House
extensions
in Rickmansworth, WD3
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Rickmansworth extensions are measured against more than the garden: the district's depth guidance, the Green Belt floorspace test at the edges of town, the Moor Park appraisal on the estate, and floor levels near the rivers.
Rickmansworth's extension questions change from one neighbourhood to the next. Behind the High Street the houses sit on tight plots with small front gardens; in Moneyhill the residential streets generally have long plots with large back gardens; and on the Moor Park estate the appraisal describes large houses on individual plots, set back in spacious surroundings. At the 2021 Census, 966 of the Rickmansworth Town ward's households lived in detached houses and 634 in semi-detached ones, the two types that most often extend sideways as well as back.
Green Belt land wraps around parts of the town, the river valleys run through it, and a made neighbourhood plan adds its own design policy. So before we sketch an extension we establish which of those layers apply to the plot, and how far the original house has already been extended.
Sources: Batchworth Community Council / Three Rivers DC: Batchworth Neighbourhood Design Code (Appendix 1 to the Batchworth Neighbourhood Plan); Three Rivers District Council: Moor Park Conservation Area Appraisal (October 2006, adopted November 2006); ONS/Nomis: Census 2021 TS044, Rickmansworth Town ward
Depth, neighbours and the planning route for a Rickmansworth extension
The district's design criteria start at the back of the house. The Development Management Policies say a single-storey rear extension should generally go no deeper than 3.6m, or 4m on a detached house, and that the depth may be reduced if the extension would harm the neighbours or be unduly prominent. Two-storey rear additions should stay behind a 45-degree splay line drawn across the garden from a point on the shared boundary level with the neighbour's rear wall.
At the side, a first-floor extension should be at least 1.2 metres from the flank boundary, though a two-storey side extension may reach the boundary if its first floor is set in by 1.2 metres. Every extension should respect the roof form, windows, doors and materials of the house and street, and should neither take light from neighbouring windows nor allow overlooking.
Conservation areas narrow what a householder can build without an application. Article 2(3) land, which takes in conservation areas, gets no permitted development for an extension beyond a side wall, or for a rear extension of more than one storey. With six conservation areas inside Batchworth, we check the council's map for the address before anything else.
The Batchworth Neighbourhood Plan adds a design test of its own. Development should respond to the Design Code on heights, materials, frontages, plot widths and boundaries, and through its scale and siting keep the sense of openness and separation between Moor Park and Rickmansworth.
Sources: Three Rivers District Council: Development Management Policies LDD (adopted July 2013); 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); legislation.gov.uk: GPDO 2015 (SI 2015/596), Schedule 2 Part 1, development within the curtilage of a dwellinghouse (latest available revised); Batchworth Community Council / Three Rivers DC: Batchworth Neighbourhood Design Code (Appendix 1 to the Batchworth Neighbourhood Plan); Three Rivers District Council: Rickmansworth conservation area; Batchworth Community Council / Three Rivers DC: Batchworth Neighbourhood Plan 2023-2038, Referendum Version
Green Belt edges and the Moor Park estate
Where a house is in the Green Belt, Policy DM2 refuses extensions that are disproportionate, individually or cumulatively, to the original building. The council's Green Belt guidance measures this by floorspace: the base is the house as built, or as at 1948 if older, every earlier extension is added in, and a cumulative increase of over 40% will normally be unacceptable. National Green Belt policy takes the same baseline, defining the original building as it existed on 1 July 1948, or as first built if constructed later.
The same guidance favours rear extensions over side ones and treats single-storey side extensions as harming openness once buildings would cover more than 80% of the frontage. Batchworth Heath conservation area lies wholly in the Green Belt, and its appraisal expects Green Belt policy to be what limits the size of extensions there.
On the Moor Park estate the council's appraisal sets its own measures. Buildings, garages and car ports included, should cover no more than 15% of the plot; at least 20% of the frontage at the building line should stay clear along the flanks; and where existing buildings already span 80% or more of the plot width at the building line, the appraisal does not accept further extension towards the boundaries or upwards.
It also treats building forward of the existing building line as unacceptable, finds deep floor plans unlikely to be acceptable, and expects ridges no taller than the neighbours'. Small extensions may be permitted development, but the appraisal says that is only likely for houses not previously extended, and the 2006 appraisal asks for a Design and Access Statement with every application in the conservation area, along with existing and proposed plans, elevations and sections.
