House
extensions
in Chalfont St Giles, HP8
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How an extension in Chalfont St Giles is judged depends first on where the house stands: inside the village's built-up area, on one of the Green Belt rows, or out in the open Green Belt around them.
The village lies in two halves on either side of the Misbourne valley, and the Green Belt wraps right round it. Strings of houses inside the Green Belt, the Neighbourhood Plan notes, run up to the built-up boundary at Three Households and at Bowstridge Lane, while Jordans, further south, is a Green Belt settlement in its own right.
Plot sizes vary as much as the planning position. Detached houses stand in large plots east of the valley; two north-western neighbourhoods are named Areas of Special Character for their low-density detached houses in big gardens; and the parish plan sees the rest of the village as two-storey for the most part, broken by the odd three-storey building and a few bungalows.
Sources: Chiltern and South Bucks District Councils (hosted by Buckinghamshire Council): Chiltern & South Bucks Townscape Character Study Part 3, Chiltern Settlements (November 2017); Chalfont St Giles Parish Council: Neighbourhood Plan for Chalfont St Giles Parish 2014-2036
Three planning positions within one village
Inside the built-up area that sits outside the Green Belt, Local Plan Policy H13 grants an extension that does no significant harm to the neighbours' amenities and leaves the character of the street scene or locality intact, while Policies H14 to H17 fill in the detail.
The Local Plan also lists existing rows of dwellings in the Green Belt; in this parish they are Three Households, Dibden Hill/Bowstridge Lane and Narcot Lane. Policy GB12 lets an extension there proceed when H13 to H17 are met, but refuses one on a settlement's edge that pushes built form significantly out into the open land alongside.
Across the rest of the Green Belt, Policy GB13 looks for an extension that stays subordinate in size and scale to the house as first built and does not intrude on the landscape, with any earlier additions weighed together. National policy frames the test as whether the extension is disproportionate to the original building, taken as it stood on 1 July 1948 or, for anything newer, as built. The full wording lives on our Chiltern page.
We settle which of the three applies before any sketch is made, from the council's policy mapping and, where a boundary runs close to the house, a written question to its planning officers.
Sources: Chalfont St Giles Parish Council: Neighbourhood Plan for Chalfont St Giles Parish 2014-2036; Chiltern District Council: Chiltern District Local Plan, adopted 1 September 1997 (alterations 29 May 2001; consolidated September 2007 and November 2011); MHCLG (GOV.UK): NPPF Chapter 13, Protecting Green Belt land (GB1-8) (August 2026)
What the Local Plan and the parish ask of an extension
Policy H14 turns on the people next door: no overbearing bulk, no significant loss of daylight to their garden or to the principal windows of their habitable rooms or kitchen, and no significant cut in their privacy. Policy H15 turns on the house itself, asking an extension to respect its scale and proportions, to stay below its height, to use outside materials, windows and doors that match or harmonise, and to carry any two-storey or first-floor addition under a hipped or pitched roof joined into the existing one. Single-storey additions can take a flat roof unless they would be prominent in the street scene or another public place.
Where wide spacing between houses is part of the street scene, Policy H17 asks a single-storey side extension to stop short of the boundary. That matters on the large plots east of the valley and in the two Areas of Special Character, where the townscape study already counts buildings placed closer to the boundary among the things eroding their character.
The Neighbourhood Plan's Design Policy 1 is written for new development, infill included. Its tests, respecting an established building line, keeping spacing like that along the street, reflecting nearby heights and not seriously reducing a neighbour's daylight, are the ones we also measure an extension against.
The plan also warns that overdeveloping garden land can harm residential amenity and the character of an area, and it sets down local opposition to modest houses being bought, demolished and rebuilt so large that they swamp the houses beside them.
Sources: Chiltern District Council: Chiltern District Local Plan, adopted 1 September 1997 (alterations 29 May 2001; consolidated September 2007 and November 2011); Chiltern and South Bucks District Councils (hosted by Buckinghamshire Council): Chiltern & South Bucks Townscape Character Study Part 3, Chiltern Settlements (November 2017); Chalfont St Giles Parish Council: Neighbourhood Plan for Chalfont St Giles Parish 2014-2036
The village centre, Jordans and the Misbourne
The conservation area at the centre of the village counts as article 2(3) land, which takes two kinds of extension out of the householder rules: anything built out past a side wall of the original house, and anything over one storey that projects behind its rear wall. Both need an application, tested under Policy CA1 against siting, the established pattern, density, scale, bulk, height, design and appearance.
Under the householder rules a single-storey addition at the back may reach no further than 4 metres past the original rear wall on a detached house, or 3 metres on any other, before its other conditions are even reached.
