In Focus: Basements

ABOVE: It is possible to build a new basement storey from scratch beneath an existing house — but this is expensive at £3,000-4,000/m², depending on access and ground conditions. Due to the cost it is only likely to make financial sense in high-value areas such as Central London, where basement extensions are increasingly popular. A basement can be extended under the garden at the front and rear – as well as beneath the house – and the garden reinstated on top.

Q: I would like to convert the dark, unused basement in my detached Victorian home, to provide space for a further living room, utility area and office. However, I’m a little overwhelmed as to how to go about it. Do I need planning permission? How do I make the space habitable? I would also like to introduce natural light where possible.

A: Basement conversions can create great rooms, providing you introduce lots of natural light via lightwells and make maximum use of available light through open plan design. Victorian and Georgian townhouses were frequently built with a basement level, typically with a sunken lightwell with railings around, and a sunken courtyard at the back, with conventional sash windows and French doors. With the lightwell painted white or similar, the rooms can still feel very light and you can open a window or door to let in fresh air.

Converting an existing basement which already has sufficient headroom costs around £850-1,050/m² using a main contractor. If you need to lower the existing floor level, or make structural alterations, this will add to the cost quite considerably — as it usually involves underpinning the walls at the same time as excavating the floor.

Waterproofing measures are critical to the success of a conversion. You should use either a chemical tanking system, or a textured membrane designed to trap any penetrating damp behind it, and direct it into a sump from where it can flow safely away, possibly using a pump depending on the level of the mains sewer. As long as the space is dry, well ventilated, light and warm, it can be used for any purpose from living space, to bedroom accommodation, a home office or studio.

Whether or not planning permission is required depends on whether the work involves increasing the volume of the existing house and, if so, by how much. In most instances (except listed buildings) a basement conversion that does not enlarge a dwelling will not require planning permission. Most minor extensions to enlarge the basement to increase the ceiling height, or to add space at the rear, or create more space or a lightwell, will fall within Permitted Development (PD) rights (see the Supplementary Report: Basement Extensions – Householder Development Consents Review at www.communities.gov.uk for further details), and so will not require planning permission. Larger basement extensions will require planning permission depending on their length, width and depth. It is always worth contacting your local authority’s planning department to find out whether or not a planning application will be necessary or whether your proposals fall within the definition of Permitted Development.

In all instances, Building Regulations approval will be required for structural alterations and a material change of use. The work is best left to a specialist basement contractor/conversion company who will be able to offer a full design and build service, including engineering calculations, and a guarantee on their work.

Further reading:

• Basements: The pros and cons
• How to convert your cellar
• A Three Storey Self-build with a Basement