Sources: 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); MHCLG (GOV.UK): NPPF Chapter 13, Protecting Green Belt land (GB1-8) (August 2026); Three Rivers District Council: Batchworth Heath Conservation Area Appraisal (January 2013); Three Rivers District Council: Moor Park Conservation Area Appraisal (October 2006, adopted November 2006)
Floor levels, drainage and the neighbours' foundations
The town's river setting brings flood risk with it. The 2026 flood assessment finds the district's most extensive river flood risk in the Colne valley, affecting Rickmansworth and Batchworth. The Environment Agency's flood map for planning shows an address's zone and whether an application must include a flood risk assessment; for a simple, low-risk extension to a house, the agency says owners may be able to prepare one themselves.
Where floor levels are set for flood risk, the assessment says the Environment Agency generally advises finished floors 600mm above the 1% AEP fluvial plus climate change flood level, and that lowering existing floor levels within that floodplain would not be acceptable. A step down into a garden-level extension is therefore a design decision, not a default.
Close to the water, the Batchworth plan asks for sustainable drainage where development is in an area at risk of flooding, and development that might discharge surface water to the canal needs the Canal & River Trust's prior consent. Beside a main river the Environment Agency recommends a green buffer strip, 8 metres wide for a fluvial river.
New foundations near a neighbour bring in the Party Wall Act. Digging within 3 metres of a neighbouring building to below its foundations, or within 6 metres where the excavation meets a 45-degree line drawn down from them, needs notice, as does building at or on the boundary.
Sources: Batchworth Community Council / Three Rivers DC: Batchworth Neighbourhood Plan 2023-2038, Referendum Version; 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); GOV.UK: The Party Wall etc. Act 1996 explanatory booklet; GOV.UK: Party walls and building work - overview
Designing and building a Rickmansworth extension
Our London team takes on extensions in Rickmansworth: in-house architects and designers draw the scheme, our structural engineers design the foundations and steelwork, and Hampstead Chartered Surveyors & Building Consultancy supports the measured survey. The application goes to Three Rivers District Council, and the Building Regulations, which cover the construction and extension of buildings, are checked by Hertfordshire Building Control or a private registered building control approver.
We start by measuring the house, the garden and the neighbouring rear walls and windows, because depth and outlook are judged from those positions, and by establishing what earlier additions the original house has already had.
Foundations are designed once trial holes have shown the ground, and existing drains are surveyed by camera before anything is built over them.
Where you stay at home during the build, the new structure is put up and made weathertight before the old rear wall is opened, so the house stays secure and dry throughout.
Outside Rickmansworth, 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: GOV.UK: Building regulations approval - when you need approval; Three Rivers District Council: Building control; GOV.UK: Building regulations approval (printable guide); Hampstead Renovations: Meadway, NW11 case study
House Extensions in Rickmansworth: Questions
Is my Rickmansworth house in the Green Belt, and what is the council's test for an extension?
Some addresses are: Batchworth Heath conservation area lies wholly in it, and Green Belt countryside surrounds the Moor Park estate on three sides. For a house in the Green Belt, Policy DM2 refuses extensions disproportionate to the original building, individually or cumulatively, and the council's guidance treats a cumulative floorspace increase of over 40% on the original dwelling, as built or as at 1948, as normally unacceptable.
Does an extension in one of Rickmansworth's conservation areas need planning permission?
Often it does. On article 2(3) land, which includes conservation areas, householder permitted development does not cover an extension beyond a side wall or a rear extension of more than one storey. The council considers every application in a conservation area from a conservation point of view, so we design and document the scheme with that in mind.
What does the Batchworth Neighbourhood Plan say about extensions?
Its design policy asks development to respond to the Batchworth Neighbourhood Design Code on building heights, materials, frontages, plot widths, boundary treatments and street enclosure, and says departures need full justification. It also asks design, scale and siting to keep the openness and separation between Moor Park and Rickmansworth, and its drainage policy expects sustainable drainage where development is in an area at risk of flooding.
How does flood risk affect an extension in Rickmansworth?
The Colne valley is the district's most extensive area of river flood risk, and Rickmansworth is among the settlements at highest risk of groundwater flooding. We check the Environment Agency's flood map for the address; where a flood risk assessment is required for the address, floor levels follow the guidance, which generally puts finished floors 600mm above the design flood level and discourages lowering existing floors within the floodplain.
Who signs off building regulations for an extension in Rickmansworth?
Hertfordshire Building Control, which the council says it owns jointly with seven other Hertfordshire authorities, or a private registered building control approver of your choosing. Building regulations approval is separate from planning permission, and an extension usually needs both.
How should I prepare my Rickmansworth home before you come to look at an extension?
Have any existing drawings, past planning permissions and completion certificates to hand, and list earlier extensions, because the Green Belt test counts them cumulatively against the original house. Clear access to the garden and side passages so we can see the neighbours' rear walls and windows, and point out any drains, trees or changes in level near where you want to build.
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