Basement Impact Assessment: screening and scoping stages

A Basement Impact Assessment is a staged document, and most of the work that decides whether a scheme is acceptable happens in the first two stages, before anyone sinks a borehole. Screening asks whether the proposal could affect groundwater, surface water or ground stability. Scoping decides what has to be investigated to answer the questions screening raised. Get those basement impact assessment stages wrong and the site investigation looks for the wrong things.

Several London boroughs require the assessment through adopted policy and a supplementary planning document, and the structure below follows the methodology those documents use.

The four stages councils expect

Stage 1 is screening: a series of yes, no or uncertain answers about the site and the proposal, covering hydrogeology, hydrology and land stability.

Stage 2 is scoping: for every question answered yes or uncertain, a statement of what investigation or assessment will resolve it.

Stage 3 is the site investigation and study: the boreholes, the groundwater monitoring, the structural survey of neighbouring buildings and the desk-based research that the scoping identified.

Stage 4 is the impact assessment itself: the ground movement analysis, the predicted effects on neighbouring structures and on the water environment, and the mitigation and monitoring proposed.

Most councils then require an independent audit of the assessment, commissioned through the authority and paid for by the applicant. Budget for it and for the time it takes.

Screening questions on hydrogeology

The hydrogeology questions ask whether the basement will intercept groundwater or alter its flow.

Typical questions cover whether the site lies above an aquifer, whether groundwater has been recorded within the proposed excavation depth, whether springs or historic watercourses are known nearby, whether the basement would extend below the water table, and whether it would sit within a source protection zone.

Answering these properly means looking at published geological records, historic maps for culverted watercourses, and any groundwater monitoring on adjoining sites. A screening answer of "no" with no evidence behind it is the single most common reason an audit sends an assessment back.

Screening questions on hydrology and stability

Hydrology covers surface water: whether the site falls within a flood zone, whether it sits in a critical drainage area, how much of the garden will be lost to hardstanding or to the basement footprint, and whether the surface water discharge rate will change. Basements under gardens reduce infiltration, and boroughs commonly require a minimum depth of permeable soil above the slab and a drainage design that maintains greenfield run-off rates. Which document carries which part of that argument is worth settling early, and the distinction is the same one covered in what goes in a flood risk assessment and what goes in a drainage strategy.

Stability covers slopes, made ground, shrink-swell clays, trees within influencing distance, historic mining, and the condition and foundation depth of adjoining buildings. In much of London the controlling factor is London Clay and its response to changes in moisture and unloading.

Scoping the site investigation

Scoping should name each investigation and tie it to the screening question it answers. A scoping section that lists a generic ground investigation is not scoping, it is a shopping list.

1. Boreholes and window samples, with locations, depths and rationale.
2. Groundwater monitoring standpipes, with a monitoring period long enough to capture seasonal variation.
3. In situ and laboratory testing appropriate to the ground model being tested.
4. A structural condition survey of adjoining properties, with photographic records.
5. Topographical and drainage surveys, including invert levels of nearby sewers.

Where access to neighbouring properties is refused, say so and explain what assumptions were made instead. Auditors accept conservative assumptions. They do not accept silence.

Monitoring periods deserve particular thought. Groundwater in London Clay and in the river terrace gravels above it behaves differently through the year, and a single dip taken in August tells you very little about February. Where the programme cannot accommodate a full seasonal record, say what the shortened period covers and how the design allows for the range it did not capture.

Ground movement modelling and damage categories

Stage 4 predicts how much the ground and the neighbouring buildings will move as a result of the excavation and the permanent works.

The output is normally expressed using the established burden of damage categories running from negligible, through very slight and slight, to moderate and above. Most boroughs expect the predicted damage to neighbouring properties to fall within the lower categories, typically no worse than slight, and will ask for the assumptions behind any prediction close to the boundary between categories.

Two things get challenged at audit. The soil parameters, particularly stiffness, where a value taken from a textbook range rather than from site testing produces a flattering answer. And the construction sequence, since predictions depend on the propping regime and the length of time an excavation stands unsupported.

Present the analysis with the input parameters visible. An assessment that reports a damage category without the inputs is asking the auditor to take it on trust, and they will not.

Neighbours, the party wall boundary and what planning cannot do

Planning and party wall are different regimes and the assessment should say so. The Party Wall etc. Act 1996 governs the private relationship between owners, including notices, surveyors and the award. It is not a planning matter, the council does not administer it, and a planning permission grants no right to underpin or excavate next to a neighbour's wall.

What planning can do is require the ground movement analysis, the monitoring and the method statements. What it cannot do is resolve a dispute about compensation for damage. Applicants who understand that distinction early tend to handle the neighbour relationship better, because they stop expecting the council to arbitrate.

A Structural Survey of adjoining properties before work starts protects both sides, and it is the evidence that settles arguments later about pre-existing cracking.

Conditions to expect

Monitoring regimes with agreed trigger levels and a stated response when a trigger is reached. Method statements and a construction management plan submitted before work starts. Restrictions on working hours, lorry routes and the size of vehicles. Retention of the approved permeable soil depth above the slab. Sometimes a requirement to appoint a named engineer with specified experience.

An illustrative example. A single-storey basement under the footprint and rear garden of a Victorian terrace, in London Clay, with no recorded groundwater in the excavation depth and a mature street tree 9 m away, would typically screen in on stability and surface water, scope two boreholes with standpipes and a tree root investigation, and conclude at very slight predicted damage with monitoring conditioned. Those figures are illustrative only.

The Basement Impact Assessment sits alongside a Geotechnical Assessment and the Drainage Strategy, and councils expect a suitably qualified author for all three, with professional review strongly advised. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval, and for specialist technical reports qualified professional input may be required.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified engineer before relying on it.