House
extensions
in Chorleywood, WD3
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Three lines on a map usually settle whether a Chorleywood extension can go to the side, the rear or upwards: the Green Belt, a conservation area and the edge of the Chilterns National Landscape.
Plenty of Chorleywood plots look as though they have room to build. The Station Estate appraisal describes large detached houses on large plots, often behind substantial hedges, at a low density of dwellings per hectare; Heronsgate's appraisal puts its own density at less than two dwellings per hectare.
Room is not the same as permission. Those appraisals value the spaces between houses as much as the houses, the Station Estate's saying that every open gap and space there is important, and the Neighbourhood Plan records a trend of smaller properties being replaced with significantly larger ones.
The Green Belt test for a Chorleywood extension
The Neighbourhood Plan says the parish includes Metropolitan Green Belt. A large proportion of the Chorleywood Common conservation area lies within it, and the whole of Heronsgate's. National policy accepts an extension there only if it is not a disproportionate increase on the original building, meaning the building as it stood on 1 July 1948, or as first built if later.
Locally, the Development Management Policies refuse extensions disproportionate in size, taken alone or together. The council's Green Belt guidance treats a cumulative floorspace increase of over 40% on the original house as normally unacceptable, takes the base as the floorspace when built or as at 1948, leaves out garages and outbuildings unless they were built integral with the house, and lets small houses with limited basic amenities extend further to reach contemporary standards.
The guidance normally looks more kindly on rear extensions than side ones, and regards a single-storey side extension as harming openness once buildings would cover more than 80% of the frontage. At Heronsgate the appraisal expects Green Belt policy, more than conservation policy, to limit how large an extension can be.
So we begin with the original house. Archive drawings, past planning decisions and the fabric itself tell us what stood there at the base date, and every later addition is counted against it before any sketch is made.
Sources: Chorleywood Parish Council / Three Rivers DC: Chorleywood Neighbourhood Plan (linked as Updated July 2026); Three Rivers District Council: Chorleywood Common Conservation Area Appraisal, 06 Audit, Development Criteria and Management Strategy; Three Rivers District Council: Heronsgate Conservation Area Appraisal (April 2012); MHCLG (GOV.UK): NPPF Chapter 13, Protecting Green Belt land (GB1-8) (August 2026); Three Rivers District Council: Development Management Policies LDD (adopted July 2013); Three Rivers District Council: SPG No.3 Extensions to Dwellings in the Green Belt (August 2003)
Conservation areas, the National Landscape and permitted development
Class A permits a one-storey addition at the back reaching up to 4 metres past the original rear wall of a detached house, and 3 metres past that of any other house. The longer allowance in paragraph (g) is reserved for houses off article 2(3) land.
That category takes in conservation areas and areas of outstanding natural beauty, which national policy now calls National Landscapes. On it, building out beyond a side wall, a rear addition taller than a single storey, and render or timber cladding all fall outside the householder rights. Of the four Chorleywood postcodes checked on Natural England's map, the two in the Chorleywood North & Sarratt ward lie inside the Chilterns National Landscape and the two in Chorleywood South & Maple Cross outside.
For land within or near the National Landscape, Policy DM7 supports development unless it fails to conserve the area's landscape character, harms its setting and the views into and out of it, or detracts from the public's enjoyment of it. Neighbourhood Plan Policy 8.1 asks building on adjoining land not to intrude on the landscape.
Throughout the parish, Policy 2.2 asks every development to add to the street scene through frontage, building line, scale and design, Policy 2.4 expects tree, hedge and soft planting, and Policy 2.6 asks applicants to show how light pollution has been kept down. We weigh that last one whenever a scheme proposes a large glazed rear wall.
Sources: legislation.gov.uk: GPDO 2015 (SI 2015/596), Schedule 2 Part 1, development within the curtilage of a dwellinghouse (latest available revised); 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 1 Part 1, Article 2(3) land (latest available revised); MHCLG (GOV.UK): NPPF Annex B, Glossary (August 2026); Natural England: National Landscapes (AONB) England feature service, point query at WD3 5SL; Natural England: National Landscapes (AONB) England feature service, point query at WD3 5SS; postcodes.io (ONSPD-derived): postcode WD3 5SL; postcodes.io (ONSPD-derived): postcode WD3 5SS; Natural England: National Landscapes (AONB) England feature service, point query at WD3 5HZ; Natural England: National Landscapes (AONB) England feature service, point query at WD3 5ET; postcodes.io (ONSPD-derived): postcode WD3 5HZ; postcodes.io (ONSPD-derived): postcode WD3 5ET; Three Rivers District Council: Development Management Policies LDD (adopted July 2013); Chorleywood Parish Council / Three Rivers DC: Chorleywood Neighbourhood Plan (linked as Updated July 2026)
Light, spacing and party walls
The district's design guidance shapes the form wherever the house stands. A single-storey rear extension should generally be no deeper than 3.6m, or 4m on a detached house; two-storey rear additions should not cross a 45-degree splay line taken from the neighbour's rear wall; and first-floor side additions should sit at least 1.2 metres off the flank boundary.
