Basement
conversions
in Chalfont St Giles, HP8
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We will be straight with you: on a plot in the Misbourne flood zones we would normally advise against a new basement, and on higher ground a basement still has to prove itself against water before it is designed.
The village sits on both sides of the Misbourne, whose valley floor has been left undeveloped. Beneath it, the county's landscape assessment says, the valley is cut into the Upper Chalk, with alluvium filling the central floodplain, and the Environment Agency classifies the river as Flood Zones 2 and 3.
The 2018 flood assessment records properties and farm buildings in the village inside Flood Zone 3a, finds the 3b zone reaching properties along the High Street, and logs groundwater flooding close to the telephone exchange during March and April 2014.
Sources: Chiltern and South Bucks District Councils (hosted by Buckinghamshire Council): Chiltern & South Bucks Townscape Character Study Part 3, Chiltern Settlements (November 2017); Buckinghamshire County Council and Chiltern District Council (hosted by Buckinghamshire Council): Landscape Character Area 19.2, Lower Misbourne Settled River Valley; Chiltern and South Bucks District Councils: Strategic Flood Risk Assessment Level 1 (November 2018)
What the Misbourne and the chalk mean for a basement
The assessment warns that local flooding after intense rain can arrive rapidly and endanger life, most of all in enclosed spaces like basement properties, and it flags flooding from the groundwater-bearing chalk aquifers that run under the Chiltern and South Bucks areas.
Chalk bedrock is where groundwater flooding happens most, in the Environment Agency's own account. Close to a public supply it goes further: in source protection zone 1, the inner ring around an abstraction serving homes or food production, the Agency will normally oppose in principle a planning application for anything that may physically disturb an aquifer.
The village has seen it happen. The Misbourne came into the High Street in 2001, and in February 2014 it left its banks upstream, north-west of the village, flooded across Upper Stone Meadow and the Blizzards Yard car park, and carried on over the High Street into Lower Stone Meadow.
The open ground beside the river is part of its flood plain. The Neighbourhood Plan says so of Upper Stone Meadow, one of the Local Green Spaces it designates, where development will only be permitted in very special circumstances. The assessment adds that a blockage in the village's critical flood relief culvert could raise flood risk, and in 2018 it reported the Environment Agency investigating fluvial risk along the Misbourne, work that could identify ways to reduce flood risk in Chalfont St Giles among other places.
So our advice is direct. Where a plot lies within Flood Zone 3a, or the 3b extents along the High Street, or where groundwater has risen close by, we will normally recommend against a new basement and look at gaining the space above ground; on higher, drier plots we only move to design once the ground investigation supports it.
Sources: Chiltern and South Bucks District Councils: Strategic Flood Risk Assessment Level 1 (November 2018); Environment Agency: The Environment Agency's approach to groundwater protection (LIT 7660; under review); Environment Agency (GOV.UK): Groundwater source protection zones (SPZs) (last updated 1 August 2024); Chalfont St Giles Parish Council: Neighbourhood Plan for Chalfont St Giles Parish 2014-2036
Planning permission for a basement in Chalfont St Giles
The 1990 Act's definition of development leaves out works that touch nothing but a building's interior, except where the works, begun since 5 December 1968, alter a building by giving it additional space underground. A new basement therefore cannot rely on that interior exemption.
The flood assessment calls a fuller review of the individual site, meaning a detailed Flood Risk Assessment, essential to the planning application and the design, and the Agency's planning flood map is where that information begins.
At the historic heart of the village, Policy CA1 applies to whatever shows of a basement above ground, such as lightwells, railings or new steps, and if the building is listed, the works may also call for listed building consent.
Beyond the village, in open Green Belt, Policy GB13 sets an extension's size and scale against the original dwelling and adds earlier extensions to the count, so before any design we put the question of how a basement's floorspace will be treated to the council's planners. The full test is set out on our Chiltern page.
Sources: legislation.gov.uk (The National Archives): Town and Country Planning Act 1990, section 55 (meaning of development); Chiltern and South Bucks District Councils: Strategic Flood Risk Assessment Level 1 (November 2018); Environment Agency (GOV.UK service): Get flood risk information for planning in England (Flood map for planning); Chiltern District Council: Chiltern District Local Plan, adopted 1 September 1997 (alterations 29 May 2001; consolidated September 2007 and November 2011); GOV.UK (Planning Practice Guidance): When is permission required?
Investigation, party walls and sign-off
Higher up, the undulating farmland shared by the village, Jordans and Seer Green is underlain by Thames River Terrace Deposits, with narrow swathes of Upper Chalk along the Alder Bourne valley to the east. Trial pits and boreholes show which ground a plot actually sits on, and where the water stands, before any structural scheme is drawn.
An excavation that comes within three or six metres of next door's building or structure, the distance turning on how deep the hole or foundations go, falls under the Party Wall Act, and so does digging below, and close to, the level of the neighbour's foundations.
Basement appraisals and conversions in Chalfont St Giles are carried out by our London team, whose architects, designers and structural engineers are all in-house and work with Hampstead Chartered Surveyors & Building Consultancy, from our studio on Finchley Road, Hampstead, NW3. Buckinghamshire Council decides any application. The 2010 Building Regulations apply to building and extending, so the waterproofing, structure and escape details are checked by Buckinghamshire Council's building control service or by a registered approver working privately.
If the investigation rules a basement out, we put that in writing and show what the same floor area could look like above ground instead.
Outside Chalfont St Giles, and in London rather than Buckinghamshire: 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: Buckinghamshire County Council and Chiltern District Council (hosted by Buckinghamshire Council): Landscape Character Area 20.1, St Giles Undulating Farmland; GOV.UK: The Party Wall etc. Act 1996 explanatory booklet; GOV.UK: Party walls and building work - work you must tell your neighbour about; GOV.UK: Building regulations approval - when you need approval; Hampstead Renovations: Fitzjohns Avenue, NW3 case study
Basement Conversions in Chalfont St Giles: Questions
Does being in a Misbourne flood zone rule out a basement?
Properties and farm buildings along the Misbourne sit in Flood Zone 3a, and the 3b extents touch properties on the High Street. The flood assessment warns that flooding can endanger life in enclosed spaces such as basements, so on a plot in those zones we would normally advise against one. Your address is checked on the Agency's planning flood map first.
How does groundwater in the chalk affect a basement in Chalfont St Giles?
Directly. The valley is cut into the Upper Chalk, groundwater flooding happens most on chalk bedrock, and groundwater flooding occurred near the telephone exchange in March and April 2014. Inside source protection zone 1 the Environment Agency will normally oppose in principle any development that may physically disturb an aquifer.
How would a basement be weighed under the Green Belt policy?
In open Green Belt, Policy GB13 measures an extension's size and scale against the original dwelling and counts earlier extensions with it. We ask the council how a basement will be treated under that test before any design, because the answer decides whether the scheme is worth pursuing.
Does digging a basement bring in the Party Wall Act?
Usually it does: excavating within three or six metres of a neighbouring building, depending on depth, and digging below and near the level of next door's foundations both require notice. Your neighbour has no power to stop lawful work, though they can influence the way and the timing of it.
Who checks a Chalfont St Giles basement against the building regulations?
Buckinghamshire Council's building control service, or a private registered building control approver you appoint, who gives the council an initial notice. Our engineers prepare the waterproofing, structural and escape details that the inspections look at.
Which investigations come before any basement design?
A look at the Agency's planning flood map and at the groundwater source protection zones, which can be viewed on the Magic map, then trial pits or boreholes, a drainage survey and a survey of the neighbouring foundations. Only if those support it do we start a design.
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