Basement impact assessment: the standard four stage structure

A basement impact assessment contents list is more standardised than most planning documents, because the structure comes from a single piece of research commissioned in London and adopted almost unchanged by the boroughs that ask for it. Four stages: screening, scoping, site investigation and study, then impact assessment. Each stage feeds the next, and skipping one is the quickest way to have the document returned by the council's appointed auditor.

The assessment is not the engineering design. It is the evidence that the engineering design will not damage neighbouring property, alter groundwater flow or worsen surface water flooding. Those three questions are the whole document.

The four stage structure

Screening asks whether the proposal could affect land stability, groundwater or surface water. It is a series of yes, no or unknown answers against a standard set of questions, usually presented as a table with a reference to the evidence behind each answer. An unknown is treated as a yes.

Scoping takes every yes and unknown from screening and defines what work is needed to resolve it. This is where the ground investigation is specified, where monitoring wells are located, and where the extent of the study area is fixed. A scoping stage that specifies nothing, because screening found nothing, has to explain why in terms an auditor can test.

Site investigation and study delivers the specified work: boreholes, window samples, groundwater monitoring over a defined period, a condition survey of adjoining buildings, and desk data on geology, historic mapping and any known ground movement in the area.

Impact assessment then answers the screening questions with evidence and states residual effects after mitigation.

1. Screening, with each question answered and referenced.
2. Scoping, defining the investigation needed for every unresolved question.
3. Site investigation and study, delivering that work.
4. Impact assessment, with residual effects and mitigation.

Write the document in that order and cross-reference it. Auditors work backwards from a residual effect to the investigation that supports it, and a report where the sections do not refer to each other takes far longer to sign off.

Land stability and ground movement

The stability section covers the temporary and permanent condition. Temporary works sequencing usually governs, because the largest movements happen during excavation rather than after the structure is complete.

Predicted movements are classified against the Burland damage category scale, which runs from negligible through very slight and slight up to severe and very severe. Most boroughs expect predictions to sit within the lower categories for neighbouring structures, with monitoring to confirm. The related question of construction-induced vibration is covered in piling vibration and neighbouring structures.

State the assumptions. Retaining system stiffness, propping levels, excavation sequence and the assumed ground parameters all move the prediction, and an assessment that presents a number without them cannot be reviewed. A Geotechnical Assessment prepared to Eurocode 7 normally supplies those parameters.

Hydrogeology and groundwater flow

The hydrogeology section has to establish whether the basement sits in or near groundwater, and whether it would act as a barrier to flow. In London clay the answer is often straightforward. In river terrace deposits above the clay, a perched water table can behave very differently across a single street.

Monitoring matters more than desk study here. Standpipes read over several months through a wet period tell you something a single borehole log cannot, and boroughs frequently reject assessments that rely on one visit in August.

Where the structure would obstruct flow, the usual mitigation is a drained cavity or a land drain routed around the box, with the discharge point identified. Say where the water goes. "Groundwater will be managed" is not mitigation.

Source protection zones and any nearby abstraction should be checked and reported even where the answer is negative, because the auditor will check them anyway.

Surface water and drainage effects

Excavating a basement usually reduces the permeable area available for infiltration and adds lightwells that collect rainfall. Both change the surface water regime on a site that may already discharge to an overloaded combined sewer.

The assessment should give existing and proposed impermeable areas, the discharge route, the proposed rate and the storage provided. Where lightwells are below the level of the receiving drain, pumped drainage with storage and a failure mode has to be described. Pumps fail during the same storms that fill lightwells, which is the detail most often missing, and it is picked up in basement lightwells and surface water risk.

Basement flood resilience belongs here too. Habitable basement accommodation in a flood risk area raises vulnerability questions that the flood risk work has to answer rather than the basement assessment.

Trees and neighbouring structures

Excavation near trees affects both the tree and the building. Root protection areas calculated under BS 5837:2012 will often overlap the proposed excavation on a typical urban plot, and the arboricultural work has to be done in parallel rather than afterwards, as set out in foundations near trees and design depth.

For neighbouring structures, a pre-works condition survey is standard and should be offered rather than waited for. Photographic and written records of existing cracking protect the applicant as much as the neighbour.

Foundation depths of adjoining properties are a common unknown. Where they cannot be established by trial pit or from records, say so and carry the uncertainty into the design assumptions rather than guessing a depth.

Terraced and semi-detached plots bring a third party into the technical argument. The neighbour may commission their own engineer, and that engineer will read the assessment line by line. A document written so that an outside reviewer can follow the reasoning without a meeting saves a great deal of correspondence later.

The independent audit

Boroughs that require a Basement Impact Assessment generally appoint an independent auditor at the applicant's cost, and the audit runs during the determination period. The auditor checks the internal logic first, then the technical content.

The failures that generate audit queries are consistent. Screening answers with no evidence reference. Scoping that omits an investigation the screening implied. Groundwater monitoring over too short a period. Damage predictions with no stated assumptions. A drainage section that ignores the lightwells. None of these are hard to avoid, and all of them cost weeks.

Budget for a second round of comments. Auditors rarely sign off first time on a complex site, and a scheme programmed with no allowance for a response will miss its committee date.

Mitigation and monitoring commitments

The closing section should read as a set of commitments a condition can capture. Movement monitoring with named trigger levels and an agreed response at each level. Groundwater monitoring continuing through construction. A construction method statement matched to the assumed sequence, which usually links to the construction management plan for the excavation itself.

Write the triggers so they mean something. A green, amber, red structure with a defined action at amber, and a stop at red, gives the council something enforceable and gives the contractor a clear instruction.

And keep the commitments realistic. An assessment promising weekly precise levelling of six neighbouring properties for two years will either be conditioned as written or quietly abandoned on site, and neither outcome helps.

Browse the report types on PlanScope to see what usually accompanies a basement application.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required.