That side gap must grow in low density areas, and the Station Estate appraisal calls its own density low. The guidance also asks that an extension takes no light from neighbouring windows, allows no overlooking and keeps the parking the council's standards require.
Building right up to, or astride, the boundary line means notifying the neighbour first, and so does excavating within 3 or 6 metres of their building, depending on the depth of the new foundations. That can catch a detached house close to the next one as easily as a semi-detached pair.
Our London team, the in-house architects, designers and structural engineers who work with Hampstead Chartered Surveyors & Building Consultancy, takes on extensions in Chorleywood. Any application goes to Three Rivers District Council, and the work is checked under the Building Regulations 2010 by Hertfordshire Building Control or by a private registered building control approver.
Sources: Three Rivers District Council: Development Management Policies LDD (adopted July 2013); Three Rivers District Council: Chorleywood Station Estate Conservation Area Appraisal (November 2005); GOV.UK: Party walls and building work - overview; GOV.UK: Party walls and building work - work you must tell your neighbour about; GOV.UK: The Party Wall etc. Act 1996 explanatory booklet; Three Rivers District Council: Development Plan (Current Local Plan); GOV.UK: Building regulations approval - when you need approval; Three Rivers District Council: Building control
Surface water and the extension footprint
The district flood assessment requires a site-specific flood risk assessment for all development, minor works included, in Flood Zone 1 where the Environment Agency's flood map for planning shows a surface water risk. The Environment Agency adds that for something as simple and low-risk as a house extension, owners may be able to write that assessment themselves.
The assessment also warns that successive minor developments can add to surface water problems, especially as the lead local flood authority is not consulted on them. Neighbourhood Plan Policy 12.1 asks plans in high-risk areas to explain how water ingress and surface water drainage will be mitigated.
We therefore design the drainage of a new roof and terrace with the extension itself: where the water goes, whether a soakaway will work on that plot, and whether the new floor should sit above the garden level.
Outside Chorleywood, and in London rather than Hertfordshire: 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: Three Rivers District Council (JBA): Level 1 Strategic Flood Risk Assessment 2026; Environment Agency (GOV.UK service): Get flood risk information for planning in England (Flood map for planning); Chorleywood Parish Council / Three Rivers DC: Chorleywood Neighbourhood Plan (linked as Updated July 2026); Hampstead Renovations: Meadway, NW11 case study
House Extensions in Chorleywood: Questions
Is my Chorleywood house in the Green Belt, and what is the council's test for an extension?
Part of the parish is: the Neighbourhood Plan says it includes Metropolitan Green Belt, a large part of the Chorleywood Common conservation area lies in it, and all of Heronsgate does. For a house there, extensions disproportionate to the original building are refused, and the council's guidance normally rejects a cumulative floorspace increase over 40% on the house as built or as at 1948. We confirm the position for your address before design.
Does an extension in the Chilterns National Landscape need planning permission?
Often it does. A National Landscape address counts as article 2(3) land, so building beyond a side wall, or a rear addition rising above a single storey, needs an application. A one-storey rear addition inside the Class A limits may still qualify, and the council applies Policy DM7 to anything within or near the area that does need an application.
What does the Chorleywood Neighbourhood Plan say about extensions?
Several of its policies bear on one: Policy 2.2 on the street scene, Policy 2.4 on landscaping, Policy 2.6 on light pollution, Policy 1.4 on the setting of conservation areas, and Policy 4.1, which will usually mean suitable bungalows in groups are not converted into multi-level houses.
How does flood risk affect an extension in Chorleywood?
Surface water is the main risk the Neighbourhood Plan identifies. Where the flood map for planning shows surface water risk at the address, the district flood assessment makes a site-specific flood risk assessment necessary even when the extension is minor, and Policy 12.1 asks the plans to show how water ingress and drainage are handled.
Who signs off building regulations for an extension in Chorleywood?
Hertfordshire Building Control, unless you appoint a private registered building control approver instead. The Building Regulations 2010 cover the extension of buildings, and approval under them is separate from planning permission, so it is needed even when the extension is permitted development.
What should I have ready for a feasibility visit on a Chorleywood extension?
Gather any old plans, past planning decisions and completion certificates for earlier additions, because the Green Belt test counts everything added to the original house. Make the garden, drain covers and side passages reachable so we can see levels, boundaries and where water runs.
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