Basement Review Draft policy and other measures for public consultation

2 2- Review of the Core Strategy

2.1 The existing policy from the Core Strategy relating to basements is shown at Appendix A.

2.2 Please note, the existing policy refers to ‘subterranean development’. The revised policy and associated matters are using the term ‘basement’ in the interests of plain English. There is no difference of meaning between the two terms.


Draft Core Strategy

2.3 The adopted Core Strategy includes policies which are specific to the consideration of new basement development. These are:

  • Policy CL2, “New Buildings, Extensions and Modifications to Existing Buildings” part (g) only
  • Paragraph 34.3.20 in support of CL2(g)
  • Policy CE1, “Climate Change” part (c) only
  • Paragraph 36.3.12 in support of CE1(d)

2.4 This policy review proposes to delete the specific parts of the policies and supporting paragraphs (listed above) that relate specifically to basements and replace them with a separate, new, basement policy, CL7, and supporting text.

2.5 The new policy will contain all matters which apply only to basement developments. Other policies may also be relevant to basement developments, and the plan should be read as a whole.

2.6 The rest of Policies CL2 and CE1, which are not specific to basements, will remain, and are not part of this review.  

2.7 The text below is the proposed text to be included in the Core Strategy. It will be inserted into the third section of Chapter 34 of the Core Strategy.

  • The first box sets out the supporting text, or reasoned justification, that explains or justifies the proposed policy
  • The second box sets out the proposed policy.

Please note, the paragraph numbering shows how this would slot into the existing Core Strategy. In the light of the current review looking at other policies in this chapter, this is subject to change.

Box 1: Supporting text

34.          Renewing the Legacy

34.3.    Policies


34.3.56.  The term basement includes all forms of development that will take place below ground, whether under a building or a garden. The policy applies to all than the most minor of extensions to existing basements. The term garden is used to include front, side and rear gardens, and other private open areas that may not be in use as gardens, because, for example, they are related to commercial premises.

34.3.57.  Basements pose particular problems not raised by above ground extensions and developments. Whilst largely out of sight, basements normally have some elements, for example roof lights or light wells, which individually and cumulatively, can have an impact upon both the character or appearance of the property, townscape and garden. Where under gardens, basements can create an artificially level landscape. There are also concerns in relation to drainage and flooding and the considerable impacts that the construction process can have upon neighbours. Neighbours may also have concerns about the impact on the structural stability of properties in the vicinity. Basements can, however, provide benefits to residents, by adding extra living accommodation.

34.3.58.  For all these reasons, there is a need for a bespoke basement policy. However, the plan must be read as a whole, and other policies will also be relevant to basement development.

34.3.59.  Surface water drainage of gardens, allowing rain to drain naturally into the subsoil and the upper aquifer is important in mitigating climate change, and minimising the risk of flooding. In order to ensure that the status quo in relation to surface water drainage is maintained without increasing surface water flows onto adjoining properties, no basement should exceed 75% of the garden, and could be significantly less than this. The amount of land to be left unaffected will depend on the results of the analysis of the surface water conditions on site.[1] Other factors – set out below – may also result in the size of the basement being constrained to significantly below 75%.

34.3.60.  The unaffected area of a garden needs to be in a single area, and when relating to a back garden it should normally be at the end of the garden. This has the advantage of being adjacent to other similar areas in other plots, and the cumulative size of these areas free of basements is important in their function to provide areas for natural drainage. This has the additional advantage of enabling larger scale planting.

1. This will form part of the Engineering Design and Construction Statement – see below, and details will be set out in the revised Basements SPD. [back]

34.3.61.  Given the duration of building works for the construction of basements, the tight urban grain and the constrained nature of many of the Borough’s roads, the impact of the construction phase of a deep basement can be tantamount to being a ‘bad neighbour’ use. Basements beneath existing buildings or their gardens, or in small scale developments, will therefore be limited to a single storey which is not of a depth that may be suitable for further horizontal subdivision in the future. Deeper basement extensions may be acceptable on larger sites which are less constrained where impacts can be successfully mitigated. In addition, in order to reflect the particular impact that the construction phase of a basement dig can have, the Council will normally limit the construction of proposals which include a significant element of basement development to weekdays only.

34.3.62.  No mature trees should be removed, felled, uprooted, topped, damaged, harmed or put at risk in the long term to make way for a basement development. BS 5837 2012 indicates that tunnelling under trees can be an option. Whilst feasible, it will put the tree at risk, and the Council does not judge the benefits that may be gained from a larger basement outweigh the benefits of minimising the disturbance and risk to protected trees. This approach will therefore not be permitted. Otherwise works should be carried out in accordance with BS 5837 2012 and the Council’s Trees and Development SPD. All applications for basements where there are trees will be accompanied by a full tree survey and tree protection proposal. These must include consideration of the construction phase of the proposal as well as the completed development


34.3.63.  In relation to heritage assets[2] , there are a number of different factors. Archaeological remains are a finite and fragile resource. The conservation, protection or setting of such remains must not be threatened by development, directly or indirectly, to ensure the Borough’s past is not lost forever.

