House
extensions
in Amersham, HP6
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Extending an Amersham house means answering three questions before any sketch: does the plot lie inside the built-up area or in the Green Belt, is it in a conservation area, and do the Misbourne's flood zones reach it.
Most Amersham households live in detached or semi-detached houses: 1,673 and 957 respectively in the Amersham and Chesham Bois ward at the 2021 Census, and 2,235 and 890 in Penn Wood and Old Amersham. Plots run from the medieval burgage strips behind the High Street to the large, secluded gardens of the woodland roads on the hill.
Room to extend varies street by street. Since 1978 much new building in Amersham has gone onto backland, and some of those schemes hold a significant number of houses on small building plots.
Sources: ONS/Nomis: Census 2021 TS044 Accommodation type, 2022 ward Amersham and Chesham Bois (E05013121); ONS/Nomis: Census 2021 TS044 Accommodation type, 2022 ward Penn Wood and Old Amersham (E05013154); Buckinghamshire County Council with English Heritage: Amersham, Buckinghamshire Historic Towns Assessment Report (2009); Buckinghamshire Council (Chiltern and South Bucks District Councils): Chiltern & South Bucks Townscape Character Study Part 3, Chiltern Settlements (Chris Blandford Associates, November 2017)
Built-up area or Green Belt: the first test for an Amersham extension
The built-up area of Amersham is excluded from the Green Belt. Within it, Policy H13 grants permission for extensions provided neighbours' amenities do not suffer significantly and the street scene nearby is not harmed, with Policies H14 to H17 setting out the detail.
Those companion policies are specific. H14 guards against a significant loss of daylight to a neighbour's garden or main windows; H15 wants materials, windows and doors that match or harmonise, no extension taller than the house, and pitched or hipped roofs on two-storey additions, tied into the existing roof.
Side space is regulated too. H16 applies H11's one-metre gap between a first-floor flank wall and the boundary to two-storey side and rear extensions, and H17 asks single-storey side extensions in spacious streets to keep a gap that suits the street.
Beyond the built-up edge the Green Belt applies. Houses in the named rows at London Road East and Cokes Lane (south) are judged under GB12, and other Green Belt houses under GB13, which permits extensions subordinate to the original dwelling and not intrusive in the landscape. Our Chiltern page quotes both alongside the national test against disproportionate additions.
Sources: 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)
Extending in Old Amersham, Elm Close and the Weller Estate
In the four conservation areas the householder rights shrink. On article 2(3) land an extension is not permitted development if it would go beyond a side wall of the original house, or if it would have more than one storey and project past the rear wall.
Those schemes need an application, judged under Policy CA1 for whether they preserve or enhance the area with regard to siting, the established pattern of development, density, scale, bulk, height, design and external appearance.
Old Town plots are long and narrow behind the High Street, and on the north side they are hemmed in between the road and the Misbourne. With 48 listed buildings on that side alone, a rear addition there often meets a listed neighbour, the conservation area and the flood zone together.
Elm Close was built as thirty Arts and Crafts semis grouped round a central green, and Highfield Close on the Weller Estate as matching semis round a triangular green, so an extension on either is drawn to read as part of the set.
Sources: legislation.gov.uk: Town and Country Planning (General Permitted Development) (England) Order 2015 (SI 2015/596), article 2 (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); Buckinghamshire County Council with English Heritage: Amersham, Buckinghamshire Historic Towns Assessment Report (2009); Chiltern and South Bucks District Councils: Strategic Flood Risk Assessment Level 1 (November 2018)
Flood zones, surface water and the ground beneath an extension
The Misbourne's Flood Zone 2 takes in many properties on Broadway, Market Square, Church Street and High Street, with Flood Zone 3a in parts of the Old Town. The Environment Agency notes that a flood risk assessment usually needs a consultant, though for a simple, low risk development like a house extension you may be able to prepare it yourself.
Surface water is a separate risk. The High Street and Broadway flood in heavy rain, and on the hill the assessment maps flow paths broadly following the railway, with some roads, and possibly properties, already flooded from surface water.
