House
extensions
in Cobham, KT11
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Extending a Cobham house starts with three facts about the address: Green Belt, flood zone and conservation area. Together they decide the planning route before the design does.
The council's 2012 companion guide, which the Design Code has since replaced as guidance, found that the 1930s and post-war semi-detached houses of Northfields have space for extensions and alterations, and expected change around Tartar Hill to come from extending existing houses rather than replacing them. In the Cobham Conservation Area the appraisal reports considerable pressure to extend.
The limits come at the edges. Green Belt surrounds the settlement, the Mole and its floodplain lie alongside the town, and the appraisals for The Tilt and Downside ask that cottage-scale houses stay cottage-scale.
Which route an extension in Cobham takes
Conservation areas count as article 2(3) land under the permitted development order, and there householder rights are narrower. Class A does not permit an extension that goes beyond a wall forming a side elevation, one of more than a single storey that goes beyond the rear wall, or cladding in stone, artificial stone, pebble dash, render, timber, plastic or tiles. Five conservation areas lie within the settlement.
Outside a conservation area we test the design against the national permitted development classes, which our Surrey page explains, and we draw the original house separately from any later additions so the limits are measured from the right starting point.
Where an application is needed, a householder application goes in with the council's Design Code Checklist. The Design Code asks that single or two-storey side extensions either integrate fully with the house or read clearly as additions, and that two-storey rear extensions be very carefully sited and designed.
Green Belt land surrounds the settlement area, and the Downside Village, The Tilt and Plough Corner conservation areas lie wholly within it. For a house in the Green Belt, policy DM18 measures an extension against the original building, which the plan takes as the house on 1 July 1948 or as first built if later. Our Elmbridge page quotes the test in full.
Sources: legislation.gov.uk (The National Archives): GPDO 2015, Schedule 1, Part 1 - Article 2(3) land; legislation.gov.uk (The National Archives): The Town and Country Planning (General Permitted Development) (England) Order 2015, Schedule 2, Part 1 (development within the curtilage of a dwellinghouse) - latest available (revised); Elmbridge Borough Council: Elmbridge Core Strategy; Elmbridge Borough Council: Design Code SPD page; Elmbridge Borough Council: Elmbridge Design Code 2024 (SPD); Elmbridge Borough Council: Downside Village conservation area; Elmbridge Borough Council: The Tilt Conservation Area Character Appraisal and Management Plan (2015); Elmbridge Borough Council: Cobham (Plough Corner) Conservation Area statement of designation; Elmbridge Borough Council: Development Management Plan (April 2015)
Extending in Cobham's conservation areas
In Northfields and around Tartar Hill the 2012 guide treated extension as the expected way to add space; inside the conservation area, where the appraisal reports considerable pressure to extend, each proposal is weighed against the appraisal.
The Cobham appraisal expects most change in its conservation area to be extensions to existing buildings, and says new work should generally match the height of the buildings next to it. Applicants are asked to justify their design decisions and show how the scheme relates to its context.
Both The Tilt and Downside appraisals say that extensions which take a house from cottage scale to something larger should be resisted. Downside's suggests pantiles often work well on outbuildings or small extensions, and the Cobham appraisal's palette runs from red stock brick and handmade rubbed brick to weatherboarding, usually stained black, handmade clay tiles and pantiles, and Welsh slate.
Brickwork in The Tilt is generally Flemish bond, and its appraisal asks for the correct bond and a traditional mortar mix wherever new work can be seen from the public realm. We set out a sample panel for approval before the walls go up.
Red brick boundary walls are part of the Cobham Conservation Area's character, and the council will resist proposals to remove or significantly alter them, which affects where a side extension or a new access can go.
Sources: Elmbridge Borough Council: Design Code SPD page; Elmbridge Borough Council: Companion guide - Cobham, Oxshott, Stoke d'Abernon and Downside; Elmbridge Borough Council: Cobham Conservation Area Character Appraisal and Management Plan (endorsed 11 June 2019); Elmbridge Borough Council: The Tilt Conservation Area Character Appraisal and Management Plan (2015); Elmbridge Borough Council: Downside Village Conservation Area Character Appraisal and Management Plan (2014)
Flood zones and foundations for a Cobham extension
Most of the settlement area is Flood Zone 1, but the town is set alongside the River Mole and its floodplain. For householder development in Flood Zone 3, the council's assessment asks for evidence either that floor levels will be no lower than existing, with flood proofing where appropriate, or that the extension's floors will sit 300mm above the known or modelled 1 in 100 year river flood level including climate change. In Flood Zone 2 it asks for floor levels no lower than existing, with flood proofing where appropriate.
