Structural survey or basement impact assessment

A structural survey tells you what the existing building can take. A basement impact assessment tells you what your excavation will do to the ground, the water and the buildings next door. They answer different questions, they are written by different people, and on a basement scheme you will usually need both.

The confusion arises because both involve a structural engineer and both talk about stability. The distinction to hold onto is direction of travel: one looks inward at your building, the other looks outward at everything your works will disturb.

What each report covers

A structural survey examines the fabric and the frame of an existing building. It records the construction, identifies defects, traces load paths, assesses foundations where they can be inspected, and gives a view on whether the building can accommodate the proposed alterations. On a conversion it answers whether the floors can carry residential loading, whether the roof can take a dormer, and whether removing a spine wall is feasible.

A basement impact assessment examines the effect of excavation on its surroundings. It covers three strands: land stability and ground movement, subsurface water flow, and surface water and flooding. It usually follows a screening, scoping, site investigation and impact assessment sequence, with a construction method and a monitoring regime at the end.

Both are report types where councils expect a suitably qualified author and where professional review is strongly advised. A draft prepared with AI assistance can carry the structure and the policy framework. It cannot make the engineering judgement, and it cannot replace the site investigation.

Building condition against ground and water effects

The overlap people notice is party wall condition, and it is shared ground. A structural survey often includes a condition schedule of the host building. A basement impact assessment predicts movement in the neighbouring building. Neither of those is the same as the schedule of condition prepared under the party wall process, which is a separate civil regime with its own surveyors and its own timetable.

What the planning officer wants from each is different too. From the structural survey, on a conversion or a retention case, the officer wants to know whether the building can be reused and what that involves. From the basement assessment, the officer wants to know whether the proposal will cause unacceptable harm to land stability, to neighbouring structures, to groundwater flow or to flood risk.

Where the application is for the demolition or partial demolition of a building in a designated area, the structural evidence carries a different weight again, and our note on demolition in a conservation area covers which consent applies.

Where a basement impact assessment is required

There is no national requirement. The trigger is local policy, and it concentrates in places where basement construction is common and the ground is difficult.

Several London boroughs require one through adopted local plan policy supported by detailed supplementary guidance, and the methodology used across London derives from a common technical approach to screening, scoping and assessment. Outside London, authorities in areas of made ground, shrinkable clay, coal mining legacy, high groundwater or landslip risk impose their own requirements.

Check three things before you commission. The adopted local plan policy that names the requirement. The validation checklist, since that determines whether the application will be registered without one. And any supplementary planning document that prescribes the method, because where one exists it will set the screening questions, the report structure and often the auditor arrangements.

Where the ground conditions rather than the basement are the issue, a Geotechnical Assessment may be the proportionate answer instead, particularly for a scheme with piled foundations and no subterranean space.

Ground movement and the neighbouring building

This is the heart of the assessment and the part that decides applications.

The engineer predicts the movement the excavation and its temporary works will cause, expresses it as settlement and strain at the neighbouring structures, and classifies the likely damage against the standard damage categories used in the field, which run from negligible through very slight and slight to severe. The usual expectation is that predicted damage to third party property should sit at the lower end of that scale.

The prediction is only as good as the inputs. That means a real ground investigation with boreholes at the right depth, groundwater monitoring over a period rather than a single reading, and information about the neighbouring foundations. Where the adjoining building is Victorian with shallow footings on clay, assumptions matter more than modelling sophistication.

The construction method is part of the answer. Sequenced underpinning in short bays, propping design, a temporary works scheme signed by an engineer, and a monitoring regime with trigger levels and defined actions if a trigger is exceeded. Officers read the monitoring section closely, because it is the part that protects the neighbour after permission is granted.

The Party Wall etc. Act 1996 runs alongside all of this. Excavating within the distances set by the Act, or working on a party structure, requires notice to the adjoining owner and usually the appointment of surveyors. It is a civil matter between owners, it is not dealt with through the planning application, and planning permission does not authorise it.

Audit and independent review

Authorities that take basements seriously commonly appoint an independent auditor to review the assessment, with the cost recovered from the applicant. The auditor checks the screening logic, the adequacy of the site investigation, the ground movement prediction and the construction method, and issues a report the case officer relies on.

Three things make an audit go smoothly. Answer the screening questions explicitly rather than in narrative, so the auditor can follow the logic. Include the raw data, including borehole logs and groundwater readings, rather than summary tables. And where you have departed from the prescribed method, say so and explain why, because an unexplained departure reads as an omission.

Budget time as well as money. An audit round trip plus a response can add weeks to the determination period, and a second round is common where the site investigation was thin.

Conditions imposed on basement works

Expect a cluster of conditions, and design for them rather than arguing about them afterwards.

• A construction method statement approved before works begin, consistent with the assessment.
• A structural monitoring regime with trigger levels, reporting frequency and named actions.
• A construction traffic management plan covering spoil removal, wheel washing and delivery routing.
• Hours of construction, often tighter than the authority's general standard.
• Groundwater and drainage requirements, including where dewatering is proposed.
• Tree protection where root systems fall within the excavation zone of influence.
• Sometimes a requirement that the works are supervised by the engineer who signed the assessment.

That last one is worth noting at the outset, because it affects the fee you agree and the continuity of your team.

Commissioning both without paying twice

One ground investigation should serve both documents and the eventual structural design. Scope it once, with the geotechnical specialist and the structural engineer in the same conversation, and specify the boreholes, the depths, the groundwater monitoring period and the laboratory testing to suit all three uses.

A workable sequence, and this is illustrative rather than prescriptive. Desk study and screening first, which establishes whether the full assessment is needed at all. Ground investigation second, designed against the screening outcome. Structural survey of the host building in parallel, since it needs no ground data. Basement impact assessment third, using the ground data. Structural scheme and temporary works last.

Running the structural survey early has a second benefit. If it shows the building cannot take the proposed alterations without work that makes the scheme uneconomic, you find out before spending on boreholes.

This is general guidance, not legal advice. Basement requirements are set locally and the engineering is site specific, so check the position with your local planning authority and take advice from 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.

Browse all 66 PlanScope report types to see how the structural and ground reports sit alongside the rest of a basement application pack.