34.3.64.  For listed buildings, the addition of a new floor level underneath the original lowest floor, or the extension of an original basement, cellar or vault has a detrimental impact on the hierarchy of the historic floor levels, and the original building’s historic integrity, scale and layout, harming its special architectural or historic interest. It is only in exceptional circumstances that this is not the case.

34.3.65.  Basements beneath the garden of a listed building may, however, be acceptable where the impact on the character of the garden does not harm the setting of the listed building and where the connection to the listed building is of an insubstantial nature and of an appropriate design, located so that it does not harm the significance of the listed building.

2. A heritage asset is a building, monument, site, place, area or landscape identified as having a degree of significance meriting consideration in planning decisions, because of its heritage interest. This definition will be included in the glossary of the Core Strategy. [back]

34.3.66. In conservation areas, works inside buildings do not require planning permission or conservation area consent so basements may be constructed under buildings and gardens. However, the visual impact of a basements should preserve or enhance the character or appearance of the conservation area.

34.3.67.  The impact of basements on other heritage assets and on non-designated heritage assets[3] must be assessed on their merits to assess the level of harm, if any, to their significance.

34.3.68.  The design of the externally visible elements of a basement – such as light wells, roof lights, railings, steps, emergency accesses and plant – is very important in minimising their impact both in the street scene, and in terms of the urbanisation of gardens, both individually and cumulatively. They must be constructed to a high standard of design and be of a scale that is sensitive to the character of the building and of the surrounding area. In some areas of the Borough, features such as light wells are part of the existing urban scene. However, that is not the case in all areas. Any externally visible elements need to be sensitively designed with close regard to the character and appearance of the existing townscape and garden areas.  

34.3.69.  Externally visible elements should not be introduced to the front of the property where they are not already a feature visible from the street. They should not be based on examples elsewhere in the Borough or on general assumptions about what happens in the Borough as a whole.

3. In addition to the national and statutory designations, a local planning authority may formally identify heritage assets that are important to the area. Such a designation will be material when assessing an application. A non-designated heritage asset may also be of value, and make an important positive contribution to the environment. Guidance is available in English Heritage’s practice guide to PPS5. [back]

34.3.70.  Basements can have a significant impact on the character and quality of the garden. The townscape of the Borough is largely formal and urban in character, but rear gardens are often a complete contrast. Even if small, they often have an informal picturesque and tranquil ambience in contrast to the busy city. Gardens above basements can result in the garden being artificially level. In addition there are external elements, such as light wells, roof lights, railings, steps, emergency accesses and plant. These have direct visible impacts, and indirect impacts such as light pollution. On a cumulative basis these lead to the incremental urbanisation of the informal garden character. To minimise this urbanisation if the external elements associated with a basement beneath a garden need to be discreetly located near the rear of the building. Whilst residents may introduce a formal character to their garden without a basement beneath, such a change is reversible. However, that consequent on the construction of a basement is permanent. Basement development must therefore be of a size that does not harm the character of the garden, the local environment, or preclude mature planting in the future.

34.3.71.  In order to protect the long term future of shrub and other garden planting as well as reducing the amount and speed of water run-off to the drainage system, the Council will require the provision of a minimum of one metre of suitably drained permeable soil above a basement within a garden.  In order to ensure that the character of the existing property is protected the original garden level should not be altered. Near the dwelling this permeable soil could be substituted for permeable paving to provide a patio area. This policy will be applied even if the garden is already hard paved, as the aim of the policy is to ensure that basement developments improve upon the existing amount and speed of water run-off.

34.3.72.  In addition to the 1m permeable soil other sustainable urban drainage measures are likely to be required to reduce both the volume and speed of water run-off to the drainage system and ensuring that surface water run-off is managed as close to its source as possible required by Policy CE2. 

34.3.73.  The carbon emissions of basement developments are greater than the equivalent above ground development because of the excavation and transportation of spoil and the use of concrete. Given the nature of basement developments and the complexity of calculating and assessing CO2 emissions and savings, the Council will take a pragmatic approach using the “very good” BREEAM Domestic for Refurbishment standard, or the equivalent BREEAM for non-residential uses, as a proxy to achieve energy savings across the whole building. Some flexibility will be allowed within a listed building, where it is demonstrated that the works needed to reach the necessary standard are incompatible with the special character of the listed building. In these cases applicants will be expected to demonstrate that every effort has been made to make the necessary carbon savings.

