Basement Impact Assessment
 Risks and issues caused by basement developments
Basement works are usually large and complex undertakings. In carrying out excavation works there is a risk that nearby buildings will be undermined. The potential for collapse also gives rise to high levels of health and safety risks that need to be managed during the construction process.
Large amounts of excavated earth may displace underground water and impact on the aquifer and ground water flows. The nature of basements also makes them susceptible to surface water and sewerage flooding.
There are also concerns that subterranean development can lead to greater environmental impacts such as loss of trees, garden spaces, and biodiversity. The process of constructing a basement is also very intensive and involves the transportation of a lot of construction waste. This represents a large carbon footprint and contributes towards climate change.
 Control of basement development
Councils impose a certain amount of control over basement development by reviewing planning applications for basements and awarding permission where applicants demonstrate that policy has been met, and the proposal can be seen as appropriate.
Also, whilst planning policy plays a role in controlling basement development and encouraging a certain standard, there are many other pieces of legislation, regulations, and codes of practice, that cover basement development and so play a part in controlling development and setting standards.
- The Party Wall etc. Act
- Building Regulations
- Highways Act
- Environmental Health (e.g. Control of Pollution Act, Environmental Protection Act)
- Housing Act
- Utility providers such as Thames Water
- Transport for London and London Underground
- Freeholder permission
- Tree Preservation Orders
A BIA may also be used by developers to ascertain and demonstrate the most suitable engineering solution for undertaking the works and to understand the likely impacts to the surrounding area or nearby buildings and structures. Likewise, the BIA may be used to justify the proposal to other parties such as neighbouring owners under a Party Wall agreement or to authorities such as Highways Authorities or nearby utility providers such as Thames Water.
 Planning application requirements
Planning authorities set the requirements for any basement planning application submission and publish these on their website. Requirements vary. Central London boroughs, where basement development is popular, such as Westminster, Camden, and the Royal Borough of Kensington and Chelsea, set high requirement standards. These Local Planning Authorities alongside others, stipulate that a BIA is submitted to support an application for any new basement development.
 Purpose of basement impact assessments
A basement impact assessment should be specific to the site and the particular development proposed. Through collation of information and detailed analysis the BIA sets out to evaluate the direct and indirect implications of a proposed development.
- Site history, underlying geology and groundwater.
- The impact of the subterranean development on drainage, sewage, surface water and ground water flows and levels.
- Ground movement due to the proposed development (including demolition of any existing structures) and corresponding damage to neighbouring properties and infrastructure.
In instances where the BIA identifies risk of damage to properties, the risk is described using the Burland Scale. This is an internationally adopted methodology that is recognised and used by the Building Research Establishment and the Institution of Structural Engineers. The Burland Scale categories damage and is based on ease of repair of visible damage.
 Contents of basement impact assessments
Although there is no standard template for the contents of a BIA, some Councils will publish their own and encourage applicants to use their templates when making planning applications so that their review process can be more easily standardised.
Camden Council outline 5 stages:
- Stage 1, Screening: Screening stages involve the identification of matters that are of concern. This stage will determine whether or not a full BIA is required. Planning authorities often provide screening questions and flowcharts so that the applicant can be seen to establish this systematically.
- Stage 2, Scoping: For each of the matters of concern identified in the screening stage, potential impacts are then identified. Filed work and data collection may be needed.
- Stage 3, Site investigation and study: Desk studies and field investigations, including monitoring and reporting, are used to develop an understanding of the specific site and its surroundings.
- Stage 4, Impact assessment: Evaluation of direct and indirect implications.
- Stage 5, Review & decision making: With planning authorities that require a BIA, this stage will involve the Council’s appraisal of the BIA and subsequent decision. Some Councils may require independent verification.
There are a multitude of elements that will comprise a full BIA. The inclusion and level of detail of each will depend on the particular site circumstances as well as the nature of the development proposal itself. The screening stage will identify the particularly relevant matters and so detailed studies and investigations can be directed towards these accordingly.
Requirements from planning authorities may also contribute towards the inclusion of certain matters within the BIA. The Royal Borough if Kensington and Chelsea for example have strict requirements for submission of any basement application.
RBKC list of matters for inclusion within a BIA:
- Engineering Design and Construction Statement (EDCS)
- Demolition and Construction Management Plan (DCMP)
- Construction Traffic Management Plan (CTMP)
- BREAAM Assessment
- Flood Risk Assessment
- Site Waste Management Plan
- SuDs (Sustainable Urban Drainage Systems)
- Arboricultural Report
Investigative reports are needed to evidence findings and to enable the detailed analysis of these matters. There are many desk-based studies that can be carried out, including:
- Site history
- Survey information of existing buildings and structures
- Underlying geology
- Groundwater levels
- Historic / current watercourses
- Underground infrastructures
 Physical site investigations
- Trial pits: Trial pits are often required to enable engineering designs to be put forward and for structural methodologies to be appraised. These may also be a requirement by a Council. Westminster Council for example require a Structural Methodology Statement that is assisted with trial pits alongside “all walls which may be impacted by the proposals”
- Ground water monitoring: An investigation may be needed to establish the ground conditions and groundwater levels. This may change over time and can be very specific to the location.
- Boreholes: Boreholes are often required to establish the ground conditions and so that an analysis of the soil type can be undertaken.
--Ellis McCarthy 17:22, 14 Nov 2018 (BST)
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