Basement
conversions
in Virginia Water, GU25

The 2018 flood study notes that it has become increasingly popular in Runnymede to build basements that extend beyond the footprint of the house and under the surrounding amenity area, most commonly in the Virginia Water area. It describes basement work as either extending an existing habitable basement under a house or building a completely new one.

Following national planning practice guidance, it says basement extensions to existing dwellings in Flood Zone 3 should not be permitted, because of the vulnerability of the people using them, and in Flood Zone 2 the exception test would have to be passed first.

Applications for basements in Flood Zones 2, 3a and 3b, and on sites of 1ha or more in Flood Zone 1, should come with a flood risk assessment, as Local Plan Policy EE13 also requires for development in those zones. That assessment has to detail a sustainable drainage system for the site and investigate whether a perimeter drain or similar measure is needed so that existing sub-surface water flows are not interrupted.

The study also records the sewerage company's advice that basements can by their nature be vulnerable to internal sewer flooding, and its recommendation that new basements incorporate positive pumped devices.

Sources: Runnymede Borough Council: Runnymede 2030 Strategic Flood Risk Assessment (Level 1, January 2018); Runnymede Borough Council: Runnymede 2030 Local Plan (adopted document, PDF)

The neighbourhood plan says the area is generally protected from rising rivers by its height, between 29m and 50m AOD, and the flood study puts 90% of the ward in Flood Zone 1 only. The 1% of the ward shown at risk in a 1 in 20 year flood includes Green Belt land along the banks of the Chertsey Bourne.

Most of the ward is underlain by Bagshot Formation sand, with pockets of Windlesham Formation sand and an isolated pocket of Taplow Gravel in the Virginia Water area. The study describes the Bagshot Formation as fine-grained quartz sand with frequent clay laminations, some silt layers and flint pebble beds in its upper levels.

The potential for groundwater flooding is mostly limited across the ward, the most notable exception being parts of Virginia Water near the Chertsey Bourne, while the north eastern corner of the ward has potential for groundwater flooding at the surface.

A basement can divert groundwater, and the study warns that the displaced water, finding a new course, may affect nearby buildings or trees. Where the water table is high the council may ask for a groundwater survey with an assessment of the cumulative impact on ground water conditions, and the study says this should be discussed with the council before an application goes in.

Sources: Runnymede Borough Council: Virginia Water Neighbourhood Plan 2024-2035, adoption version, with the Virginia Water Design Code (Appendix 1); Runnymede Borough Council: Runnymede 2030 Strategic Flood Risk Assessment (Level 1, January 2018)

The Local Plan notes that a basement is often seen as a way of adding floorspace without impacting on the openness of the Green Belt, then states that a basement is still an addition to the size of the original or existing building, and as such may result in disproportionality or a materially larger replacement, contrary to Green Belt policy.

Policy EE14 names basements and sunken areas, light wells, changes to ground levels, exposed parts of buildings and ramps among the features the council weighs for their effect on openness.

In the design code areas of Callow Hill and Stroude Valley, every proposal must be assessed against national and Local Plan Green Belt policy as applicable. The borough-wide test is set out on our Runnymede page.

We therefore treat light wells, external stairs and any change in garden level as part of the Green Belt case from the first sketch, and keep them as small and as hidden as the rooms below allow.

Sources: Runnymede Borough Council: Runnymede 2030 Local Plan (adopted document, PDF); Runnymede Borough Council: Virginia Water Neighbourhood Plan 2024-2035, adoption version, with the Virginia Water Design Code (Appendix 1)

Policy VW1 expects proposals to reflect the area's character in bulk, massing and landscaping, and to keep trees, hedgerows and green spaces where possible. In Trumps Green the code protects the clusters of mature trees in the back gardens of Trumps Green Avenue.

The Local Plan requires sustainable drainage for the management of surface water unless shown to be inappropriate, and the neighbourhood plan asks new buildings to harvest grey and rainwater where possible.

Below ground, the flood study finds most of the ward compatible with infiltration drainage or open to bespoke infiltration designs, but some areas have very significant constraints, the largest in the Green Belt around Stroude Road, and the known wetspots it records in the ward include Christchurch Road, Cabrera Avenue, Trumpsgreen Road, Sandhills Road and Stroude Road.

The structural engineer designs the retaining walls, waterproofing and temporary works from borehole and trial pit results, and we serve party wall notices, agree monitoring with neighbours and plan spoil removal before excavation starts.

Outside Virginia Water, 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: Runnymede Borough Council: Virginia Water Neighbourhood Plan 2024-2035, adoption version, with the Virginia Water Design Code (Appendix 1); Runnymede Borough Council: Runnymede 2030 Local Plan (adopted document, PDF); Runnymede Borough Council: Runnymede 2030 Strategic Flood Risk Assessment (Level 1, January 2018); Hampstead Renovations: Holland Avenue, Wimbledon SW20 case study