Basement Impact Assessment or Geotechnical Assessment
A Basement Impact Assessment is a planning document about effects on neighbours, groundwater and stability. A Geotechnical Assessment is an engineering document about design parameters. Most basement schemes need both.
A Basement Impact Assessment asks what the excavation does to the neighbours, the water and the ground around it. A Geotechnical Assessment asks what the ground will do to the structure you intend to build in it. The basement impact assessment vs geotechnical assessment question matters because the two overlap in their site investigation and diverge completely in who reads them and what they have to prove.
If the site is in a borough with a basement policy, you will need both, and the geotechnical work feeds the basement assessment rather than replacing it. Clients often assume the reverse, having been told by a contractor that the ground investigation covers everything. It does not, and the difference is usually discovered at validation.
The questions each assessment answers
The basement document is a planning document. It was developed to answer the specific concerns that arose when residential basement excavation became common in dense urban areas: subsidence of neighbouring properties, interruption of groundwater flow, loss of ground infiltration, surface water flooding, and the loss of trees whose roots sit in the affected soil.
The geotechnical document is an engineering document. It characterises the ground, gives design parameters for foundations and retaining structures, and supports design to Eurocode 7 and compliance with Part A of the Building Regulations. It is usually written for the structural engineer and the building control body.
Where the planning system takes an interest in ground conditions on a site without a basement policy, a Geotechnical Assessment is normally what the officer means, covering land stability, made ground and the feasibility of the proposed foundation solution.
Screening, scoping and investigation
The basement assessment follows a staged structure that most boroughs with a policy have adopted in similar terms.
- Screening, a desk exercise that asks a series of yes or no questions about ground conditions, surface water, slope stability and the local hydrogeology.
- Scoping, which takes the questions answered yes at screening and defines the investigation needed to resolve them.
- Site investigation and study, including boreholes, groundwater monitoring over a period, and a structural survey of neighbouring properties.
- Impact assessment, which sets out predicted effects and the mitigation and monitoring proposed.
The geotechnical investigation sits inside step three. Boreholes sunk for the basement assessment give the same data the structural engineer needs, so commission them once and write the brief to serve both purposes. Splitting the two procurements is how sites end up with two sets of holes and two sets of fees.
Groundwater monitoring is the part that controls the programme. Standpipe readings over a winter period tell you far more than a single reading in August, and boroughs that audit basement assessments know it.
Ground movement and damage categories
Predicted ground movement is expressed as a damage category using the Burland scale, which runs from negligible through very slight, slight, moderate, severe and very severe. Most policies and most audits expect the predicted damage to neighbouring properties to be no worse than the slight category.
The calculation depends on the retaining solution, the construction sequence and the stiffness of the ground. A contiguous piled wall installed in short bays behaves differently from underpinning in a hit and miss sequence, so the assessment has to describe a buildable method rather than an idealised one.
Two practical points. The assessment should test the sequence that the contractor will actually use, because an audit will ask. And predicted movement is only half the story, since monitoring conditions with trigger levels are what the borough relies on during construction.
Groundwater, flow paths and surface water
This is where the two documents are furthest apart. A geotechnical report treats groundwater as a design input, something to be dewatered, resisted or drained. A basement assessment treats it as a receptor, asking whether the structure will dam a shallow aquifer, divert flow towards a neighbouring cellar, or reduce the volume of ground available for infiltration.
Sites on sloping ground with permeable superficial deposits over clay are the ones where this matters most, and the hydrogeology of the basement usually needs its own conceptual model. Surface water sits alongside it: removing garden to build a basement box reduces infiltration and increases run-off, which brings the drainage strategy into the same conversation.
Where borough policy sets the requirements
Basement policy is local. A number of London boroughs have adopted development plan policies and supplementary planning documents dealing specifically with basements, setting limits on extent beneath gardens, depth, the proportion of the plot that may be excavated, and requirements for soft landscape above the structure. Others have nothing beyond general amenity and drainage policy.
Read the borough's own adopted policy and its supplementary guidance before scoping anything, because the requirements vary in substance as well as in wording. Some cap the excavation at a single storey below ground, some restrict the footprint beneath the rear garden to a proportion of the plot, and some require a minimum depth of soil over any structure that sits under soft landscape so that trees can be planted above it.
That last requirement has a habit of appearing late. A basement designed to the plot boundary with a shallow slab over it can fail a soil depth policy even where the ground movement and hydrogeology work is faultless, and the fix is a design change rather than more evidence. Outside London, basement policies exist in some authorities and not in others, and a Basement Impact Assessment may still be requested where the ground conditions justify it.
Audit by the council's own consultant
Boroughs with basement policies commonly appoint an independent auditor to review the submitted assessment, with the cost recovered from the applicant. The auditor works to a checklist and reports to the case officer, and their queries have to be answered before the application can be recommended.
Audits are not adversarial, though they feel that way when the third round of questions arrives. Most queries come from the same places: an unclear construction sequence, groundwater monitoring over too short a period, ground movement predicted for a different method from the one described elsewhere in the application, or missing structural survey information for the adjoining properties. Anticipating them shortens the process considerably, and there is a settled approach to responding to a basement audit that keeps the exchange to two rounds.
Programme and cost across the two
The geotechnical work can usually be delivered in weeks once access is arranged. The basement assessment cannot, because the groundwater record needs time and the audit adds rounds after submission.
Plan on the basis that the basement assessment sets the critical path. Start the monitoring before the design is finished, agree the investigation brief with both the structural engineer and the basement specialist, and budget for the audit fee as a separate line. Both documents need suitably qualified authors, and councils expect a chartered engineer or equivalent to sign the work.
Work out which documents the scheme needs before you commission any of them: browse the report types on PlanScope.
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.
Frequently asked questions
Do I need both a basement assessment and a geotechnical report?
On a site in an authority with a basement policy, usually yes. The geotechnical work characterises the ground and supports the structural design, and the basement assessment uses that data to answer planning questions about neighbours, groundwater and surface water. Commission the ground investigation once, with a brief written to serve both documents.
What is the Burland scale?
It is the classification of building damage used to express predicted ground movement effects, running from negligible through very slight, slight, moderate, severe and very severe. Basement policies and audits commonly expect predicted damage to neighbouring properties to be no worse than the slight category, based on the construction sequence actually proposed.
How long does groundwater monitoring need to run?
Long enough to capture seasonal variation, which in practice means standpipe readings across a winter period rather than a single reading. A short monitoring record is one of the most common reasons an auditor asks for more information, and it is the item most likely to delay determination.
What is a basement audit and who pays for it?
Authorities with basement policies often appoint an independent consultant to review the submitted assessment against a checklist and report to the case officer, with the cost recovered from the applicant. Queries have to be resolved before the application can be recommended, so budget for the fee and for at least one round of responses.
Does a basement need a geotechnical report if there is no basement policy?
Ground conditions can still be a planning matter where stability, made ground or mining legacy affect whether the development is deliverable. In that situation a geotechnical assessment is usually what the officer is asking for. Check the authority's local validation list, because the trigger differs between areas.
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.