01 The two phases, and what each one is for
A Phase 1 is a desk study, also called a Preliminary Risk Assessment (PRA): historical Ordnance Survey maps, land-use history, environmental database searches and a site walkover. No intrusive works happen. It builds a conceptual site model (CSM) on the source-pathway-receptor pollutant-linkage model, and concludes whether plausible contamination risks exist and whether a Phase 2 is needed.
A Phase 2 is an intrusive site investigation: trial pits, boreholes, sampling of soil, soil gas and groundwater, laboratory analysis and ground-gas monitoring. It quantifies the risk and can lead to a remediation strategy.
The governing approach in England is "suitable for use", set by the National Planning Policy Framework (NPPF) for the planning route and by Part 2A of the Environmental Protection Act 1990 for the contaminated-land regime. The point is not a pristine site but a site fit for its intended use.
02 What a Phase 1 desk study actually tells you
A Phase 1 reconstructs the history of the site and its surroundings: historical land uses from Ordnance Survey historical maps, past industrial activity, infilled ground (ponds, pits, quarries), former tanks, mining legacy and nearby landfills, plus environmental data such as licensed discharges and recorded pollution incidents.
From that it identifies potential contaminant sources, pathways and receptors - site users, neighbours, controlled waters such as aquifers, buildings and buried services, and ecology - and sets out a preliminary conceptual site model.
The crucial limit: a Phase 1 establishes plausibility, not fact. It cannot confirm whether contamination is actually present, or at what concentration. Its key output is whether plausible pollutant linkages exist and whether intrusive investigation is recommended. Read it for that conclusion first, then check whether the work it recommends has actually been carried out.
03 What a Phase 2 site investigation adds
A Phase 2 puts numbers on the risk. Intrusive sampling and laboratory testing are compared against generic assessment criteria for human health - for example the LQM/CIEH Suitable 4 Use Levels, or the Environment Agency Category 4 Screening Levels - and against water-quality standards for controlled waters.
Ground-gas monitoring classifies the site into a "characteristic situation" (CS1 to CS6, after Wilson and Card, used in BS 8485), which sets the level of gas protection a building needs. A higher characteristic situation means more protection and more cost.
The output is a quantified risk and, where needed, a remediation strategy. Watch the end-use sensitivity: criteria for a residential end use are far stricter than for a commercial or industrial one, so the same ground can pass for one scheme and fail for another.
04 Ground gas, asbestos and the things that cost real money
A handful of findings drive remediation cost: asbestos in made ground; ground gas (methane and carbon dioxide from made ground or a nearby landfill) requiring membranes and venting; hydrocarbons from former fuel tanks; and heavy metals.
Each converts into a real cost line: gas protection systems, validation, soil treatment or disposal of contaminated material to licensed landfill (which carries landfill tax), and import of clean cover. None of these is a footnote - they are budget items that belong in the bid.
For an industrial or logistics end use the human-health bar is lower than residential, but controlled waters - an aquifer or a Source Protection Zone - can still drive significant remediation regardless of the building use. A "less sensitive" end use does not make a sensitive water receptor go away.
05 Where the unpriced risk usually hides
The most common gap is the simplest to miss: only a Phase 1 in the data room, recommending a Phase 2 that has not been carried out. That is unquantified risk - treat it as cost plus programme risk, not a clean site.
Other gaps recur. Reports written against a different end use than yours, so the conclusions may not transfer. A CSM that predates the current scheme and no longer matches the layout or the receptors. Remediation done but no validation or verification report to prove it worked. And reliance on an old "no further action" letter that may not reflect today's criteria or your intended use. Each is a question to ask before you price, not after.
06 Red flags to catch before you bid
Some findings should change your number. Watch for: a Phase 2 recommended but absent; asbestos identified; ground gas at characteristic situation CS2 or above; former underground tanks; infilled ponds, pits or quarries; a historic or operational landfill close by; controlled waters (an aquifer or Source Protection Zone) as a sensitive receptor; a remediation strategy with no cost attached; and a "suitable for use" conclusion tied to a different end use than the one you intend.
None of these is automatically fatal, but each turns a clean site into a conditional one that needs a number against it before you bid. The job is to convert every caveat into a cost, a programme risk, or a reason to walk - and to make sure none of them are buried in an appendix.
07 Frequently asked questions
What is the difference between a Phase 1 and a Phase 2 contamination report?
A Phase 1 is a desk-based preliminary risk assessment: historical maps, land-use history and a walkover that identify whether plausible contamination risks exist and whether intrusive work is needed. A Phase 2 is an intrusive site investigation: boreholes, trial pits and sampling of soil, gas and groundwater, tested in a laboratory, that quantify the risk and inform any remediation. Phase 1 flags plausibility; Phase 2 establishes fact.
Do I need a Phase 2 site investigation?
If the Phase 1 identifies plausible pollutant linkages - for example former industrial use, made ground, tanks or a nearby landfill - it will normally recommend a Phase 2, and a local authority will usually condition one before development. If a Phase 1 has been done and recommends intrusive work that has not yet happened, the contamination risk on that site is still unquantified.
What is a conceptual site model?
A conceptual site model, or CSM, is the assessor's working picture of how contamination could cause harm on a site, built on the source-pathway-receptor model: a source of contamination, a pathway to reach something, and a receptor that could be harmed such as people, controlled waters or buildings. If any link in that chain is broken there is no viable pollutant linkage. The CSM drives what, if anything, needs investigating or remediating.
Who is liable for contaminated land after buying a site?
Under Part 2A of the Environmental Protection Act 1990 the original polluter is liable first, but where they cannot be found the liability can fall to the current owner or occupier, so contamination can become the buyer's problem. Purchases are generally on a caveat emptor basis, which is why pre-bid review of the contamination reports, and appropriate warranties or indemnities, matter.
What are the stages of a site investigation?
Site investigation runs in stages: Phase 1 is the desk study and walkover, a preliminary risk assessment; Phase 2 is the intrusive investigation that samples soil, gas and groundwater; and where contamination is confirmed, later stages cover the remediation strategy and a verification (or validation) report proving the work was done. These UK phases map to what the US calls Phase I and Phase II Environmental Site Assessments. For a bid, the key question is which stage the data room has actually reached.
How long does a Phase 1 desk study take?
A Phase 1 desk study is usually quick once commissioned, often a week or two, because it is desk-based and needs no site works or laboratory time. The longer, more uncertain part is what it triggers: if it recommends a Phase 2 intrusive investigation, that adds fieldwork, laboratory analysis and reporting, and any ground-gas monitoring runs over several weeks.
How much does a Phase 2 site investigation cost?
It depends on the size of the site and how many boreholes, trial pits and samples the conceptual site model calls for, plus the laboratory suite and any ground-gas monitoring. For a bid the investigation fee is rarely the point: what matters is what it finds, because asbestos, ground gas or contamination can carry remediation costs many times the cost of the survey itself.
Turn the contamination reports into a position, in hours
Plumb reads the Phase 1 and Phase 2 reports in a deal's data room alongside the planning, geotechnical and title material, pulls out the conceptual site model, the contaminants found, the ground-gas situation and the remediation strategy, and flags where a Phase 2 is still outstanding or the risk is unquantified - every point cited back to the page it came from, so your read stands up in committee.