Services | Basement Impact Assessments

Croft has extensive experience working with both clients and professions on Basement Impact Assessment projects of varying scale. For information on other Boroughs, please speak to one of the team today on 020 8684 4744.

Basement Impact Assessments

A Basement Impact Assessment is a comprehensive technical report submitted alongside a planning application to demonstrate that a proposed basement will not cause harm to the structural stability of the property or neighbouring buildings, nor worsen local groundwater, surface water drainage, or cause subsidence or flooding.

Site and Ground Conditions

Geological context: Identify the site's geology (e.g., London Clay, Lambeth Group, gravels).
Made ground: Presence of fill, rubble, or anthropogenic materials.
Historical land use: Past land development or industrial use that may influence soil quality.
Borehole logs and trial pits: Ground investigation results with soil profiles and Standard Penetration Tests (SPT).

Hydrogeology and Groundwater

Groundwater table level: Determine its depth and seasonal fluctuation.
Permeability: How well water flows through the soil (important for flood and drainage assessments).
Aquifer vulnerability: If construction might affect underlying aquifers.
Groundwater flow direction: To assess if the basement could block or divert water.
Potential for perched water tables in granular layers above impermeable clay.

Flood Risk and Drainage

Surface water drainage: How water will be managed during and after construction.
Sustainable Drainage Systems (SuDS): Proposals to limit runoff and recharge groundwater.
Flood zones: Proximity to rivers, sewers, or surface water flood paths.

Soil and Slope Stability

Bearing capacity: Can the soil safely support the basement structure?
Settlement risks: Risk to adjacent structures from movement or dewatering.
Slope angle and stability (especially for sites on a gradient).

Impact on Trees and Root Protection Areas

Tree roots can destabilise retaining walls and vice versa.
The assessment should evaluate the likely extent of root systems and how they will be protected or worked around.

Impact on Nearby Structures (Structural-Stability Considerations)

Geotechnical zone of influence: How far excavation effects may reach.
Party Wall implications: Possible effect on shared structures.
Vibration and settlement risk from piling or excavation.
Require reference to BS 6031 (Earthworks) and CIRIA C760 (Control of Groundwater).

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The Hydrogeology

The councils want to confirm that when you excavate a sizeable hole that its affects have been considered. A sensible approach really. Different boroughs have different criteria and levels of information, but they mainly want to know:

Will the basement cause flooding?
Will the basement affect water flows around the area?
Will the basement affect neighbouring properties?
Will the basement affect any historic structures?
Will it affect the local highway, or infrastructure (Rail or underground)?
Has a trained professional looked at the project and considered the pitfalls?
Ensuring the basement is inspected by a trained professional during the works

Ground Movement Assessment

‍Any excavation in the ground distributes stresses. The ground movement assessment looks to gauge expected movement to strains in local building. This is then given a Burland Scale reference. In line with the CIRIA C820 report Croft have developed their own software for movement of small basements in clay.

Hydrogeology

‍The study of water flow through the ground is Hydrogeology. Croft look at how water flow may be affect around the new basement. 1 Basement is of little effect; Constructing all the basements in a terrace will create a dam forcing water to flow into different areas.