Save this picture! Section of a proposed basement extension in Knightsbridge, London. Image via The Daily Mail

The London Borough of Kensington and Chelsea is set to pass new legislation aimed at curbing the spate of large basement extensions in the area. The trend for these "mega-basements" is a result of the strict planning guidelines applied to the borough's many historic buildings, forcing the area's wealthy and space-hungry residents to extend downwards instead of upwards or outwards. However, with a ten-fold increase in the number of basement extension plans since 2001, work on these complex underground projects was becoming a nuisance, causing Kensington and Chelsea Council to freeze the planning applications of 220 basement proposals while it sought a resolution.

Planning inspector David Vickery gave his seal of approval to the proposed guidelines in a report, saying: "I am satisfied that the council has identified deep concerns amongst its local residents about alleged adverse impacts on living conditions from noise, vibration, dirt, and dust from construction and from associated traffic, in addition to concerns about impacts on drainage, on appearance and landscape, on structural stability, and on historic buildings."

The borough's proposed new guidelines on basement extensions involve:

A restriction of underground extensions to a single storey, unless on an exceptionally large site.

A reduction in how far an extension can undermine a property's garden, from 85% of its area to 50%.

An total ban on basements under listed buildings.

A requirement to include a construction traffic management plan as part of any planning applications for underground extensions.

The approval by the planning inspector means that the 220 frozen applications can now be resumed, however according to Kensington and Chelsea Council "the vast majority of these cases would not comply with the new policy," and will probably be rejected.

Cabinet member for planning policy Tim Coleridge described the basement extensions as "the single greatest planning concern our residents have expressed to us in living memory," and the concern over the phenomenon doesn't stop there: mega-basements garnered the public's disbelief earlier this year when New Statesman discovered that it was common practice to bury construction machinery in its own hole after completing a basement extension, because it was cheaper to abandon the £5,000 machines than to lift them back out. One estimate given by New Statesman put the total value of buried diggers in London at £5 million. Even worse, some residents for whom one extension had proved too small were apparently now unearthing these sacrificial machines, causing unexpected project delays.

The new planning guidelines are expected to be adopted officially at the council's meeting on January 21st.

Story via the Architects' Journal, BD Online, Planning Resource, New Statesman