Analysis of the Royal Borough of Kensington and Chelsea’s planning records show that between June 1999 and January 2014, there were 594 planning applications involving basement extensions. Of these applications, 386 were fully granted, 126 were withdrawn, 64 were refused and a further 18 had conditions imposed. The results below show that the vast majority of applications involving basement extensions – over 65% – are approved. However, this may be set to change.

Fig.1 – Outcome of planning assessments by Kensington and Chelsea involving basement extensions

London boroughs set to crack down on mega-basements

A Guardian report from October of last year indicates that the current high approval rate for basement extensions may be about to change. Controversial “mega-basements”, built by wealthy home owners, have “caused flooding, sink holes and structural damage to neighbouring buildings in the past – as well as endless disruption during the construction process.”

As a result of these issues, new regulations may soon be introduced limiting any extensions to 50% of the garden size with depths of no more than one storey.

Basement Impact Assessments

Architects, property developers and planning consultants who are about to embark on London based projects involving basement extensions, will need to consider a Basement Impact Assessment (BIA). A BIA comprises three strands: hydrology (flooding), sub-surface flow (groundwater) and slope stability.

Whilst the BIA process varies from one borough to another, there is usually a set procedure involving: screening, scoping, investigation and risk assessment. Some basement extension applications, when accompanied by appropriate site investigation data, won’t pass the screening stage, whilst others may need a few simple scoping calculations to demonstrate that there will be no impact.

Contact our team for advice

With regulations on basement extensions set to change and more planning applications likely to be rejected in the future, it’s important to ensure that you seek advice from suitably qualified professionals regarding your BIA. Ambiental work closely with leading hydrogeologists and geotechnical consultants; our team can provide advice on screening assessments and site investigations.