Basement policy in London and what the assessment covers

A Basement Impact Assessment in London is not a single report. It is a staged process that begins with three screening exercises, narrows to a scoped investigation, produces a ground movement prediction with a damage category for each neighbouring building, and is then audited by an engineer appointed by the borough at your expense. A number of inner and west London boroughs adopted detailed basement policies after a decade of residential excavation, and the assessment process they require has no real counterpart elsewhere in England.

If you are planning a basement under a London house, the policy limits in the borough's local plan will shape the design before any engineer gets involved. Read those first.

Why London boroughs adopted basement policies

The pressure came from the ground up. In areas where extending outwards or upwards was constrained by conservation area policy and plot size, and where values justified the cost, basements became the obvious way to add space. Neighbours experienced two years of excavation, lorries in narrow streets, cracked plaster and arguments about groundwater.

Boroughs responded with policies limiting extent and depth, and with an assessment regime designed to put the engineering evidence in front of the planning officer rather than leaving it to building control and the party wall surveyors. Whether that was the right place to deal with it is arguable. It is the position, and it means a basement application needs engineering input at planning stage.

The London Plan addresses the related matters of flood risk, design and sustainable drainage, and the detailed basement policies sit in borough local plans and supplementary guidance. Download both for your site.

Policy limits on extent, depth and siting

Adopted basement policies differ in detail and converge on a familiar set of limits. Treat the following as the pattern to check against your borough's own wording rather than as the rule.

Permitted development is a separate question. An enlargement of a dwellinghouse below ground can in principle fall within the householder rights, subject to the limits in Class A of Part 1 of Schedule 2 to the General Permitted Development Order, and in practice a basement involving a light well, an external stair or any change to external appearance is usually outside them. Several boroughs have also made Article 4 directions withdrawing the relevant rights. Article 4 directions are the mechanism boroughs use for this, and the way they operate is set out in the note on Article 4 directions and office to residential conversion. Check the direction before assuming any permitted development route is open.

Screening, scoping and the staged assessment

The methodology most boroughs adopt follows a sequence, and skipping a stage is the commonest reason an assessment is sent back.

1. Screening. Three sets of questions are worked through, covering subterranean groundwater flow, surface water flow and flooding, and land stability. Each question is answered yes, no or uncertain, with the evidence cited. A yes or an uncertain answer carries forward.
2. Scoping. The questions carried forward define what has to be investigated, and the scoping section states what work will be done and why, including the extent of ground investigation and any monitoring period.
3. Site investigation and desk study. Boreholes and trial pits, groundwater monitoring over a stated period, geological information including the published mapping and borehole records, historical mapping, records of culverted watercourses, and a condition survey of neighbouring buildings.
4. Impact assessment. A ground movement assessment, a hydrogeological assessment, a surface water and flooding assessment, and an assessment of effects on trees and on neighbouring structures.
5. Non-technical summary, written so that a neighbour and a committee member can understand the conclusions.

The screening stage is where applicants try to save money and where the audit catches them. An assessment that answers no to every screening question on a clay site with a culverted stream two streets away will not survive review.

The structure the assessment follows is set out on the basement impact assessment page, and it is worth laying the report out in that order so the auditor can follow it.

Ground movement and damage categories

This is the technical heart of the submission. A ground movement assessment predicts the vertical and horizontal displacement of the ground and of neighbouring structures caused by the excavation and the permanent works, using the retaining wall design, the construction sequence and the ground parameters from the investigation.

The predicted movements are then translated into a damage category for each affected building, using the established classification that runs from negligible, through very slight and slight, to moderate, severe and very severe. Boroughs typically expect predicted damage to neighbouring property to be no worse than the slight category, and many look for very slight.

Three things make the difference between categories. The stiffness and embedment of the retaining system, usually contiguous or secant piles or a sequential underpinning scheme. The construction sequence, and in particular whether propping is installed before excavation proceeds. And the condition of the neighbouring building, since an unreinforced masonry wall with existing cracking tolerates far less movement than a modern frame.

Design the retaining system for the movement target rather than for economy, because a scheme that predicts moderate damage will not get permission and redesigning it after an audit costs more than specifying it properly at the outset.

The authority's independent audit

Most boroughs with a basement policy require the assessment to be audited by a chartered engineer appointed by the council, with the cost recovered from the applicant.

The audit checks whether the screening was done properly, whether the scoping followed from it, whether the investigation supports the parameters used, whether the ground movement assessment is methodologically sound, and whether the conclusions follow. It produces a report that the planning officer relies on, and it commonly returns a list of queries requiring a response from your engineer.

Budget both the fee and the time. An audit round trip of four to eight weeks is normal, and two rounds are not unusual where the first submission was thin. The way to minimise it is to brief your engineer that the report will be audited, and to ask them to make the evidence chain explicit: each screening answer referenced to a source, each parameter referenced to a borehole log or a published value, each assumption stated.

Appoint an engineer who has done London basements and been audited before. The methodology is specific and experience of it shows in the report.

Construction method statements and monitoring

Planning permission for a basement is normally conditioned on a construction method statement, and on larger schemes it may be secured by a planning obligation.

The statement covers the construction sequence and temporary works, the retaining wall and propping design intent, the dewatering approach if any, a monitoring regime with instrument positions, trigger levels and the actions to be taken at each trigger, hours of working, vehicle routing and the number of lorry movements, wheel washing, hoarding, and the contact arrangements for neighbours. Much of this overlaps with a construction management plan, and some boroughs ask for both, which is one reason to check the full set of report types before scoping the submission.

The monitoring regime is the part neighbours care about most. Set out where the targets and precise levelling points are, how often they will be read, what the amber and red trigger values are, who is notified, and what work stops when a red trigger is reached. A statement that commits to monitoring without trigger values commits to nothing.

The Party Wall etc. Act 1996 runs alongside all of this with its own notices and awards. It is a separate statutory process and planning permission does not substitute for it.

Trees, gardens and drainage

Basements under gardens reduce the volume of soil available to trees and change how water moves through the ground.

Keep the excavation outside root protection areas calculated to the standard method for trees to be retained, and remember that the protection area extends well beyond the canopy for a mature tree. Where the basement sits under a garden, the policy requirement for a minimum depth of soil above the slab exists so that planting can survive, and that depth has to be real soil above drainage layers rather than a nominal dimension on a drawing.

Drainage needs addressing twice. The permanent condition has to show that surface water can still infiltrate or be attenuated across the reduced permeable area, and the temporary condition has to show that construction dewatering will not draw down groundwater beneath neighbouring foundations. Where the site sits over a public sewer, a build-over agreement with the sewerage undertaker will be needed independently of planning.

Where a basement is not acceptable at all

Some sites cannot support a basement, and recognising that early saves a great deal of money.

A site in the functional floodplain. A site beneath or immediately adjacent to a listed building where the works would harm significance and no public benefit outweighs it. A site over or close to underground rail or utility infrastructure where the asset owner's protection requirements cannot be met. A site where the ground conditions and the neighbouring building condition mean that predicted damage cannot be kept within the acceptable categories. A site where the works would materially divert groundwater flow or intercept a culverted watercourse.

An honest early appraisal, using the published geological mapping, the flood maps, the borough policy and a short conversation with a geotechnical engineer, will identify most of these for the cost of a day's work.

Enter your postcode on PlanScope to see which documents a basement proposal on your property is likely to need.

This is general guidance, not legal advice. Basement policies, audit arrangements and permitted development positions differ between boroughs, so check the adopted local plan, any supplementary guidance and any Article 4 direction before relying on anything here. For specialist technical reports, qualified professional input may be required. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.