A Basement Impact Assessment, or BIA, explains how proposed excavation could affect ground stability, groundwater, drainage and neighbouring buildings. It should identify the relevant risks and the measures needed to manage them, using evidence specific to the site rather than a generic basement report. [1]
When is it needed?
The requirement comes from the council's planning policy and submission requirements. Camden expressly requires a BIA for a new or extended basement. Its published guidance says the level of technical information depends on the development, but engineering input is needed regardless of the scale of the proposed basement work. [1]
Do not assume another borough uses exactly the same report format. Kensington & Chelsea's basement guidance and planning reporting also require technical evidence addressing structural and ground conditions, but the names and stages of the documents differ. [2] [3]
What should the assessment establish?
Camden's guidance describes a staged process: screening, scoping, investigation and assessment, followed by independent checking where required. Screening identifies the issues that need further work; it is not a licence to leave unanswered risks out of the submission. [4]
Ask the team to explain four practical outputs:
- what is known about the ground and water conditions;
- which existing and neighbouring structures could be affected;
- what assumptions remain and how they will be tested;
- what changes to the design, construction method or monitoring are necessary.
These questions help distinguish evidence from an assurance that a competent contractor will deal with everything later. HSE's structural-stability guidance likewise requires the risks of alteration work to be established and the support arrangements planned. [5]
Who prepares it?
The relevant contributions may need structural, geotechnical, geological and water expertise. Camden identifies qualifications for the different technical disciplines and expects authors to have appropriate professional competence. An architectural drawing package alone does not provide those assessments. [4]
Clarify appointments before commissioning the work. Ask who is responsible for the combined report, who will answer the council or auditor, and who will revise the engineering if the planning design changes. Clear appointments help competent members of the project team coordinate the design and respond to changes. [6]
What does an independent audit do?
Camden uses Campbell Reith to audit BIAs against its technical policy requirements. An audit checks the submission; it does not turn the council or auditor into the project's structural designer or relieve the appointed team of its own responsibilities. [1] [6]
Allow for a response and revision stage. Where further investigation is requested, the design may need to change before the assessment can be closed out. Camden's guidance sets out the staged evidence and checking process rather than treating the first report as automatically complete. [4]
What the BIA does not replace
A BIA is not a building regulations approval, party wall award, construction contract or waterproofing guarantee. Keep those workstreams visible in the project plan. Groundwater assessment and waterproofing design inform one another, but a product specification cannot substitute for assessing effects on the site and neighbours. [7] [8] [9]
Camden's proposed replacement Policy D6 remains under examination as checked on 2 October 2026. Confirm the adopted policy and audit requirements again when preparing a later application. [10]