Construction method statements for basement excavation

A basement construction method statement answers a different question from a basement impact assessment. The impact assessment predicts what the excavation will do to groundwater, ground movement and neighbouring structures. The method statement says how the work will be carried out so those predictions hold. Councils that require both will refuse an application that submits only the first, and the commonest reason a method statement is sent back is that it describes an outcome rather than a sequence.

Write it as a construction narrative with dimensions and dates in it. An engineer should be able to read it and price the temporary works.

What the statement has to describe

Six things, in this order, cover almost every authority's requirement.

The existing ground and groundwater conditions the method assumes, cross-referenced to the ground investigation rather than restated. The form of retaining structure and its temporary support. The sequence of excavation, in numbered stages, with the depth reached at each stage. Spoil volumes, vehicle movements and the routes they take. The monitoring regime, with instruments, locations, frequency and trigger levels. And the protection offered to third parties, covering both physical measures and insurance.

Anything beyond that tends to be padding, with one exception worth adding: a paragraph naming the design standards and the competencies of the people who will produce the temporary works design. A Geotechnical Assessment prepared to Eurocode 7 is the normal foundation for the numbers, and the method statement should say which report and which revision it relies on.

Keep the statement consistent with the Basement Impact Assessment. Where the two disagree on a depth, a level or a wall type, the officer will assume the worse of the two applies.

Sequence of excavation and temporary support

Councils ask for the sequence because the sequence is where neighbouring structures are put at risk. Set it out stage by stage.

For an underpinned scheme beneath an existing house, that means the bay sequence: bay widths, usually around one metre, the hit and miss pattern, the minimum curing period before adjacent bays are opened, and the pinning detail at the interface. State the maximum length of unsupported excavation at any moment. For an embedded retaining wall, give the wall type, whether contiguous, secant or a king post arrangement, the installation method, and the propping arrangement at each level with the depth at which each prop is installed and the depth to which excavation proceeds before the next prop goes in. CIRIA guidance on embedded retaining wall design is the usual reference point.

Then deal with water. Say whether the excavation is expected to be dry, whether sump pumping or a wellpoint system is proposed, where the discharge goes and under what consent. Groundwater control that assumes a discharge to the public sewer without the undertaker's agreement is a programme risk rather than a method.

One honest uncertainty belongs in every basement method statement: made ground is variable, and the sequence should say what happens if the excavation meets conditions that differ from the investigation. Name the person who stops the works and the design review that follows.

Spoil removal and vehicle movements

This is the part neighbours read. It is also the part most often left as a placeholder, and a method statement with an unquantified spoil figure invites a condition requiring the whole document to be resubmitted.

A Construction Traffic Management Plan usually carries this material on a larger site, and on a single dwelling basement it can sit inside the method statement. Either way the highway authority will want the route drawn, not described. Where a crane or hoist is involved, remember that planning permission gives no right to swing a jib over adjoining land, a point covered in more detail under crane oversail and what planning permission does not grant.

Noise, vibration and working hours

Basement works are noisy and the noise is concentrated in the months when neighbours are at home. Assess construction noise and vibration to BS 5228 Parts 1 and 2, and give predicted levels at the nearest facade for the noisiest activities rather than an average across the programme. Breaking out, piling and concrete cutting are the ones that matter.

For vibration, human response is assessed to BS 6472-1 and structural risk is a separate question again. Where the adjoining property is listed or has known structural defects, a condition survey before work starts is worth doing whether or not the council asks.

Propose working hours in the statement rather than waiting to be told. A common pattern is 8am to 6pm on weekdays and 8am to 1pm on Saturdays with no Sunday or bank holiday working, and noisier operations restricted to a narrower window inside that. Offering the restriction is more persuasive than accepting it.

Monitoring regime and trigger levels

A monitoring regime without trigger levels is a data collection exercise. Set out the instruments, the baseline period, the thresholds and the response.

1. Instruments and locations: precise levelling points on the neighbouring structures, crack monitors on existing cracks, inclinometers or tilt meters where a retaining wall is close to a structure, and groundwater standpipes where the assessment relies on a groundwater level.
2. Baseline: readings taken before any works, over a stated period, so seasonal movement is separated from construction movement.
3. Frequency: daily during the most sensitive stages, weekly during the rest, and the point at which monitoring stops.
4. Trigger levels: a first threshold that prompts increased frequency and notification, and a second that stops work pending a design review. Express both in millimetres of movement or millimetres per week, with the predicted movement from the impact assessment alongside.
5. Reporting: who receives the readings, how quickly, and whether the council or the adjoining owner gets them.
6. Contingency: the measures held in reserve, such as additional propping or grouting, and who authorises them.

Trigger levels should relate to the damage categories used in the impact assessment, so a reader can see that the first threshold sits well below the onset of aesthetic damage.

Insurance and third party protection

Confirm that the contractor carries public liability and contract works cover, and state the limit of indemnity. Where the design is by a specialist, professional indemnity cover for the temporary works designer belongs in the same paragraph. Keep the figures current, because a statement quoting expired cover is worse than one quoting none.

Separately, the Party Wall etc. Act 1996 applies to excavation within three metres of a neighbouring structure below the level of its foundations, and within six metres where a line drawn down at 45 degrees would strike the foundation. That is a civil process with appointed surveyors, and it runs alongside planning rather than inside it. Make that distinction in the statement so nobody assumes the planning consent settles it. A structural survey of the host and adjoining properties usually gets commissioned for both processes at once.

Securing the statement by condition

Most authorities condition the method statement rather than approve it with the permission, which gives the applicant a choice. Submitting a full statement with the application takes longer up front and removes an eight week discharge step from the programme. Submitting an outline statement and accepting a condition is faster to validation and slower to start on site.

For a basement, submitting the detail up front is usually the better trade, because the discharge application will attract consultation with the highway authority and sometimes the lead local flood authority, and that consultation happens whether you front-load it or not. Ask the case officer at pre-application which they prefer, then propose the condition wording yourself.

Draft the basement documents for an application on PlanScope by entering the postcode, then have the temporary works sections reviewed by a chartered engineer before submission.

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