34.3.74.  Construction traffic can cause nuisance and disturbance for neighbours and others in the vicinity. The applicant must demonstrate that an appropriate approach has been taken to reduce this impact to acceptable levels, taking the cumulative impacts of other development proposals into account. The Council would expect the applicant to demonstrate that the building compound and the skip location can be accommodated on site or in the highway immediately outside the application site.[4]


4. The details of what is required will be set out in the Basements Supplementary Planning Document. [back]


34.3.75.  The methods used in construction can have a significant bearing on the quality of life of residents and businesses in the vicinity, in terms of issues such as noise, air quality, dust and vibration. The applicant must demonstrate that an appropriate approach will be taken, taking the cumulative impacts of other development proposals into account.[5]


5. The details of what is required will be set out in the Basements and Transportation Supplementary Planning Documents. [back]


34.3.76.  The structural implications of the construction of basements below existing buildings, both on the site and nearby, are of particular importance to local residents. The applicant must demonstrate the ground and hydrological conditions of sites (including whether the surface subsoil is gravel or clay), how they intend to carry out the excavation, demolition, and construction work associated with their proposed development whilst safeguarding the structural stability of the buildings around it.[6] The structural integrity of the development itself is not controlled through the planning system but through the Building Regulations and the Party Wall Act.

6. The details of what is required will be set out in the Basements Supplementary Planning Document. [back]

34.3.77.  Given their nature, basements are more susceptible to flooding, both from surface water and sewerage, than conventional extensions, and applicants are advised to see Policy CE2[7] . In particular,

  • self contained basement dwellings are not permitted in Flood Risk Zones 3 of the Environmental Agency’s tidal flood risk zones map[8] .
  • self contained basement dwellings in Flood Risk Zone 2 will only be permitted where they pass the ‘exceptions test’. This is a test which forms part of the technical guidance on flooding[9] .
  • a flood risk assessment will be required in Flood Zones 2 and 3, in Critical Drainage Areas/Local Flood Risk Zones, and for sites greater than 1 ha in Flood Risk Zone 1, demonstrating how the development will be made safe, and the flood risk reduced, and incorporating mitigation measures into the proposal. This will include the installation of a ‘positively pumped device’[10] .
7. Further information will be provided in the Basements SPD. [back]
8. [back]
9. Technical Guidance to the National Planning Policy Framework, CLG, 2012. [back]
10. A positively pumped device is a non-return valve and pump device installed to prevent sewage back-surging into basements in times of heavy rain and allow the property’s sewage to flow properly into the sewer network. [back]

34.3.78.  Applicants wishing to undertake basements are strongly advised to discuss the proposals with neighbours and others who will be affected. Plans should be shared and party wall negotiations should be well underway with those affected before the planning application is submitted. Emerging proposals related to traffic and construction should also be shared with residents and businesses in the vicinity so that their local knowledge and their needs can be more readily taken into account.



Proposed Policy

Policy CL7


Basement development must be of the highest quality. The Council will require  Basement development to adhere to the following requirements:


a.   The basement must not exceed 75% of each garden of the property. Where the findings of the analysis of the surface water conditions of the site demonstrate surface water drainage will not be maintained, this percentage will be reduced. The unaffected garden must be in a single area.



b.   The basement must not comprise more than one additional storey except on larger sites which are less constrained and where it can be demonstrated that traffic and construction impacts can be successfully mitigated


c.   There must be no loss, damage or long term threat to trees of townscape or amenity value, and the ability of future tree planting of a suitable size and scale both on site and in neighbouring gardens must not be prejudiced.


d.   The scheme must not cause substantial harm to heritage assets[11] .

11. A heritage asset is a building, monument, site, place, area or landscape identified as having a degree of significance meriting consideration in planning decisions, because of its heritage interest. This definition will be included in the glossary of the Core Strategy. [back]

e.    The development must maintain and take opportunities to improve the character of the building, garden or wider area, with external elements such as light wells, roof lights, plant and means of escape being sensitively designed and discreetly sited.

f.   The basement must not introduce light wells and railings to the front or side of the property which are visible from the street, where these are not a feature of that street.


g.   The development must include a sustainable urban drainage scheme including a minimum of one metre of permeable soil above any part of the basement beneath a garden.


h.     Where the basement is to be constructed under an existing building, the dwelling or commercial property to which the basement relates must be adapted to a high level of performance in respect of carbon emissions and this must be verified at pre-assessment stage and after construction has been completed. Where a new building with a basement is proposed, the same applies to the entire building.


i.    The submitted application must demonstrate how traffic and construction activity will be organised so as not to harm road safety, significantly increase traffic congestion, nor place unreasonable inconvenience on the day to day life of those living and working nearby.


j.   The submitted application must demonstrate how the construction will be carried out in such a way as to minimise potential impacts such as noise, vibration and dust for the duration of the works;


k.     The submitted application must demonstrate how it is intended to safeguard the structural stability of the application building and nearby buildings.

Applicants are also advised to look at policy CE2, Flooding, in considering basement proposals.


Other options

2.8 The Council has considered a number of further options as part of the formulation of this draft policy, options that have not been taken forward for a number of reasons. The principal alternative options and the reasons that they have been rejected are set out in Appendix B. These options have been considered as part of the Council’s Sustainability Appraisal of the policy.