Amersham's built-up area is excluded from the Green Belt, which changes the question you are asking. Inside the town the local plan's ordinary extension policies apply rather than the tighter Green Belt rules. Those policies are specific about roof form, materials and side boundaries, so the design work is worth doing properly before anyone prices a foundation.
The Green Belt stops at the edge of the built-up area
Amersham's built-up area is excluded from the Green Belt, though it is enclosed by it. The adopted local plan lists the built-up areas of Amersham and Chesham Bois among those excluded, so the plan's ordinary built-up area extension policies apply inside the town rather than the Green Belt rules that apply outside it.
There is also no neighbourhood plan for Amersham. Buckinghamshire Council's list of neighbourhood plans carries no Amersham entry, while neighbouring Chesham Bois and Little Chalfont sit at the neighbourhood area designation stage. That leaves the Chiltern District Local Plan, adopted on 1 September 1997, as the document your design is measured against.
What Policies H15 and H17 ask of an extension
Roof form comes first. Under adopted Policy H15 a two storey or first floor extension should normally have a hipped or pitched roof integrated into the existing roof. A flat roofed one will not be permitted unless it is surrounded by existing pitched roofs, is subordinate to the main roof, and is not prominent in the street scene or locality.
Policy H15 also requires that external materials, windows and doors match or harmonise with those of the existing dwelling. Policy H17 allows a single storey side extension to be built up to the side boundary. The exception is areas characterised by spacious layouts, where relatively large distances matter in the street scene and a gap must be kept.
The ground the foundations go into
It depends which of the two Amershams you are in. Across Amersham-on-the-Hill the British Geological Survey maps the Clay-with-flints Formation over chalk bedrock, at Sycamore Road, Hill Avenue, Woodside Road, Chestnut Lane and Highfield Close. BGS describes that formation as orange-brown and red-brown sandy clay with abundant nodules and rounded pebbles of flint, up to about 10 metres thick.
BGS also notes that the thickness is very variable where the formation has been let down into dissolution pipes in the chalk beneath. On the valley floor the picture differs. Under Amersham High Street the mapped superficial deposit is Alluvium, clay, silt, sand and gravel laid down by the River Misbourne, over New Pit Chalk Formation bedrock. Mapping tells you what to expect. A trial hole on your plot tells you what you have.
Extending a house in Elm Close
The council has already set out what has worked there. Its Elm Close appraisal records that opportunities for sympathetic extension are limited. It also records that several properties have been given two storey side extensions with pitched roofs, set back from the front elevation and with ridge lines below that of the main roof. That is how they stayed subordinate to the original building.
Garages are treated the same way. The houses of Elm Close were built without garages, and every garage added since the 1920s has been placed in a rear garden, which the appraisal asks any further garage to follow. Points like these belong in the conversation with the council before drawings are finalised.
- First fix and second fix plumbing and electrics throughout
- Party Wall Act notices for excavation near a neighbour's wall
- Scaffolding, site set-up and safe access on tight London plots
- Roof works and structural alterations at roof level
- Flat and pitched roofs with rooflights or lanterns
Trusted House Extensions in Amersham
Because an extension here is a design conversation with the council before it is a building job. We appoint the structural engineer for the foundations, beams and openings, build to the calculations, and coordinate the building control application and inspections. Where the local plan's roof and materials policies or a conservation area shape the design, we build what has been approved rather than improvising on site. Fully insured, with free no obligation quotations.
- Weathertight shell reached before the fit-out begins
- Registered in England, Companies House no. 14611254
- Photographs kept at each stage for your records
- Permitted development checked before you pay for a planning application
- Clean, tidy and reliable tradespeople
- Materials priced at options, so you choose the finish