Although the issue of excavating basements under London townhouses has been a hot topic in conservation circles in London for some time it has rarely been an issue that we have been asked to consider. However a recent case allowed us the opportunity to formulate our views on this issue.  Essentially, the SPAB is concerned that excavating beneath a property to create a new basement will have a profound impact upon the historic fabric and character of the heritage asset and also on the legibility of the building’s development, even though the changes will not be very visible other than from within the basement itself.

In many cases the basement may be one of the few parts of the building that has not seen a great deal of alteration and, as such, it provides valuable evidence of the building’s original plan form, history and development. Basements may also be one of the last areas to retain any surviving original finishes, which should therefore be protected and retained rather than destroyed. The SPAB also takes the view that traditional buildings have an intimate connection with the land on which they are constructed and the relationship between the building and the ground below its foundations should not be lightly dismissed.  Excavating beneath the ground floor of the building breaks this intrinsic and important connection with the land and would result in a building that is no longer in its original context. In many ways excavating below a building and severing its bond with the soil is no less destructive to the building’s integrity than relocating a building to a new site and should therefore be firmly resisted.

There are, of course, also a whole host of related and very serious technical concerns regarding the physical impact on the neighbouring properties. It is extremely difficult to predict accurately how such a large and invasive excavation might affect the water table and the movement of water within the ground around a property, but it seems likely that there is at least some level of risk to the building and to any neighbouring structures.

The SPAB is also concerned about the introduction of additional light wells, as these will inevitably change the appearance of the outside of the building and will have an adverse impact on its setting due to the light that will wash upwards across the elevations when the lights are used in the basement in the evening.

As a final point, we note that the Royal Borough of Kensington and Chelsea has adopted a new local planning policy (CL7) which indicates that basement developments should not extend under listed properties. The SPAB wholeheartedly endorses this policy and hopes to see a similar approach taken in the other London boroughs in due course.