Foundations follow the geology. The Old Town stands on chalk overlain by river alluvium, with soils prone to waterlogging as the groundwater table rises and falls; the ridge is chalk capped with clay with flints, under slowly permeable soils.
Our structural engineers size foundations from trial holes dug on the plot, not from assumptions, and set the new floor level and drainage so that the extension sheds water away from the house.
Sources: 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); Buckinghamshire County Council with English Heritage: Amersham, Buckinghamshire Historic Towns Assessment Report (2009)
Neighbours, party walls and sign-off for an Amersham extension
An extension in Amersham is drawn and engineered by our London team, with in-house architects, designers and structural engineers working alongside Hampstead Chartered Surveyors & Building Consultancy, who survey the existing house. Buckinghamshire Council decides the planning application; Buckinghamshire Council's building control service or a private registered building control approver inspects the build, because the Building Regulations cover the extension of buildings.
Building on the line between two properties, working on an existing party wall or excavating close to a neighbour's foundations and deeper than them means serving notice under the Party Wall Act, a process kept apart from planning permission and building regulations.
Where permitted development does cover a scheme, a Lawful Development Certificate from the council confirms that it is lawful without a planning application.
In the Areas of Special Character the Townscape Character Study finds the character weakened where buildings move closer to the boundary and front gardens are hard-surfaced, so we keep side space and planting wherever the brief allows.
Outside Amersham, 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 Council: Building regulations and control; GOV.UK: Building regulations approval - when you need approval; GOV.UK: Party walls and building work - work you must tell your neighbour about; GOV.UK: Party walls and building work - overview; GOV.UK (MHCLG): Permitted development rights for householders: technical guidance (September 2019); Buckinghamshire Council (Chiltern and South Bucks District Councils): Chiltern & South Bucks Townscape Character Study Part 3, Chiltern Settlements (Chris Blandford Associates, November 2017); Hampstead Renovations: Meadway, NW11 case study
House Extensions in Amersham: Questions
Is my Amersham house in the Green Belt, and what is the council's test for an extension?
Houses within the built-up area of Amersham are not: it is excluded from the Green Belt, and extensions there are judged under Policy H13 on neighbours and the street scene. Houses beyond that edge are in it. Those in the named rows at London Road East and Cokes Lane (south) fall under GB12; the rest under GB13, which allows extensions subordinate to the size and scale of the original dwelling and not intrusive in the landscape.
Does an extension in one of Amersham's conservation areas need planning permission?
Often it does. On article 2(3) land, which includes conservation areas, an extension beyond a side wall of the original house, or one of more than one storey beyond the rear wall, is not permitted development. Policy CA1 then tests the design on siting, scale, bulk, height and appearance. A modest single-storey rear addition is not caught by those two exclusions, so we check it against the remaining limits before deciding the route.
How does flood risk affect an extension in Amersham?
In the Old Town it can shape the scheme. Many properties on Broadway, Market Square, Church Street and High Street are in Flood Zone 2, with Flood Zone 3a in parts, and the High Street and Broadway also flood from surface water. A planning application there carries a flood risk assessment, which for a simple house extension you may be able to prepare yourself. On the hill, surface water flow paths following the railway are the thing to check.
Who signs off building regulations for an extension in Amersham?
Buckinghamshire Council's building control service, or a private registered building control approver who informs the council through an initial notice. The Building Regulations 2010 cover the extension of buildings, so foundations, structure, insulation, drainage and fire safety are all inspected. Approval is separate from planning permission and from any party wall agreement.
Do I need to tell my neighbours about an extension in Amersham?
If you build on or at the boundary, work on an existing party wall or excavate deeper than their foundations close by, the Party Wall Act requires you to serve notice. Separately, Policy H14 protects neighbours against a significant loss of daylight to their garden or main windows, and the council considers that when it decides the application.
How should I prepare my Amersham home for a feasibility visit about an extension?
Send the address and any title plan, earlier planning decisions or drawings for the house, and tell us about extensions already built, because in the Green Belt the council counts their cumulative effect. Clear access to the garden and the side passage so we can measure the boundaries and look at the ground and drainage where the extension would go.
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