UK: Basement Extensions In London

Last Updated: 9 December 2013
Article by Helen Hutton

Proposals to dig basements under existing buildings in built-up areas often meet much local opposition. This is especially so in Westminster City Council ("WCC") and the Royal Borough of Kensington & Chelsea ("RBKC"), where the number of basement applications, including many double and some triple basements, has risen dramatically. This increase may be due to the fact that some people want to avoid the cost and hassle involved in moving to larger premises, yet would still like to increase their available living accommodation (and where there is no possibility of increasing the building upwards or outwards, there is no option but to dig down). For some, a basement provides the chance to add a swimming pool, gym or a media room; and, for others, it provides an opportunity to increase the value of their investment property. The cost of underground construction is around twice as high as above ground construction, so there are far fewer basements appearing in the lower value areas of London and outside the capital, than in its central and western areas.

There have been a number of instances reported in recent years where neighbours have been badly affected by basement excavations, not only in structural terms where cracks have appeared in neighbouring properties but also in terms of the disruption caused during months of noisy excavation works. The strength of feeling amongst neighbours about proposed excavations often results in strong objections to planning applications. Current local policies are often seen as too weak to prevent questionable schemes from progressing.

HSE Inspectors took enforcement action in November 2011 at a number of sites in London which were visited during a 2 day initiative to improve standards in basement construction. Inspectors visited 109 sites in Hammersmith & Fulham, RBKC, WCC and Wandsworth. They took enforcement action at 40 sites, serving 78 notices. Four sites were so dangerous that they were closed.

Current law and policy

Excavation works to create a new basement under a property and part of the garden can sometimes be carried out by exercising permitted development rights, without planning permission. However, if the basement is intended to extend beyond the original footprint of the building, create a separate unit or result in alterations to the external appearance of the building, then planning consent is usually required, to be assessed under local policies. A local authority can remove certain permitted development rights, including those allowing excavations, by issuing an Article 4 Direction and bringing the works under local planning control. Local authorities can also develop local planning policies to restrict what is permitted.

If the original property is listed, then any alterations (internal or external) which affect its character, need listed building consent.

In all cases, Building Regulation controls will also apply, dealing with structural and engineering issues, health and safety, on-site operations, etc.

If the proposed works (including foundations or underpinning) are close to any neighbouring properties, then the Party Wall etc. Act 1996 might well also apply, to protect the rights of neighbouring owners and occupiers.

While the Government is widening permitted development rights for some above-ground domestic extensions, it is likely that permitted development rights currently available for basements will become more restrictive in the future. On 12th November 2013, the Labour MP for Westminster North (Karen Buck) introduced a Private Members' Bill to "regulate the construction of new basements and extensions to basements...". The second reading of the Bill is expected to take place on 22nd November 2013. Without Government backing (which has not been forthcoming yet), it is unlikely to proceed beyond that. The proposals in the Bill restrict basements to one storey and to no more than 50% of the garden. They also require construction traffic management plans to be in place.

Royal Borough of Kensington and Chelsea

The rise in basement applications in RBKC recently has been very significant, amounting to 13 in 2001, 186 in 2011 and 307 in 2012.

RBKC has been looking at basement policy while undertaking a partial review of its Core Strategy. The revised policy was approved in a vote of the Council in June 2013 and is to be adopted in early 2014.

The number of basement planning applications in RBKC is expected to rise over the next few months, before the new (more restrictive) policy begins.

Proposed Policy

The proposed basement policy would apply to all basements (meaning any floor level substantially below the prevailing level of back gardens) across all land uses. Basements would be limited to one storey; an additional basement would not be allowed where there is one already.

The height of basements would be restricted usually to 3-4m, which would prevent future horizontal subdivisions. Basements containing swimming pools may be allowed to be a little deeper. The height restriction aims to reduce the impact of excavations on neighbours and avoid the greater structural risks involved in construction.

Basements would be limited to 50% of the area of back gardens (including paved areas, but not land under existing outbuildings). The unaffected garden must be a single area. This should allow more natural landscape in RBKC and more flexible planting in gardens, including the retention of major trees. Trees of townscape or amenity value must not be lost or damaged in basement works. The area restriction would also ensure that water could drain through to the Upper Aquifer.

It is recognised that new basements under listed buildings affect the hierarchy of historic floor levels and the original buildings' historic integrity. Under the new policy, basements would be severely restricted under or near listed buildings. Care would also need to be taken of historic foundations. Basements would only be allowed under gardens where there would be no harm to the structure or setting of listed buildings and the basements would be "substantially separate" from buildings. Basements would therefore only be possible where gardens are large enough to locate them away from historic buildings. Any link between listed structures and basements must be discreet. Any externally visible elements must preserve or enhance the character and appearance of Conservation Areas.

The impact of any externally visible basement elements must be minimised generally, including appropriate siting of lights and plant.

Surface water run-off must be managed properly. At least 1m of soil must remain above a basement under a garden. All basements must be fitted with suitable pumps to deal with potential sewer flooding. The impact of carbon emissions of basements should also be mitigated, including meeting high BREEAM ratings.

Traffic and construction activity must not harm road safety nor place unreasonable inconvenience on local residents. The impact of noise, dust and disturbance from basement construction must be kept to a minimum. These policies are to be worded more strongly than currently. The structural stability of the property and nearby buildings must be safeguarded.

Some of these issues were formerly in RBKC's 2009 Supplementary Planning Document, so their importance is being recognised in them being moved into the proposed Core Strategy.

During consultation on the above, residents pressed for the area of basements within gardens to be restricted, to ensure that larger garden areas could be retained as proper gardens and for disruption to the area to be reduced. They also pressed for controls relating to listed buildings to be strengthened. Local residents generally appear to be pleased that the Council is taking a firm approach to basement development.

Westminster City Council

WCC has also seen a sharp rise in basement developments recently with 86 planning applications for basements in part of St John's Wood in the year ending September 2011, including 13 in Hamilton Terrace alone, and 518 basement applications in the last 4 years (with only 1 in 7 being refused). The cumulative impact of such extensive development on nearby occupiers can be severe.

WCC does not have specific adopted Core Strategy (or saved Unitary Development Plan) planning policies relating to basements, although a number of other policies are taken into account (relating to flooding, noise, sustainable design, etc). WCC generally requires a construction methodology statement approved by a chartered surveyor or structural engineer, to show the basement works can be carried out without risk or that risks could be mitigated. The Council is however now developing a specific policy for basement excavations in its emerging Local Plan. WCC's proposed policy currently follows similar lines to that proposed by RBKC. Depending on the outcome of the consultation, which is taking place until 29 November 2013 in the Borough, it intends to implement the new rules in Spring 2015.

There may well be an increase in applications for basements over the next few months, as residents apply for planning consent before the policy is tightened-up.

Other central London areas

Other boroughs are also now reviewing basement development controls. Changes are likely to restrict the extent of current basement developments.

The restrictive nature of such proposed new policies is likely to curtail the trend in central and western London basement development, but it will hopefully also provide a sound basis for regulating the competing interests of neighbouring landowners. It will be interesting to observe whether the proposed changes will have the effect of making existing larger London homes even more expensive, as the opportunities for future extensions will be more limited.

The content of this article is intended to provide a general guide to the subject matter. Specialist advice should be sought about your specific circumstances.

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