Appendix C: Requirements of the Basement Impact Assessment

C.1 The Council intends to amend the requirements on what information is necessary when an application for a basement is submitted. This information is set out on its Local List, and changes to that list will be notified in due course, once the feedback from this consultation has been assessed..

C.2 The Council's intention is that any planning application that includes a new basement will need to include a Basement Impact Assessment (BIA). A BIA comprises a number of elements, which are set out below. Templates for the BIA will be able to be downloaded from the Council's planning website, for inclusion with applications. Its purpose is to require the applicant to confirm that the individual elements of the assessment are all included. A draft is included at the end of the Appendix.

ENGINEERING DESIGN AND CONSTRUCTION STATEMENT (EDCS)

C.3 The Council recognises that the construction of basements in a dense urban environment is a technically challenging exercise that will require great care throughout the process. Such developments can carry a risk to both existing and neighbouring structures if ill planned, poorly constructed, poorly managed, or not properly considering local geology and hydrology.

C.4 In order to minimise such risks, the Council will require applicants to provide a detailed EDCS before any application which includes an element of basement excavation is registered.

C.5 The purpose of the EDCS is not to spell out one particular engineering solution. It is to demonstrate how the applicants 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 and avoiding long term harm to the character and quality of their local setting. The responsibility to ensure that the development is structurally sound remains that of the developer.

C.6 It will also ensure that the applicant's approach is in the public domain, and thus neighbours can see exactly what is proposed, and, where appropriate, refer to the EDCS in any Party Wall Award.

C.7 The detailed requirements for the EDCS will be set out within the Basement Development SPD. It is based upon the Basements report carried out by Alan Baxter Associates, for the Council. It must show how all relevant design issues have been addressed.

C.8 The key principles will include:

  • The EDCS will require a clear and unequivocal conclusion that the applicant's proposal, as outlined in the EDCS, would, if carried out in that manner, safeguard the structural stability of the buildings around it.
  • The EDSC will require a clear sequence and method of construction to be developed that is specific to the project and that reflects its design. A generic EDCS will not be appropriate.
  • The EDCS must be prepared and signed off by a Chartered or Civil Engineer, and further checked by an appropriately qualified independent third party.
  • A Chartered or Civil Engineer must be retained to detail the structural works, review the contractor's proposals, method statements and temporary works proposals and monitor the construction

C.9 The EDCS will include:

  • A desk study to establish the site history, age of property, topography, geology and ground conditions, rivers and watercourses whether existing or old, the ground water regime, flood risk issues and underground infrastructure.
  • Physical investigations, clearly presented with accompanying drawings, with engineering interpretation of the results. This must include establishing the ground conditions including the geological strata and presence of the Upper Aquifer, ground water monitoring where the Upper Aquifer is present; trial pits on walls to be underpinned or to have piled walls built close to them to establish the details of the existing foundations and their condition.
  • An appraisal of the existing building structure and understanding of relation (and condition) of adjoining buildings.
  • Statement on groundwater when part of the proposal is below the water table. To include (as appropriate) consideration of cumulative impact and potential of new basement to cause a local rise in water level of Upper Aquifer.
  • Consideration of how the basement structure is likely to be built. Include envisaged sequence of construction, temporary propping, how vertical and lateral loads are to be supported and what must be done to limit the movements of the existing structure and adjoining buildings.
  • Assessment of movements expected and statement of how these will affect the existing property, adjoining buildings or other adjacent structures. To based on calculations or empirical means with appropriate justification.
  • Building specific issues that are relevant, such as adjoining properties that are particularly susceptible to movement, with proposals for their protection.
  • Details of movement monitoring to be carried out during construction works including "traffic light" trigger levels and actions to be followed by the contractor.
  • The engineering design has to be advanced to a "Detailed Proposal Stage", as set out in the Services ACE.

C.10 The Council will develop a pro forma, which will be included in the SPD to assist applicants in the production of a EDCS.

DEMOLITION AND CONSTRUCTION MANAGEMENT PLAN (DCMP)

C.11 The implementation of any planning permission can disturb those living in the vicinity. This is particularly the case for basement development given the type of work, its duration and extent and the proximity of neighbours. Control of the noise, vibration and dust associated with the implementation of such a permission is addressed through the provisions of the Control of Pollution Act (1974) and Environmental Protection Act (1990).

C.12 The Council will require the submission of a DCMP at validation stage which would set out the methodology the applicant (and in particular a contractor) intends to use to implement the permission. It will include consideration of the site and its setting, a description of the works, including timescales for demolition, piling (where used) and construction; details of the mitigation measures proposed and details of the proposed monitoring.

C.13 The DCMP would be approved by the Bi-Borough Director for Environmental Health prior to being submitted with the planning application.

C.14 The detailed requirements of the DCMP will be set out in the basements SPD. The key principles will include an agreement:

  • that the contractor will abide by the Borough's working hours
  • that the best practical means are used for the construction works. These will include:
  • reference to the relevant Codes of Practice ( BS 5228:1997 Code of Practice for Noise and Vibration Control on Construction and Open Sites, BS 7385:1993 Evaluation and measurement of vibration in buildings ).
  • Identification of appropriate methods of piling that minimise noise and vibration
  • Identification potential mitigation measures (acoustic screening etc)
  • to register the site with the Considerate Contractors Scheme
  • to undertake a dust risk assessment in accordance with the Mayor for London's Best Practice Guidance Document 'Control of dust and emissions from construction sites
  • for larger sites, agree to consider the submission of a s61 application

C.15 The DCMP will be enforced by planning condition. Where the applicant intends to amend its provisions from those submitted at the validation stage, an updated DCMP will be required.

