Guide
Site investigation stages: phase 1, phase 2 and what each one buys
Updated
Site investigations are phased so that cheap information is bought before expensive information. Understanding the stages is the easiest way to avoid paying for intrusive work the desk study could have descoped.
The phases
- Phase 1: desk study and walkover
- Historical maps, geological and environmental records, a site walkover, and a preliminary risk assessment. Typically £500–£1,200 (typical 2026 quotes). For contaminated land planning conditions this is the stage the council expects first.
- Phase 2: intrusive investigation
- Trial pits, window sampling or boreholes; in-situ and laboratory testing; a factual and usually an interpretative report. Typically £1,500–£4,000 for a single house or self-build plot (typical 2026 quotes). This is the stage that informs foundation design.
- Further phases where needed
- Remediation strategy and verification (contaminated sites) or supplementary investigation (unexpected ground). Scoped per site; costs depend entirely on what phase 2 found.
Who asks for what
- Structural engineers want phase 2 data (soil strength, groundwater, sulfates) to design foundations without guessing conservatively at your expense.
- Building control and warranty providers (for example NHBC) commonly require GI evidence on suspect ground: shrinkable clay with trees, made ground, slopes. Common practice, not a blanket statutory rule.
- Planning authorities attach contaminated land conditions on sites with a suspect history; those follow the phased land contamination guidance on gov.uk.
- Lenders and purchasers of development land sometimes require both phases before money moves.
The standards worth knowing by name
UK ground investigations are planned, carried out and reported under BS 5930 (the code of practice for ground investigations) and Eurocode 7 (geotechnical design). You do not need to read either; you do want them referenced in the proposal, because they are the shorthand for an investigation your engineer, building control and warranty provider will accept without re-doing.
The efficient order for most self-builds: appoint the structural engineer first, let them specify the investigation, then get competing quotes for that scope. Quotes for a named scope are comparable; quotes for "a site investigation" are not.