Basement Impact Assessment outline for a London borough
Boroughs audit basement assessments against a fixed sequence. Here is the outline that survives that audit, section by section, with the failures that send documents back.
A Basement Impact Assessment outline that a London borough will accept has a fixed shape, and it is not the shape of an ordinary technical report. It runs screening, scoping, site investigation, impact assessment, then mitigation and monitoring, in that order, with the screening questions answered one by one and the evidence for each answer named. Boroughs with basement policies commonly appoint their own consultant to audit the document against that sequence, and an audit that finds a screening question answered from assumption sends the whole thing back.
Write it so the auditor can follow the logic without asking you anything.
Screening against the borough's questions
Start with the borough's own screening question set, reproduced in full, with your answer and the evidence beside each one. The questions divide into three families: surface water and flooding, subterranean groundwater flow, and land stability.
Typical surface water questions ask whether the site is in a flood zone, whether it lies in a critical drainage area, whether surface water flows across it, whether it sits within 100 m of a watercourse including a culverted or historic one, and whether impermeable area is increasing. Groundwater questions ask whether the site is above a productive aquifer, whether springs or wells are recorded nearby, whether the basement will extend below the water table and whether dewatering is proposed. Stability questions ask about slopes, made ground, previous mining or quarrying, shrinkable clay, nearby tunnels and existing basements.
Three rules make screening auditable. Answer yes, no or unknown and never maybe. Cite the source for each answer with its date, whether that is geological mapping, the flood map for planning, a borehole record, a historic map or a site walkover. And treat unknown as a trigger for scoping rather than a reason to move on.
Where the borough publishes a screening flowchart, follow it exactly and include the completed version as a figure.
Scoping the further work required
Scoping turns every yes and every unknown into a defined piece of investigation, and it should read as a table of question, reason and proposed work.
The work usually falls into a small set: intrusive investigation with boreholes and window samples, groundwater monitoring in standpipes or piezometers over a stated period, permeability or falling head testing, a condition survey of the host and adjoining buildings, topographic and level survey, a ground movement assessment, a drainage survey and capacity check, and an arboricultural survey where trees are retained.
Name the standards. Site investigation to BS 5930, geotechnical design to Eurocode 7, tree survey to BS 5837:2012. A Geotechnical Assessment prepared to those standards is the document the structural section will rely on, so scope it once and properly.
Say what you are not doing and why. An auditor is more comfortable with a reasoned exclusion than with a silence.
Hydrogeology and groundwater flow
This section needs a conceptual ground model, drawn rather than described. A section through the site showing strata, the proposed basement box, recorded groundwater levels with dates, and the direction of flow does more than three pages of text.
The analysis then addresses whether the structure will obstruct flow, how much of the flow path cross-section it occupies, what that does to levels upslope and downslope, and whether the effect is material at the nearest receptors. Where obstruction is significant, the usual mitigation is a drainage layer or land drain around the structure returning flow to the downstream side, which has to be drawn and specified rather than mentioned.
Dewatering deserves its own subsection where it is proposed: predicted drawdown and its radius of influence, duration, discharge route and consent, settlement risk from drawdown in compressible soils, trigger levels and a contingency.
Surface water goes here too, cross-referenced to the drainage design rather than repeated. The approach in a drainage strategy for a minor residential scheme is a reasonable model for the level of detail a single basement needs.
Land stability and ground movement
The ground movement assessment is the section auditors read most closely. It should state the analysis method, the soil parameters used and their source, the excavation sequence and temporary works principle assumed, the predicted vertical and horizontal movements at the party walls and at any highway or utility asset, and the resulting damage category for each adjoining building against a recognised classification.
Two points separate a good assessment from a weak one. The prediction should be tied to a specific construction sequence, because underpinning in short bays with immediate propping gives a different answer from a continuous excavation. And the assessment should identify the most sensitive receptor and say how close it is to the next damage category up.
Utilities and the highway need naming. A basement wall under a footway or close to a main sewer brings the undertaker and the highway authority into the picture, each with its own approval.
Structural monitoring proposals
Monitoring is the mechanism that converts a prediction into a controlled risk, so set it out as a regime rather than an intention.
- Baseline readings taken before any excavation, at defined monitoring points on the host and adjoining buildings, the highway and any retained tree.