C.16 For the largest of proposals an applicant may seek a s61 "prior consent" Notice under the Control of Pollution Act rather than a DCMP. This enables a contractor to seek prior consent to conduct their works in a particular way. They must submit an application to the Council's Environmental Health department that sets out how the works will be completed and how noise will be minimised. Specifically, they include:

  • Methods of work
  • Plant to be used
  • Noise emissions of such plant
  • Estimates of noise produced
  • Likely duration of each phase
  • Mitigation

C.17 Once the Council is satisfied that the application contains sufficient information, it can issue the Prior Consent Notice with various conditions attached, which must be complied with. Applications are tailored for specific phases of development and consents must be issued prior to works commencing. If the works are carried out in accordance with the s61 consent the developer is immune from enforcement action relating to noise If a s61 application is not requested by the Council, it is likely that a s60 notice (under COPA 1974) will be served instead. This will specify the Council's working hours and recommended best practicable means.

CONSTRUCTION TRAFFIC MANAGEMENT PLAN (CTMP)

C.18 In order to minimise the impact of the traffic associated with the construction phase of a development, the Council will require the submission of a CTMP at the validation stage of an application. This must be approved by the Director of Transportation and Highways prior to being submitted with the planning application. This will ensure that applicants consider how they intend to implement a scheme and service a site before permission is applied for.

C.19 The function of the CTMP is to ensure that construction traffic will not jeopardise road safety, will not significantly increase traffic congestion, nor place unreasonable inconvenience on the day to day life of those living and working nearby.

C.20The detailed requirements of the CTMP will be set out in the Basements SPD. They will be governed by the following principles.

  • The routing of vehicles will be planned to ensure minimal disruption to residents and to avoid sensitive locations that generate large number of pedestrian or vehicle trips
  • The size of vehicles used will appropriate for the streets within which the development is located
  • Vehicles' access to the site will only be permitted between the hours of 10 and 4 pm
  • Parking suspensions will be avoided unless absolutely necessary
  • The building compound and the skip location should be accommodated on site or in the highway immediately outside the application site.
  • The development works will be coordinated with adjacent development sites
  • Local residents are kept informed of development works and complaints will be dealt with promptly and effectively

C.21 The CTMP will be enforced by planning condition. Where the applicant intends to amend its provisions, a further application will be required.

BREAAM ASSESSMENT

C.22 The construction and ongoing occupation of a basement can produce a significant amount of Carbon Dioxide, which contributes to climate change. The Council requires that this impact must be mitigated, with the chosen method of this mitigation being that the entire dwelling, following a basement development, meets the the "Very Good" BREEAM Domestic for refurbishment standard. This should apply at both pre-assessment and post construction stages.

Flood Risk Assessment

C.23 The Council requires a site specific flood risk assessment for all development (including basements) in Flood Risk Zones 2 and 3 (as shown in the Environment Agency's flood zone map. A site-specific flood risk assessment is carried out by, or on behalf of, an applicant to assess the risk of flooding to a development site. The FRA must also demonstrate how flood risk (from all sources of flooding) to both the development itself and to others, will be managed now, and taking climate change into account. In some cases, an exception and sequential tests may also be required.

Site Waste Management Plan

C.24 Require applicants for development of a scale that triggers the need for a Site Waste Management Plan (as set out by the Site Waste Management Plans Regulations 2008) to prepare and implement Site Waste Management Plans to arrange for the efficient handling of construction, excavation and demolition waste and materials. These should identify of the volume and type of material to be demolished and/or excavated, opportunities for the reuse and recovery of materials and to demonstrate how off-site disposal of waste will be minimised and managed.

Sustainable Urban Drainage Systems (SuDS)

C.25 Where the basement proposed extends beyond the footprint of the existing building, the applicant will be expected demonstrate that measures have been put into place which to manage and reduce surface water run-off. The Council has developed a SuDS tool to calculate the increase of surface water run-off as a result of new impermeable surfaces associated with the development. The SuDS tool also gives different options of SuDS which can be implemented. The output of the tool is a report showing the SuDS solutions chosen for that development. This report must be submitted along with the planning application. The plans and drawings submitted with the application should show the location of the chosen SuDS for the application to be validated.

C.26 If the proposed basement will not lead to an increase in impermeable surfaces and surface water run-off SuDS are not required. In those cases an statement should be included along with the application explaining why the development will not increase impermeable surfaces.

Arboricultural report

C.27 All applications for basements where there are trees must 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.

 

Draft Basement Impact Assessment Checklist

Please tick the items that are included with your application. If the information is not submitted, you application will not be registered.

  • I have included with my application the following information:
  • I have noted the requirements of each of these items set out in the Basements SPD and confirm that I have complied with these requirement
  • Engineering Design and Construction Statement
  • Demolition and Construction Management Plan
  • Construction Traffic Management Plan
  • BREEAM for Refurbishment Assessment
  • Flood Risk Assessment
  • Demonstration of effective Sustainable Urban Drainage
  • Pre application consultation report
  • Confirmation that neighbours have been provided with copies of EDCS and notified of the provisions of Party Wall process.

Where relevant

  • Site Waste Management Plan
  • Arboricultural Report