Where the functional floodplain, Flood Zone 3b, takes in developed streets, existing building footprints that can be shown to keep floodwater out are not counted as functional floodplain, and minor development there should not affect floodplain storage or flow routes, so an extension's site plan needs to show exactly where the existing house stands.
The ground here is Taplow Gravel or alluvium over Bagshot sand and the Claygate Member, and almost all of the Cobham Conservation Area lies in an area of high archaeological potential. Our engineer designs foundations from trial holes, and we raise archaeology at design stage for any dig inside the conservation area.
Sources: Elmbridge Borough Council: Level 1 Strategic Flood Risk Assessment, April 2024 (AECOM); Elmbridge Borough Council: Elmbridge Design Code 2024 (SPD); Elmbridge Borough Council: Cobham Conservation Area Character Appraisal and Management Plan (endorsed 11 June 2019)
Building an extension in Cobham
The measured survey covers the plot as well as the house: levels, trees, drainage runs and boundaries. Our drawings separate the original house from later additions, because the permitted development limits and the Green Belt test both start from it.
Where an extension is built on or at the boundary, or its foundations go below and near the neighbour's, we prepare the party wall notices alongside the drawings so they are served in good order.
Extending your home is on the council's list of work that may need building control approval: Elmbridge Building Control Services (EBCS), for Elmbridge Borough Council, until 31 March 2027, and from 1 April 2027 the provider named by East Surrey Council, or a private registered approver if you prefer.
We set the site up so the house stays usable: a sealed line between the build and the rooms you live in, a protected route for deliveries, and a planned day for breaking through into the existing house once the new structure is weathertight.
Outside Cobham, and in London rather than Surrey: Holland Avenue, Wimbledon SW20. That completed project of ours combined a loft conversion, a double-storey extension to the rear and side, and a whole-home refurbishment, as its case study describes.
Sources: Elmbridge Borough Council: Development Management Plan (April 2015); GOV.UK: Party walls and building work (printable guide); Elmbridge Borough Council: Building control; GOV.UK: Building regulations approval; Hampstead Renovations: Holland Avenue, Wimbledon SW20 case study
House Extensions in Cobham: Questions
Is my Cobham house in the Green Belt, and how big can the extension be?
Green Belt surrounds the settlement, and the Downside Village, The Tilt and Plough Corner conservation areas are wholly inside it. If your house is in the Green Belt, policy DM18 of the Development Management Plan sets the limit by reference to the original building, the house as it stood on 1 July 1948 or as first built if later. We do not reduce the test to a rule of thumb; our Elmbridge page quotes it, and we measure your house against it.
Can I build a rear extension in Cobham under permitted development?
It can be. Where no conservation area covers the house, a design that stays within the national limits qualifies, which we check against the original house. Inside a conservation area the rights are narrower: nothing beyond a side wall, nothing over one storey beyond the rear wall, and no cladding in render, timber, stone or tiles. The two Article 4 directions the council lists for Cobham, on land off Woodlands Lane and Pointers Road, remove other rights, not the householder ones.
Does an extension in the Cobham conservation area need planning permission?
A side extension, or anything over one storey beyond the rear wall, does, because permitted development does not cover them on conservation area land. Most applications in a conservation area also need a Heritage Statement, and the Cobham appraisal asks applicants to justify their design decisions and show how the scheme relates to its context.
How does the flood zone affect an extension in Cobham?
The floor-level requirements in the council's flood assessment are set for Zones 3 and 2. In Zone 3 it asks householders to show either that floor levels will be no lower than existing, with flood proofing where appropriate, or that the extension's floors will be raised 300mm above the known or modelled 1 in 100 year river flood level including climate change. In Zone 2 it asks for floor levels no lower than existing, with flood proofing where appropriate. On developed land in the functional floodplain, Zone 3b, minor development should also not affect floodplain storage or flow routes.
Does an extension in Cobham need building regulations approval, and from whom?
Usually. The council lists extending your home among the work that may need building control approval, and the regulations apply whether or not planning permission is needed. Approval comes from Elmbridge Building Control Services (EBCS), for Elmbridge Borough Council, until 31 March 2027, and from 1 April 2027 the provider named by East Surrey Council, or from a private registered building control approver.
How should I prepare my Cobham home for a feasibility visit about an extension?
Find any plans, consents or completion certificates for earlier extensions, and anything that shows what the house looked like originally, because both the permitted development limits and the Green Belt test start from the original house. Note where the boundaries, drains and large trees are, and tell us how you use the house now and what is missing.
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