Basement
conversions
in Radlett, WD7
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A basement under a Radlett house is possible only where both the flood position and the ground allow one, and close to the Radlett Brook they may not; we establish both before anything is drawn below the ground floor.
Radlett lies in the valley of Tykes Water, the Radlett Brook, and the neighbourhood plan says the town's underlying geology is mainly chalk. The council's 2023 flood assessment adds that the soils across Radlett and Shenley are generally well-drained, loamy and sandy, and that the brook's catchment is heavily urbanised and relatively steep.
That mix is why this page is cautious. The council's design guide treats a residential basement very differently in each flood zone, the Environment Agency normally objects to development that could physically disturb an aquifer right beside a drinking-water abstraction, and a basement under a long Radlett garden must still leave room for a mature garden above it. None of that can be judged from the street.
Sources: Hertsmere Borough Council / AECOM: Level 1 Strategic Flood Risk Assessment (2023); Aldenham Parish Council / Hertsmere Borough Council: Adopted Radlett Plan 2020-2036 (low resolution)
What decides whether a Radlett basement is possible
The council's draft Part D guide, used in planning decisions since 15 September 2016, classes residential basements as highly vulnerable and says they are not appropriate in Flood Zone 3. In Flood Zone 2 they may be permitted after the sequential and exception tests; in Flood Zone 1 they will generally be acceptable, subject to the other policies.
Flood Zones 2 and 3 lie most heavily alongside a handful of rivers, the Radlett Brook among them, and the 2023 assessment records brook floods in December 1979 and September 1992. Where Flood Zone 3b, the functional floodplain, covers developed land, it says no basements will be permitted, nor basement extensions, nor conversions of basements to a higher vulnerability class.
Flood Zone 1 is not the end of the question. The guide asks for a site-specific flood risk assessment wherever a basement is planned in Flood Zone 2, in Flood Zone 3, or in an area at risk of surface water flooding, because digging out removes natural flood storage from the soil and the water has nowhere else to go. The council's assessment adds that a basement dwelling stays more vulnerable to surface water than an office.
So the first piece of work on a Radlett basement is the river and surface water mapping for the exact plot. If the house falls in Flood Zone 3 we say so plainly, and look with you at finding the space above ground instead.
Sources: Hertsmere Borough Council: Planning and Design Guide SPD; Hertsmere Borough Council: Draft Planning and Design Guide SPD Part D (draft for DM use and public consultation, October 2016); Hertsmere Borough Council / AECOM: Level 1 Strategic Flood Risk Assessment (2023)
Chalk, groundwater and source protection under Radlett
The plan describes Radlett's geology as mainly chalk, and the Environment Agency says groundwater flooding is most common where the bedrock is chalk. The council's flood assessment finds a limited potential for groundwater flooding across most of the borough, with some areas in the north-west, north-east and centre where it might occur, though no such flooding has been recorded.
The same assessment warns of several groundwater source protection zones in the area. An SPZ1 is the zone immediately around an abstraction point for domestic supply, and inside one the Environment Agency will normally object in principle to development that may physically disturb an aquifer. The zones can be viewed on the government's Magic map.
Drainage needs evidence too: the assessment calls for site-specific infiltration testing to show whether, and where on a plot, infiltration drainage can work.
Before any basement design the ground is investigated, a desk study first and then boreholes or trial pits with groundwater readings, so that the engineer designs the retaining walls and waterproofing to measured ground rather than to assumptions.
Sources: Aldenham Parish Council / Hertsmere Borough Council: Adopted Radlett Plan 2020-2036 (low resolution); Environment Agency: The Environment Agency's approach to groundwater protection (LIT 7660; under review); Hertsmere Borough Council / AECOM: Level 1 Strategic Flood Risk Assessment (2023); Environment Agency (GOV.UK): Groundwater source protection zones (SPZs) (last updated 1 August 2024)
Size, lightwells, trees and the Green Belt
The guide asks whether the space could be found above ground before a basement is accepted as the most sustainable answer. Under an existing house the basement should be entirely below ground, with no lowering of the levels around the house to suggest an extra storey, and bedrooms and main living spaces at basement level should be avoided.
