Many local planning authorities and London borough councils will only allow basement development where the applicant can prove it will not cause harm to the built and natural environment. Addressing this issue typically requires a ‘basement impact assessment’ (BIA) which is specific to the site and proposed development.
The requirements for the basement impact assessment will vary but will typically involve a phased approach:
Stage 1 – Screening
The first stage is to commission a hydrogeological and hydrological screening report to determine whether further desk study, site investigation and impact assessment is required. All basement proposals should be subjected to the screening stage to identify the matters relevant to assessment of local flooding and/or neighbouring amenity and structural risks.
Stage 2 – Scoping
The scoping stage of the basement impact assessment should build on the information obtained for the screening stage. During this phase, a desk study should be undertaken to determine the preliminary impacts of the proposed basement development. This assessment should be used to identify any site investigation works required.
Stage 3 – Site investigation
Depending on the findings of the screening and scoping stages a site investigation may be required to develop a better understanding of the site, neighbouring properties and immediate surroundings. The scope of the investigation will depend on the location of the proposed basement, its size and its relationship to adjacent structures.
The site investigation will comprise several stages, including:
- Desk study and site reconnaissance;
- Intrusive investigation;
- Reporting; and
The aim of the site investigation is to determine the geological conditions; assess the geotechnical and hydrogeological constraints; and consider the implications of the ground conditions for the proposed development.
Stage 4 – Impact analysis
The final stage is detailed impact analyses (if required) on the effects of the proposed basement construction including: calculations of predicted ground movements; structural impact; slope stability; and groundwater flows.