What a basement impact assessment condition requires

A basement impact assessment is the document most inner-London boroughs require before they will approve a new or extended basement. It answers one question in three stages: could this excavation harm the ground, the groundwater, the surface water regime or the neighbouring buildings, and if so what does the design do about it. This page sets out what the assessment covers, what the condition attached at approval then asks for, and how often a drainage condition arrives on the same notice, from the London decision notices we read every morning.

What does a basement impact assessment cover?

The structure is common to the boroughs that require one, even where the local name differs. Screening asks a fixed set of questions about the site: is it on a slope, near a watercourse, within a surface water flow path, in an area of known groundwater, next to a listed or already-undermined building. A "yes" to any of them takes the scheme into scoping, which defines the site investigation and analysis needed to answer it. The impact assessment then reports the findings under four heads: land stability, hydrogeology (groundwater flow and the effect of an obstruction in it), hydrology (surface water and flood risk), and the structural stability of adjoining property, with the construction method and monitoring proposed to control each.

The assessment is signed by a chartered engineer, and several boroughs also require an independent check of it. Camden, Kensington and Chelsea, Westminster and Islington each publish basement policy or guidance that sets the local scope; the borough's current validation list is the authoritative statement of what must be submitted, and it changes, so check it for the site rather than relying on a template from the last job.

What does the condition attached at approval require?

The approval turns the assessment's promises into obligations. In practice the condition asks for some or all of the following before work starts, which is why so many basement conditions are pre-commencement:

• A construction method statement or structural methodology statement: sequence, temporary works, propping, underpinning and the design responsibility for each, consistent with the assessment.
• Ground movement and monitoring: predicted movement of neighbouring structures, trigger levels, and the monitoring regime that proves the prediction held.
• Groundwater and drainage measures: how the excavation deals with groundwater, how the lightwells and the retained footprint drain, pumps and non-return valves, and where the surface water goes now that the garden is hard.
• Verification: confirmation, often by the same chartered engineer, that the works were built as the statement described. This is the obligation that outlives practical completion.

How often does a drainage condition arrive with it?

The share of basement approvals carrying a drainage condition appears here once the sample clears our publication floor.

The practical consequence for the engineer: the drainage submission and the method statement are usually discharged together, drawn by the same team, and it is the drainage detail that gets refused for insufficient evidence, not the structure. The refusal grounds are on their own page; prepare both submissions with the planning application rather than after approval, and the pre-commencement condition stops being a programme risk.

Where does this come from?

The description of the assessment is drawn from the published basement policies and guidance of the London boroughs that require one, and the figures from planning decision notices we read every morning, with basement schemes picked out by their proposal description. A rate is published only when its own sample clears a floor of 20. Only aggregate figures appear here: never a scheme address, an application reference or condition wording. See the methodology page or the borough table for basements.