Where a large house sits on an extensive plot, extending under part of the rear garden may be possible, provided a mature garden can be kept above; soil and drainage details are normally required with the application, and an arboricultural assessment usually is too.
Lightwells and sunken terraces should be avoided in small front gardens and in conservation areas unless already an established feature, with positions at the side or rear considered first, and any that are built need railings or grilles.
In the Green Belt that surrounds Radlett, the guide assesses a basement in exactly the same way as building above ground, and Policy SADM26 says its tests of extension, alteration and original building may include a basement level. The guide also accepts that a basement can be a more acceptable way of adding space there than extra storeys or bulky roofs.
Sources: Hertsmere Borough Council: Draft Planning and Design Guide SPD Part D (draft for DM use and public consultation, October 2016); Aldenham Parish Council / Hertsmere Borough Council: Adopted Radlett Plan 2020-2036 (low resolution); Hertsmere Borough Council: Site Allocations and Development Management Policies Plan (adopted November 2016)
Party walls, building control and how a basement goes forward
Digging below and near the foundation level of a neighbour's property is work you must tell them about, so most basements bring party wall notices. The Building Regulations 2010 cover the construction and extension of buildings, and a basement is both.
A Radlett basement is taken on by our London team of in-house architects, designers and structural engineers, working with Hampstead Chartered Surveyors & Building Consultancy. Hertsmere Borough Council decides the application, and Hertfordshire Building Control or a private registered building control approver signs the work off.
A basement stays at feasibility until the flood zone, the surface water risk and the ground investigation are all in. If any of them rules it out, you hear that before any drawings are commissioned.
Where it can proceed, the dig sequence, temporary support, movement monitoring on neighbouring walls and the removal of spoil are planned with our engineer and set out for the party wall surveyors before excavation begins.
Outside Radlett, and in London rather than Hertfordshire: Project example: Hampstead, NW3. That project of ours is a completed project whose recorded work comprised a new basement with a cinema room, gym and wine cellar, as its case study states.
Sources: GOV.UK: Party walls and building work - work you must tell your neighbour about; GOV.UK: Party walls and building work - overview; GOV.UK: Building regulations approval - when you need approval; Hertsmere Borough Council: Hertfordshire Building Control Limited; Hampstead Renovations: Fitzjohns Avenue, NW3 case study
Basement Conversions in Radlett: Questions
Is my Radlett address in a flood zone, and does that rule out a basement?
The Environment Agency's flood map for planning answers the first part for the exact address. The council's guide calls residential basements not appropriate in Flood Zone 3; in Flood Zone 2 they face the sequential and exception tests; in Flood Zone 1 they will generally be acceptable subject to other policies. Zones 2 and 3 lie most heavily along rivers that include the Radlett Brook.
Does the groundwater position under Radlett affect a basement?
It can. The neighbourhood plan describes Radlett's geology as mainly chalk, and groundwater flooding is most common over chalk bedrock. Several source protection zones lie in the area, and inside an SPZ1 the Environment Agency will normally object to development that may physically disturb an aquifer, so we check the zones and investigate the ground before any design.
Is a basement weighed in the Green Belt test for my Radlett house?
Yes, where the house is inside the Green Belt: a basement is assessed exactly as development above ground would be, and Policy SADM26 says extension, alteration or original building may include a basement level when proportion is judged.
Do I need a party wall agreement for a basement in Radlett?
You must tell your neighbour if you dig below and near the foundation level of their property, which most basements do. A neighbour cannot block lawful work, though they can shape the way it is done and its timing, so the notices go out well before excavation.
Who signs off building regulations for a basement in Radlett?
Hertfordshire Building Control or a private registered building control approver, whichever you appoint. A basement is construction and extension at once, both of which the Building Regulations 2010 cover.
What surveys come before a basement design in Radlett?
A flood check for the address, surface water included, then a ground investigation with groundwater readings, a structural survey of the house and the neighbouring walls, a drainage survey, and an arboricultural survey where trees are close. Should the plot prove to lie in Flood Zone 3, the basement work stops at that point.
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