An extension in Potters Bar is a ground and consent problem before it is a design problem. GS Renovation builds single and two storey extensions here as a contractor, appointing the structural engineer and coordinating the building control application and inspections. Quotations are free and carry no obligation.
The ground you are building on
London Clay. British Geological Survey 1:50,000 mapping sampled across the built up area of Potters Bar returns London Clay Formation bedrock at every point, with Lambeth Group appearing only at the northern and north western fringe of the town. Hertsmere's own conservation area appraisal for The Royds records the geology as Tertiary clay, the London Clay, covered by slowly permeable and seasonally waterlogged soils with some brown subsoils of the Windsor Series.
Ground like that moves with the seasons. Foundation depth is a question for the engineer and the building control surveyor, not a figure carried over from another job. We dig trial holes where the design calls for them.
Trees, and why they govern the foundation depth
Because clay shrinks as trees draw water out of it, and swells again when they go. Hertsmere Borough Council's tree advice states that subsidence is not always caused by trees, that several other factors may be involved, and that a thorough investigation should establish the cause before any trees are removed. Clearing a tree to make room for a foundation is a decision with consequences either way.
In a conservation area the tree has its own notice period. Hertsmere requires six weeks written notice before working on a tree in a conservation area. Notice is not needed for trees with a stem diameter of less than 7.5cm measured 1.5m above the ground, or less than 10cm where thinning to help the growth of other trees. The trees along The Avenue probably date from between 1905 and 1925 and stand on narrow grass verges on each side of the road.
Hertsmere's numbers for extensions
The design guide is specific, and the figures are worth having before a drawing is paid for. It says the council is likely to refuse a rear extension that breaches the 45 degree line, or that extends more than 3.35 metres beyond the original rear building line. That line is drawn at 45 degrees from the nearest edge of neighbouring front or rear facing windows, counting only windows to habitable rooms or kitchens. The council will also resist single storey rear extensions greater than 3 metres high with a flat roof, or 4 metres high with a pitched roof, measured from ground level.
Side extensions and gardens have their own thresholds. Parts of Potters Bar are named in the guide, alongside parts of Radlett, Bushey and Elstree, among the locations with significant separation between buildings. There, two storey side extensions should sit a minimum of 2 metres from the side boundary, regardless of the position of any existing ground floor extension or garage being replaced. The guide also sets minimum retained garden areas of 60 square metres for a two or three bedroom house or bungalow, 80 square metres for four bedrooms and 100 square metres for five or more.
Getting the application validated
Send what Hertsmere asks for, in the format it asks for. Its validation requirements include the application fee, a completed application form, the correct ownership certificate, compliant plans and a completed Community Infrastructure Levy Additional Information form where necessary. Plans must be at a recognised metric scale, include a scale bar and be in PDF format.
Applications go through the Planning Portal at planningportal.co.uk/apply, and Hertsmere also accepts them by email or post on downloaded paper forms. We work alongside whoever draws your plans, so that the structural design and the submitted drawings say the same thing.
- Full fit-out, finishing and making good
- Bi-fold, sliding and French door openings to the garden
- Drainage diversion and build-over agreements with the water authority
- Planning applications and permitted development assessments
- Single and double storey extensions
Why Choose GS Renovation in Potters Bar
Extensions here stand or fall on the ground conditions and the consent route, so both are settled before the foundations are priced. We appoint the structural engineer, build to their calculations, and coordinate the building control application and inspections rather than leaving them for you to chase. You deal with one point of contact from the first survey to the completion certificate. GS Renovation Ltd is fully insured and registered in England, company number 14611254.
- A dedicated project manager from first survey to handover
- Neighbours given notice before the noisy stages
- Party Wall notices served for excavation near a neighbour's wall
- One point of contact on the phone, not a call centre
- Foundation depth set after trial holes, drains and trees are checked
- Insulation and thermal detailing to current regulations