Which reports a basement under a London terrace needs

London basement planning reports come in a fixed order, and the one that governs everything else is the basement impact assessment. Boroughs with a specific basement policy generally require it to be prepared by a chartered engineer or chartered geologist, to follow a screening, scoping, investigation and assessment sequence, and to be independently audited at the applicant's expense. Everything else in the submission, from the tree report to the construction plan, hangs off its findings.

Before commissioning anything, read your borough's adopted basement policy. The requirements differ, and a scheme that is routine in one borough is outside policy in the next.

What borough basement policies typically control

There is no national basement policy. The controls sit in local plans and supplementary guidance, and while the drafting differs the common themes recur.

Policies frequently limit excavation to a single storey below ground, restrict the footprint to a proportion of the garden or of the footprint of the original building, require a minimum depth of soil above the structure where it extends under a garden, restrict or prohibit basements beneath listed buildings, and require light wells and vents to be designed so they do not harm the street scene. Some set out where basements will not be acceptable at all, such as under highways or in defined flood or groundwater areas.

Read the policy wording closely. A cap expressed as a proportion of the garden is measured differently from one expressed against the original building footprint, and schemes are refused on that distinction regularly.

The basement impact assessment and its structure

A Basement Impact Assessment is not a single report. It is a staged process, and boroughs expect the stages to be presented separately so an auditor can follow them.

1. Screening, which asks a series of yes or no questions about the site across three headings: subterranean water flow, surface water and flooding, and land stability.
2. Scoping, which takes every question answered yes or uncertain at screening and sets out what investigation is needed.
3. Site investigation and study, delivering the boreholes, trial pits, groundwater monitoring and structural inspection the scoping identified.
4. Impact assessment, predicting the effects on groundwater, drainage, slope stability and neighbouring structures, with mitigation.
5. Non-technical summary and conclusions, written so a neighbour can follow them.

A common and expensive mistake is to answer screening questions with a negative to avoid investigation. The auditor challenges it, the application stalls, and the investigation happens anyway at a later and more costly point.

Groundwater monitoring is the item that most often lengthens the programme, because a meaningful record needs readings across months rather than a single visit.

Ground movement and the damage category test

Neighbours object to basements because they fear cracks, and the assessment answers that fear numerically.

A ground movement assessment predicts settlement and horizontal strain at neighbouring structures from the excavation and the construction sequence, and expresses the result using the standard categories of building damage running from negligible upwards. London basement policies commonly require predicted damage to adjoining properties to fall no higher than the category described as very slight.

A Geotechnical Assessment supplies the ground model behind all of it. On a terrace the party wall position matters as much as the geology, because underpinning a shared wall in the wrong sequence is what causes movement.

The Party Wall etc. Act 1996 runs alongside planning and is a civil regime, not a planning one. Notices, surveyors and awards under that Act neither grant nor withhold planning permission, and a planning consent does not authorise works to a party wall. Both are needed.

Trees, root protection and soil depth

A basement extending under a garden can remove the rooting volume that keeps a mature tree alive, and it can change soil moisture across the whole plot.

A Tree Survey to BS 5837:2012 should identify every tree on the site and every tree on adjoining land whose root protection area reaches into it, with the root protection areas plotted against the proposed excavation. Where a tree is protected by a preservation order or stands in a conservation area, the separate consent regime applies.

Soil depth above the structure is the other issue. Where policy requires a minimum depth of soil over a basement under a garden, that depth supports planting and drainage, and a scheme that leaves a thin layer over a slab will not deliver the landscaping shown on the drawings. Show the build-up in section.

Water: groundwater, surface water and flood risk

Three separate questions, often conflated.

Groundwater flow is the subterranean question: whether the structure will intercept or dam a flow path, raising levels on one side and lowering them on the other. That is what screening is testing and what monitoring answers.

Surface water is about the loss of permeable ground and the increased run-off from a hard structure beneath a garden. The drainage design has to deal with it, and a basement under a garden reduces the infiltration options considerably.

Flood risk is a policy matter. Sleeping accommodation below ground is treated as highly vulnerable in national flood risk terms, and in higher-risk zones that classification can rule out a bedroom in a basement regardless of engineering. A Flood Risk Assessment has to address surface water and groundwater flooding as well as rivers, and should set out safe access and egress.

Construction management and the neighbours

Construction effects generate more objections than the finished basement ever will, and boroughs respond with detailed requirements.

Expect to submit a construction management plan covering the excavation method and sequence, spoil volumes and lorry movements, the routing and holding of vehicles, hours of work, dust and noise control, vibration monitoring with trigger levels, and a named contact for neighbours. Many boroughs operate their own construction code and require a plan approved before work starts, sometimes with a financial bond.

Be realistic about lorry numbers. A basement under a terrace generates a substantial volume of spoil on a street that may have no off-street standing at all, and an application that understates the movements loses credibility on everything else.

Where policy caps the scheme

Some schemes cannot be made acceptable. A second storey of basement where policy permits one, a footprint beyond the permitted proportion of the garden, a basement under a listed building where policy resists it, or a bedroom below ground in a high flood risk area are not engineering problems. No quality of assessment overcomes an adopted policy.

Test the scheme against the policy first, then commission the investigation. That order saves more money on basement projects than any other decision.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. A basement impact assessment and ground movement analysis require suitably qualified engineers, and 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.

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