check the address before any design work.
What do the council's flood rules say about digging a basement in Cobham?
The council's 2024 strategic flood risk assessment sets them out in its table of floor-level requirements for Flood Zones 3 and 2: under Flood Zone 3, no basements in Zone 3b or in areas prone to groundwater flooding, and no self-contained basement dwelling or basement bedroom in Zone 3a; and for basements that are allowed, internal access to a higher floor 300mm above the 1 in 100 year flood level including climate change. Applications are decided by Elmbridge Borough Council until 31 March 2027; East Surrey Council from 1 April 2027.
If my Cobham house is in the Green Belt, does a basement count against the extension limit?
Not if it meets DM18's conditions: wholly below ground, no significant extra activity on the site, no larger than the footprint of the house as it now stands, and visible only through discreet light wells, ventilation or escape. A basement that fails any of them is added to the volume and footprint the Green Belt policy controls.
Will a Cobham basement need a hydrogeological (groundwater) study?
Yes. The council's flood guidance asks every development to start with a preliminary hydrogeological risk assessment, and because a basement is work below ground, a full assessment follows. Its job is to show whether the basement would meet the water table at its highest level, using trial pits and boreholes where the ground is uncertain.
Who inspects a basement conversion in Cobham for the building regulations?
Elmbridge Building Control Services (EBCS), for Elmbridge Borough Council, until 31 March 2027, and from 1 April 2027 the provider named by East Surrey Council, or a private registered approver you appoint. Building control officers check the work against the Building Regulations, including the escape route from any habitable room below ground.
What happens at a feasibility visit for a basement conversion in Cobham?
We look at the house, its plot and its street, check the flood zone and groundwater position for the address, and see how excavation and spoil removal could be managed. If there is a cellar already, we record its depth and condition. You then get a written view on whether a basement is worth investigating further and what the site investigation should cover.
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