Ground Investigation and Soil Surveys for Extensions and New Builds

• For the majority of London householder extension projects, the structural engineer will specify foundations based on:
• •Knowledge of the local geology (London Clay is well-characterised, for example)
• •Visual inspection of the site
• •Information from NHBC Chapter 4.2 (tree proximity) or local authority records
• •Reference to adjacent properties and their foundation performance

• This approach is appropriate where:
• •The geology is well-understood and uniform
• •There are no unusual site features or history
• •The project is relatively straightforward (single-storey rear extension, loft conversion)

**When a ground investigation is specifically required**:

*Made ground*: In large areas of London, the natural ground has been built on, infilled, or engineered multiple times. Made ground is unpredictable in composition and compressibility — virgin Natural London Clay at 3m depth may overlie Victorian rubbish fill, ash, or organic material. Made ground cannot be relied upon for conventional strip or pad foundations. Areas with high made ground risk include former industrial sites, riverside areas, bomb sites (WW2 damage was often levelled rather than excavated), and areas of former market gardens or nurseries.

*Contaminated land*: Former garages, petrol stations, dry cleaners, paint works, tanneries, and many other industrial uses left ground contamination. Phase 1 Desk Study (reviewing historical maps, records, and site history) identifies whether contamination risk exists; Phase 2 intrusive investigation (soil sampling and laboratory testing) quantifies it. Planning authorities in London routinely require Phase 2 investigations for sites with identified contamination risk before planning consent is implemented.

*Unusual water conditions*: High groundwater tables, artesian pressure, or proximity to culverted watercourses can affect foundation design. A ground investigation can confirm groundwater depth and any artesian pressure risk.

*Basement and below-ground structures*: Any basement or semi-basement extension requires ground investigation to confirm the bearing strata, groundwater table, and any contamination — the investment in investigation is proportionate to the cost of getting the waterproofing and foundation design wrong.

*Proximity to railways and underground lines (London-specific)*: Many London sites are within the influence zone of Underground, Crossrail, Overground, or Network Rail infrastructure. TfL and Network Rail have specific requirements for investigations and structural design in their consultation zones — typically triggered by any excavation within 15m of a railway or tunnel structure.

*Tree influence with complex root system*: Where multiple large trees of different species are present or removed, a ground investigation can confirm the depth of desiccated clay and inform the heave and shrinkage design.

Ground investigation is typically structured in two phases:

• **Phase 1 — Desk Study**:
• •Review of historical maps (Ordnance Survey from 1890s to present)
• •Environmental database search (past industrial uses, registered contamination, landfills, mining history)
• •Review of geological maps and borehole records from BGS (British Geological Survey)
• •Review of planning records and any previous site investigations
• •Output: a Phase 1 Preliminary Risk Assessment (PRA) — categorises risk as low, medium, or high and makes recommendations for Phase 2 if required
• •Cost: £600–£2,000 for a domestic site; £2,000–£8,000 for a larger commercial site

**Phase 2 — Intrusive Investigation**: Physical exploration of the ground using one or more of:

• *Trial pits*:
• •Excavated by a small tracked excavator, typically 1.5–5m deep
• •Allows direct visual inspection of soil profile, identification of made ground or fill, and collection of samples for laboratory testing
• •Multiple pits across the site give a picture of variability
• •Cost: £200–£500 per pit (excavation and logging)

• *Borehole drilling*:
• •Hollow stem auger or rotary coring equipment advances to depths of 10–30m
• •Appropriate for deep investigations, contamination sampling at depth, or where the investigation depth exceeds the reach of trial pits
• •Continuous sampling every 1m gives a vertical profile of soil conditions
• •Can also include in-situ testing: Standard Penetration Test (SPT) for granular soils, undrained shear strength testing for clays
• •Cost: £800–£2,500 per borehole (dependent on depth and number of samples)

• *Dynamic probing / CPT (Cone Penetration Testing)*:
• •Less common for domestic sites; more common in geotechnical work for larger development
• •Fast and efficient for characterising soil strength profiles without bringing samples to surface

• **Laboratory testing**:
• Samples from trial pits or boreholes are sent to a UKAS-accredited laboratory for relevant testing:
• •Particle size distribution (soil classification)
• •Atterberg limits (plasticity of clays — critical for shrink-swell assessment)
• •Organic content (identifies peaty or made ground)
• •Chemical testing (pH, sulphates, chlorides, heavy metals) — for contamination assessment
• •Asbestos in soil (particularly relevant for former demolition sites)

• **Output of Phase 2**:
• •Ground Investigation Report: soil classification, contamination assessment, recommendations for foundation design, remediation requirements (if contamination is found)
• •The structural engineer uses this report to design the foundations with confidence
• •The planning authority (if contamination was a planning condition) reviews the report and issues a response confirming whether remediation or further investigation is required

**Typical investigation costs for a London domestic extension (2025)**:

• •Phase 1 Desk Study: £700–£2,000
• •Phase 2 with 3–4 trial pits + basic laboratory testing: £2,500–£6,000
• •Phase 2 with 2 boreholes to 15m + full laboratory testing: £4,000–£9,000
• •Full investigation for a basement extension (2 boreholes, contamination suite, groundwater monitoring): £6,000–£15,000

For a simple rear extension on a straightforward London Clay site with no contamination risk, a ground investigation is often not required — the structural engineer's assumptions are appropriate and the investigation cost is disproportionate. Discuss with your structural engineer at the outset whether a site-specific investigation is warranted.

• **Timescale**:
• •Phase 1 Desk Study: 2–4 weeks
• •Phase 2 investigation (mobilisation + fieldwork + laboratory + report): 4–10 weeks total
• •For planning-related investigations (required as planning condition): allow for the investigation to be completed and submitted before implementing the planning consent

**Interpreting the results — what matters most**:

*For foundation design*: The key output is the bearing capacity of the founding strata at the proposed foundation depth — expressed as an allowable bearing pressure (kN/m²). London Clay at 1.5m depth: typically 75–100 kN/m². London Clay at 2.5m depth: typically 100–150 kN/m². The structural engineer uses this to size pad footings or select pile dimensions.

*For contamination*: Results are compared to screening assessment criteria (Environment Agency's S4ULand tool / Generic Assessment Criteria for human health risk). Values above the screening criteria require further risk assessment or remediation. Common remediation options: remove and dispose of contaminated material as controlled waste; cap in place with clean material; in-situ treatment (limited in domestic context).

*Unexpected findings*: Ground investigations sometimes find features not anticipated by the desk study — old buried foundations, voids, or pockets of organic fill. These findings change the foundation design and potentially the project cost and programme — but it is always better to find them during investigation than during construction.