In Jordans the work may need a further approval from Jordans Village Limited, on top of planning permission and the Green Belt policy that covers the whole settlement.
Nearer the river, the flood assessment puts several properties into Flood Zone 3a and expects a 1% annual chance flood on the High Street to stay under 0.5 metres deep. Flood information for an application starts from the government's flood map for planning, which says that for a simple, low-risk job such as a house extension the owner may be able to prepare it.
Sources: legislation.gov.uk: GPDO 2015 (SI 2015/596), Schedule 1 Part 1, Article 2(3) land (latest available revised); legislation.gov.uk: GPDO 2015 (SI 2015/596), Schedule 2 Part 1, development within the curtilage of a dwellinghouse (latest available revised); Chiltern District Council: Chiltern District Local Plan, adopted 1 September 1997 (alterations 29 May 2001; consolidated September 2007 and November 2011); Chalfont St Giles Parish Council: Neighbourhood Plan for Chalfont St Giles Parish 2014-2036; 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)
Ground, party walls and sign-off
Ground conditions change across the parish. On the valley floor the chalk lies under alluvium, while the rolling farmland the village shares with Jordans and Seer Green rests mostly on Thames River Terrace Deposits, so foundations are sized from trial pits rather than assumed.
Building on or at the boundary, or cutting into a party wall to carry a new beam, means telling the neighbour first. They cannot block work that is lawful, though they can shape when and how it happens.
Extensions here are drawn and built by our London team, whose architects, designers and structural engineers are employed in-house and partner with Hampstead Chartered Surveyors & Building Consultancy; the work is run from Finchley Road in Hampstead, NW3. Buckinghamshire Council decides the application. Extensions come within the Building Regulations 2010, and you can have them checked by Buckinghamshire Council's building control service or appoint an approver registered for building control in the private sector.
The build order is agreed in advance: foundations and drainage, then a weathertight shell, and only then the opening into the existing house, so the rooms you live in stay closed off from the works for as long as possible.
Outside Chalfont St Giles, and in London rather than Buckinghamshire: Project example: Hampstead Garden Suburb, NW11. That project of ours added a rear extension of a single storey to an Arts and Crafts house, and had to win consent from both the Hampstead Garden Suburb Trust and the council, as its case study records.
Sources: Buckinghamshire County Council and Chiltern District Council (hosted by Buckinghamshire Council): Landscape Character Area 19.2, Lower Misbourne Settled River Valley; Buckinghamshire County Council and Chiltern District Council (hosted by Buckinghamshire Council): Landscape Character Area 20.1, St Giles Undulating Farmland; 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: Meadway, NW11 case study
House Extensions in Chalfont St Giles: Questions
Which Green Belt rule would my extension in Chalfont St Giles fall under?
The built-up village is outside the Green Belt, and there Policy H13 and the policies after it apply. The Green Belt rows at Three Households, Dibden Hill/Bowstridge Lane and Narcot Lane come under Policy GB12, which refuses extensions that push into the open land at a settlement edge. Elsewhere, Policy GB13 wants an extension kept subordinate to the original dwelling and unobtrusive in the landscape, counting earlier extensions too.
Is planning permission needed to extend a house in the village centre conservation area?
For a side extension, or a rear extension of more than one storey, yes, because the conservation area is article 2(3) land. A single-storey rear addition can still fall within the householder rules if it meets every condition, although cladding the house in render, timber, plastic or tiles is not covered. Where permission is needed, Policy CA1 is the test.
How does the parish's Neighbourhood Plan bear on an extension?
Its Design Policy 1 covers new development and infill, asking for respect for building lines, street spacing, neighbouring heights, daylight and frontage boundaries. It records opposition to small houses being replaced by very large dwellings that overwhelm their neighbours, and asks applicants to consider the 2013 residential extensions guidance where relevant.
My house is near the Misbourne. What flood information will an extension need?
Several properties near the river are in Flood Zone 3a, and the 3b extents reach properties on the High Street. An application then needs flood risk information, which starts from the government's flood map for planning; for a simple, low-risk extension the service says an owner may be able to prepare it.
Who carries out the building control checks on an extension?
Buckinghamshire Council's building control service or a private registered building control approver, as you prefer; an approver informs the council through an initial notice. We book each inspection as the work reaches that stage, so nothing is covered up before it has been seen.
What should I have ready before you visit to talk about an extension?
Any plans, earlier permissions and the dates of past extensions, since in the open Green Belt earlier extensions are counted cumulatively. Note where you think the boundaries run, which trees matter to you, and how you use the rooms that the extension would join.
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