- Instrument types and precision, whether precise levelling, tell-tales, crack gauges, inclinometers or groundwater loggers.
- Reading frequency by construction phase, increasing during the critical excavation and underpinning stages.
- Green, amber and red trigger levels expressed as movement in millimetres, set below the predicted values.
- The response at each trigger, including who is notified, within what period, and what work stops.
- Reporting arrangements, including a named responsible engineer and the recipients of each report.
- A post-completion period of monitoring before the regime closes.
State who pays for the monitoring and who holds the duty if the contractor changes. Conditions commonly require the regime to be approved before excavation, so submitting it with the application shortens the programme.
Cumulative effects with nearby basements
This is the weakest section in most submissions and the one boroughs with basement policies care about most. The question is whether your basement, added to those already built and those already approved, changes groundwater flow or ground movement in a way that no individual assessment captured.
Do what can actually be done. Search the borough's planning register for basement decisions in the surrounding streets and map them. Check the borough's own basement records or constraints layer if it holds one. Look at the site investigation records for neighbouring schemes where they were published with applications. Then state, in the conceptual model, how much of the relevant flow path cross-section is already occupied.
An illustrative example. A terrace of 14 houses on a gentle slope over river terrace gravels already has five consented single-storey basements, three of them within four properties of the application site. The conceptual model shows the gravel horizon is 3 m thick and the proposed box occupies roughly the upper half of it across the plot width. Adding a perimeter drainage layer with a cross-connection to the downslope side keeps the predicted level change at the nearest receptor within the natural seasonal range. The figures are illustrative and every street has its own geology.
Where the honest answer is that the cumulative position cannot be established, say so, say what you searched, and design conservatively.
Audit by the council's consultant
Expect the submitted Basement Impact Assessment to be reviewed by an independent engineer appointed by the borough, often with the fee recharged to the applicant. The audit is a compliance check as much as a technical one, and audits usually fail documents for the same handful of reasons.
Screening answered without cited evidence. A conceptual ground model missing or not drawn in section. Groundwater data from a single reading rather than a monitoring period. Movement predictions not tied to a construction sequence. Cumulative effects asserted to be negligible with nothing behind it. Mitigation described but not drawn or specified. And a monitoring regime without trigger levels.
Write the document against that list. Where an audit does go against you and the application is refused, the same structure carries straight into an appeal statement of case, which is another reason to keep the reasoning traceable from the start.
One honest uncertainty. Screening question sets and audit expectations differ between boroughs, and the method one borough treats as standard another will supplement with its own requirements. Ask for the current screening proforma and the audit brief before you scope the investigation.
Browse all 66 report types on PlanScope to see which documents a basement application is likely to need alongside the assessment.
This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. For specialist technical reports, qualified professional input may be required.
PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
What are the four stages of a Basement Impact Assessment?
Screening against the borough's question set on surface water, groundwater and land stability. Scoping the investigation for anything answered yes or unknown. Site investigation covering boreholes, groundwater monitoring, testing and condition surveys. Then impact assessment against damage and impact criteria, with drawn mitigation and a monitoring regime.
How should screening questions be answered?
Yes, no or unknown, never maybe, with a cited source and date beside each answer. Acceptable sources include geological mapping, the flood map for planning, borehole records, historic maps and a site walkover. Treat unknown as a trigger for scoping rather than a reason to move past the question.
What makes a ground movement assessment credible?
Stated analysis method, soil parameters with their source, and predictions tied to a specific excavation and underpinning sequence, because short bays with immediate propping give different results from a continuous dig. Report movements at each party wall and at highway or utility assets, with the damage category for each and the most sensitive receptor identified.
How do I evidence cumulative effects?
Search the borough's planning register for basement decisions in surrounding streets and map them, check any basement constraints layer the borough holds, and use published site investigation data from neighbouring applications. Then state in the conceptual model how much of the relevant flow path cross-section is already occupied, and design conservatively where the position cannot be established.
Who audits the assessment and who pays?
Boroughs with basement policies commonly appoint an independent engineer to review the submitted assessment, and the fee is often recharged to the applicant. The audit checks compliance with the required method as much as the technical content, so budget for it and ask for the audit brief before scoping the investigation.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.