Currently, planning permission is required for the construction and extension of basements, even when not visible above ground level. At the time of writing, the extension of a property below ground is not directly covered by permitted development rights but submissions have been made to address this apparent anomaly.
A detailed analysis of the role of basements within the planning guidelines of the UK is published in The Hidden Potential - a planning review document.
Size of development
While planning approval is required for the construction of a basement, often the size of the proposed construction below ground is less contentious than an over-ground structure. This is particularly useful for increasing the proposed floor area of an existing or new property in areas with strict planning policy controlling the construction of new buildings, such as a National Park or Conservation area. In-fill development in urban settings can also benefit from the accommodation and value added by inclusion of a basement.
A low energy house (pictures to the left), recently constructed in the New Forest National Park was limited above ground to the size of the original existing single-storey structures on site. Development of the three-bedroom family home was possible through the construction of a large basement, containing study area, two double bedrooms, wine storage and plant area, and a large library and TV room.
As described in site potential, the inclusion of a basement level can assist in obtaining planning permission by raising the density of a development through increasing the number of homes without reducing the amenity levels.
Flood risk areas
There is a resistance, through planning controls and the insurance industry, to build houses on areas prone to flooding. The provision of any habitable rooms in basements in flood risk areas is generally not supported by planning legislation but can be feasible if addressed directly. For example, the provision of an escape stair to an area above the flood risk level could be an acceptable solution, rendering the proposed development feasible with basements.
The construction of concrete ground structures or sacrificial basements is a recognised solution for construction in areas of high flood risk. The habitable spaces are raised a minimum of 600mm above the level of design flood risk, while the basement area can provide additional nonhabitable storage space. Concrete is a flood resilient material and the design and construction of the basement and ground floor can deliver ‘best practice’ both in terms of water-entry prevention to the habitable areas and recovery from the effects of flooding.