Policy DH10
Basements and lightwells
New basements, lightwells and extensions to existing basements will only be permitted where they:
a) do not extend into or underneath the garden further than 50% of the depth of the host building measured from the principal rear elevation;
b) do not extend into or underneath the garden further than 50% of the depth of the garden;
c) are designed to avoid the loss of amenity space;
d) are set back from neighbouring property boundaries where they would extend beyond the footprint of the host building;
e) do not comprise more than one storey. Exceptions may be made on large sites;
f) do not result in an unacceptable impact on the amenity of adjoining properties or on the local, natural and historic environment during and post construction;
g) are designed to minimise the risk of flooding to the property and nearby properties from all sources of flooding;
h) include a minimum of one metre of soil above any part of the basement beneath a garden;
i) ensure that the basement helps reduce the volume and flow of surface water run-off through appropriate use of SuDS and will provide active drainage devices to minimise the risk of sewer flooding;
j) ensure that lightwells at the front or side of the property are as discreet as possible and allow the scale, character and appearance of the property, street or terrace to remain largely unchanged;
k) are designed to safeguard the structural stability of the existing building, nearby buildings and other infrastructure;
l) provide a Construction Method Statement (CMS) (carried out by a qualified structural or civil engineer) to be submitted with planning applications for all basement projects; and
m) provide a construction traffic management plan as part of the CMS to ensure that traffic and construction activity does not cause unacceptable harm to pedestrian, cycle, vehicular and road safety.
New self-contained basement flats will not be permitted in the Environment Agency's Flood Zone 3 areas where there is a risk of rapid inundation by flood waters in the event of a breach of the river's flood defences, unless a satisfactory means of escape can be provided.
8.66 For the purposes of this policy, a basement is considered to be a floor of a building which is partly or entirely below ground level. A ground or lower ground floor with a floor level partly below the ground level (for example on a steeply sloping site) will therefore generally be considered basement development.
Size of basements
8.67 The council recognises the benefits of new residential basement and lightwells in meeting housing needs and residents aspirations. It will permit basements but subject to a number of criteria being met, that will safeguard the quality of life. The council will allow extensions of houses and flats into the basement below the building, providing there is no adverse impact on the amenity of neighbouring properties (such as flooding) or negative impact on the street scene due to the need for the provision or alteration of lightwells.
8.68 The policy criteria states that basements should not exceed 50% of the garden area and not exceed 50% of the depth of the host building. Restricting the extent of basement excavations to any approved extension and limiting the depth of excavation to a single storey will help to limit the extent and duration of construction. This will help to reduce the impact of basement construction on local residents. This criterion applies to the front garden, the rear garden and gardens to the side of the property individually, rather than calculated as an aggregated garden area for the whole property. The unaffected garden must be in a single area and where relevant should form a continuous area with other neighbouring gardens. Sufficient margins should be left between the site boundaries and any basement construction to sustain growth of vegetation and trees.
8.69 On large sites, basements of more than one storey may be permitted in certain circumstances. These will generally be new developments of the size of an entire or substantial part of an urban block. They should be large enough to accommodate all the plant, equipment and service requirements associated with the development within the site, and offer more opportunity to mitigate construction impacts and carbon emissions on site. These schemes will be expected to provide appropriate evidence to demonstrate to the Council's satisfaction that the development does not harm the built and natural environment or local amenity or increase flood risk. For the purposes of this policy, large sites are considered to be:
- new major developments, for example schemes which comprise 1000m2 additional non-residential floorspace or 10 or more additional dwellings;
- developments the size of an entire or substantial part of an urban block.
8.70 There may be a specific need for double basements for some commercial developments. Where these are a double basement is proposed as part of these schemes, applicants will be expected to demonstrate the need for a double basement and have explored all other alternatives. These cases will be assessed on a case-by-case basis.
8.71 A 'single storey' basement is considered to be one that cannot be subdivided in the future to create additional floors. It is generally about 3 to 4 metres floor to ceiling height but a small extra allowance for proposals with a swimming pool may be permitted.
Flood risk
8.72 It is important that proposals for new or extended basements provide clear evidence that demonstrates that there is no adverse effect on surface water drainage, the sewers and/or on groundwater flows. Opportunities should be taken to integrate sustainable drainage systems (SuDS) such as vegetation and permeable surfaces to help to control surface water runoff. Water butts can also be used to collect rainwater for later re-use. Where proposals cause the loss of vegetation, this can also affect the character of conservation areas and planted rear gardens, thereby impacting on privacy, shade and biodiversity. Therefore, where any part of a basement is constructed below a garden, a minimum of one metre of topsoil should be provided above it to promote infiltration and allow for planting. In areas at risk from flooding, new self-contained basement dwellings will not be permitted where they are at risk of rapid inundation by flood waters in the event of failure or breach of flood defences and where no adequate means of escape can be provided because of the risk to life in the event of a flood. Any new basement that is below street level should be designed to reduce flood risk and to minimise any impact from flooding from any source. To protect against sewer flooding, developments must include the provision of a pumped solution or 'active drainage devices' incorporating non-return valves to prevent water entering a property from drains and sewers.
Construction and demolition
8.73 Basement excavation often raises concerns about the structural stability of adjacent properties because of works to party walls and foundations, in particular. These issues may be properly dealt with by means of a party wall agreement under the Party Wall Act 1996. However, the council wishes to encourage good neighbourliness and avoid planning applications which cannot be implemented due to the lack of agreement between the applicant and the owners of neighbouring properties or land instability. The NPPF places significant weight on ground conditions, land stability and local environmental issues as material considerations in determining planning applications. Therefore, the council requires applicants to submit a Construction Method Statement (CMS) (carried out by a qualified structural or civil engineer) with the planning application and to make the statement available at the same time to neighbouring owners to demonstrate that the development accords with the policy.
8.74 The level of information required will be appropriate with the scale, nature and location of the scheme. Basement construction can cause nuisance and disturbance for neighbours and others in the vicinity, through construction traffic, parking suspensions and the noise, dust and vibration of construction itself. The applicant must demonstrate that these impacts are kept to acceptable levels under the relevant acts and guidance, taking the cumulative impacts of other development proposals into account. Every effort must be made to locate the building compound and the skip on site or in exceptional circumstances in the highway immediately outside the application site.
8.75 A construction management statement will be required to be submitted with the application for basement projects and would be expected to cover:
- appointment of appropriately qualified civil or structural engineer;
- a report outlining the ground and hydrological conditions of the site and dealing with groundwater flow ensuring structural stability during excavation and demolition;
- temporary propping/temporary works and construction works, minimising disturbance
- drilling of boreholes;
- impact on trees;
- sequence of temporary works to minimise the effect on neighbours and management of water flow;
- consideration of related cumulative impacts;
- professional verification of works safeguarding amenity: noise vibration and dust from construction work; and
- construction traffic management plan.
Lightwells
8.76 It is very important to minimise the visual impact of lightwells, roof lights, railings, steps, emergency accesses, plant and other externally visible elements. Care should be taken to avoid disturbance to neighbours from light pollution through roof lights and other forms of lighting.
8.77 Introducing lightwells where they are not an established and positive feature of the streetscape can harm the character or appearance of an area. Where external visible elements are allowed, they need to be sensitively designed and sited, respecting the existing character and appearance of the building, streetscape and gardens in the vicinity.
8.78 Lightwells should seek to retain as much external amenity space as possible. Excessively large lightwells will not be permitted in any garden space.