House
extensions
in Radlett, WD7
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A Radlett extension is usually weighed against three things at once: the council's extension guide, the Radlett Plan's concern for spacious plots and, at the edge of the town or close to Tykes Water, the Green Belt test or the brook.
Extending is how Radlett houses grow. The neighbourhood plan ties the town's long and narrow plots to a large number of planning applications to turn existing houses into even larger ones, and its analysis of permissions granted in 2014, 2015 and 2016 found a significant increase in floor area, plot by plot.
The same plan sets out what residents value: spacious plots, clear visual breaks between houses, mature trees visible in back gardens from the street, and a low density overall. The council's own extension guide names parts of Radlett among the places where a significant gap between buildings has to be kept, which is the first thing we measure before sketching a side extension.
Sources: Aldenham Parish Council / Hertsmere Borough Council: Adopted Radlett Plan 2020-2036 (low resolution)
Permitted development or an application for a Radlett extension
Class A of the householder rules covers the enlargement, improvement or other alteration of a house without an application, within its limits. Inside Radlett North or Radlett South the land is article 2(3) land, and there Class A allows nothing that reaches beyond a side wall, nor anything taller than one storey that extends past the rear wall.
Earlier additions count. On article 2(3) land those limits apply to the new part taken together with any existing enlargement it joins, so a house extended in the past may have little left to use.
The bungalow directions are aimed upward. They concern Class AA, the added storey, and the council says they are not intended to stop the named bungalows being extended under other permitted development rights.
Any planning application goes to Hertsmere Borough Council, and the Radlett Neighbourhood Plan is a material consideration alongside the borough's plan. Its Policy HD3 requires all development to respect local character and residential amenity.
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); Hertsmere Borough Council: Radlett Bungalows Article 4; Hertsmere Borough Council: Radlett Neighbourhood Plan; Aldenham Parish Council / Hertsmere Borough Council: Adopted Radlett Plan 2020-2036 (low resolution)
What the council's extension guide asks of a Radlett house
Part E of the council's Planning and Design Guide sets the yardsticks. An extension should sit comfortably inside a line drawn at 45 degrees from the nearest edge of a neighbour's front or rear window, and one that breaches that line, or reaches more than 3.35 metres beyond the original rear building line, is likely to be refused.
Single-storey additions should be no taller than 3 metres with a flat roof or 4 metres with a pitched one. A flat roof is normally accepted on a single-storey rear addition unless a pitched or hipped roof would suit a visible extension better, and using that roof as a balcony or terrace will normally be refused or controlled by condition.
Side extensions are where Radlett's spacing counts. A single-storey side extension should stand at least 1 metre from the boundary, should not exceed half the width of the main building seen from the front, and may need to step back from the front building line, particularly in conservation areas.
The guide also expects most of the garden to survive, resists extensions that end in crown roofs, and warns that large wrap-around schemes have a greater effect on the neighbours' outlook, privacy and light.
Green Belt, the brook and flood risk around Radlett
Radlett is surrounded by the Metropolitan Green Belt, and its boundary is drawn on the council's Policies Map. For a house within it, Policy SADM26 says an extension must not be disproportionate to the original building, singly or cumulatively, and must be subordinate to it; the original is the house as it stood on 1 July 1948, or as built if later.
To judge scale the council compares the change in floor space, in volume and in the site coverage of buildings and hardstanding. Garages and outbuildings sited more than 5 metres from the main house are not treated as part of the original building, so we measure where each one stands before any calculation is made. National policy asks the same question: an extension escapes being inappropriate Green Belt development only if it causes no disproportionate increase over the original building, dated from the same 1 July 1948.
Beside Tykes Water, Policy HD6 of the neighbourhood plan asks for a natural buffer of at least nine metres from the top of the bank, and the council's byelaws require its prior consent for work within 9m of any watercourse.
In Flood Zones 2 and 3 even minor development needs a site-specific flood risk assessment, though the council's assessment says an extension that would not significantly increase the number of people at risk generally needs a less detailed one. Where raising the new floor to the usual level is impractical, the Environment Agency or the council can be asked about a lower level with flood resistance measures.
Sources: Aldenham Parish Council / Hertsmere Borough Council: Adopted Radlett Plan 2020-2036 (low resolution); Hertsmere Borough Council: Site Allocations and Development Management Policies Plan (adopted November 2016); MHCLG (GOV.UK): NPPF Chapter 13, Protecting Green Belt land (GB1-8) (August 2026); Hertsmere Borough Council / AECOM: Level 1 Strategic Flood Risk Assessment (2023)
Structure, neighbours and sign-off for a Radlett extension
Your neighbours must hear from you before building work on or at the boundary, before work on an existing party wall, and before you dig below and near their foundation level. That catches most extensions to Radlett's semi-detached pairs and terraces, and many on detached plots where new footings come close to the house next door.
The Building Regulations 2010 cover the construction and extension of buildings, so every extension needs approval. Our London team, with in-house architects, designers and structural engineers and with Hampstead Chartered Surveyors & Building Consultancy for the survey, prepares the application to Hertsmere Borough Council and the submission to Hertfordshire Building Control or to a private registered approver.
Our engineer sizes the foundations to the ground found in trial pits, and designs the steel for the opening into the house, so that the design and the building control drawings agree before work is priced.
On site the existing rooms are sealed off first. The new structure is made weathertight before the wall between old and new comes down, which keeps the kitchen or living room in use until late in the job.
Outside Radlett, 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: Party walls and building work - overview; GOV.UK: Party walls and building work - work you must tell your neighbour about; GOV.UK: Building regulations approval - when you need approval; Hertsmere Borough Council: Hertfordshire Building Control Limited; Hampstead Renovations: Meadway, NW11 case study
House Extensions in Radlett: Questions
Is my Radlett house in the Green Belt, and what is the council's test for an extension?
That depends on which side of the boundary it stands: Radlett is surrounded by the Metropolitan Green Belt, and the line is drawn on the council's Policies Map. For a house within it, Policy SADM26 requires an extension to be subordinate to the original building and not disproportionate to it, alone or with earlier additions, judged on floor space, volume and site coverage.
Does an extension in one of Radlett's conservation areas need planning permission?
Often it does, because each conservation area is classed as article 2(3) land: there, Class A permits no extension beyond a side wall and nothing over a single storey that extends past the rear wall, and earlier enlargements count towards the limits. The council's guide adds that the extension may need to sit back from the front building line, particularly in conservation areas.
What does the Radlett Neighbourhood Plan say about extensions?
It notes that long, narrow plots have led to many applications to enlarge houses, and Policy HD4 says it will not support the loss of garden land to development that ignores the surrounding character and pattern. Its Design Code asks for ridge heights that take account of gradients and for enough space between dwellings, and applications are read against it as a material consideration.
How does flood risk affect an extension in Radlett?
Near the Radlett Brook, where Flood Zones 2 and 3 are concentrated, an application needs a site-specific flood risk assessment, although for an extension that does not significantly increase the number of people at risk the council generally needs a less detailed one. Policy SADM14 also requires that development does not increase the risk of flooding elsewhere.
Who signs off building regulations for an extension in Radlett?
Hertfordshire Building Control or a private registered building control approver, whichever you appoint. We arrange the inspections they ask for as the work goes on, and the completion certificate matters later, because without approval you will not have the certificates you may need when you sell.
How should I get ready for a feasibility visit about extending my Radlett house?
Look out any drawings or permissions for earlier extensions, because they count towards the permitted development limits and, in the Green Belt, towards the cumulative test. Note which neighbouring windows face your garden, where trees stand near the likely footprint, and whether a stream or ditch runs along the bottom of the plot.
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