Contamination Phase 1 or Phase 2: what triggers the dig
Phase 1 asks whether a source, a pathway and a receptor connect. If they plausibly do, Phase 2 goes into the ground. The trigger decides a five-figure cost.
Phase 1 versus Phase 2 contamination survey is a sequence rather than a choice. Phase 1 is a desk study and walkover that builds a conceptual model of the site and asks one question: is there a plausible link between a source of contamination, a pathway and a receptor. If the answer is no, and the desk study is thorough enough to be believed, the work stops there. If the answer is yes, Phase 2 goes into the ground to find out whether the linkage is real.
For a self-builder, that single question decides a five-figure difference in cost and several weeks of programme, so it pays to understand what makes a Phase 1 conclusive.
What a Phase 1 desk study establishes
A preliminary risk assessment, to use the term the Environment Agency's land contamination risk management guidance uses, assembles the site's history and physical setting from existing records and a site visit.
The core sources are historic Ordnance Survey mapping through every available edition, the published geology and any borehole records, landfill and waste site registers, environmental permits, the local authority's contaminated land records, radon potential, coal mining and mineral records, groundwater vulnerability and source protection zones, and aerial photography. Then a walkover recording made ground, staining, odour, tanks, asbestos-containing materials, vegetation dieback and anything the maps missed. The registers and what a commercial search does and does not cover are set out under contaminated land registers and what a search shows.
Two outputs matter. A site history narrative with dates and sources, and a conceptual site model. Everything else in the report is supporting material.
Phase 1 can reach three conclusions: no plausible pollutant linkage and no further work; plausible linkages requiring intrusive investigation; or insufficient information, which is a polite way of saying the desk study needs more work before it can conclude either way.
The source, pathway and receptor model
A pollutant linkage needs all three elements present. Remove any one and there is no risk to manage, which is why the model is the pivot of the whole process. The detail of how linkages are built and tested is covered under the conceptual site model and how pollutant linkages work.
For a single plot or a small residential scheme, the model usually looks like this.
| Element | Typical examples on a small site |
|---|---|
| Sources | Made ground, a former filling station, an old orchard, a demolished workshop, an oil tank, ash and clinker fill |
| Pathways | Ingestion of soil, dermal contact, inhalation of dust and vapour, ground gas ingress, plant uptake, leaching to groundwater |
| Receptors | Future residents including children, gardeners growing produce, construction workers, controlled waters, buried concrete and services |
The residential end use is what makes small sites sensitive. A gravelled commercial yard with the same soil chemistry as a proposed garden presents a fraction of the risk, because there is no soil contact and nobody grows vegetables in it. Change the use to housing with private gardens and the same made ground becomes the reason for a clean cover system.
When a plausible linkage triggers intrusive work
Plausible means credible on the available evidence rather than proven. The threshold is deliberately low, because the point of Phase 1 is to screen.
In practice these findings almost always lead to Phase 2 on a residential scheme:
- Made ground of unknown origin across the site, which is the commonest trigger of all and covers most brownfield plots and many garden plots in old settlements.
- A former industrial, commercial or agricultural use on or immediately adjacent to the site, including workshops, garages, tanneries, gasworks, scrapyards, sheep dips and farmyards.
- Infilled ground: a quarry, pond, watercourse, pit or landfill within or close to the boundary.
- A residential end use with private gardens over any of the above.
- A site over a principal aquifer or in a source protection zone, where controlled waters are the receptor.
Where none of those applies, a well-evidenced Phase 1 concluding no further work is accepted routinely. The failures are usually thin desk studies: two map editions instead of the full sequence, no walkover, or a conclusion that does not follow from the site history.
The NPPF places responsibility for securing safe development on the developer or landowner, and requires the site to be suitable for its proposed use. That is the test the council is applying, so a Phase 1 that says the site is probably fine will not discharge it.
Sampling strategy and analysis suites
Phase 2 is an intrusive investigation carried out in accordance with BS 10175, and its design follows the conceptual model rather than a standard template.
The strategy combines targeted locations on the sources the Phase 1 identified with a systematic spread across the rest of the site, so that both known hotspots and unknown contamination are picked up. On a small plot that might be four to six trial pits and a couple of window sample holes. On a hectare of former industrial land it is a much larger exercise. Exploratory holes should reach natural ground, and samples should be taken at regular intervals and at every change of strata, with the logs recorded to the standard.
The analysis suite comes from the model too. A typical residential suite covers a metals and metalloids range, polycyclic aromatic hydrocarbons, total petroleum hydrocarbons with aliphatic and aromatic banding, asbestos screening and identification, pH and sulfate for concrete design, and speciated phenols. Where solvents, dry cleaning or fuel are suspected, add volatile and semi-volatile organics and consider soil vapour. Where controlled waters are a receptor, add groundwater samples.
Results are compared against published generic assessment criteria for the proposed land use. Exceedances do not automatically mean remediation. They mean the next step is either a site-specific quantitative risk assessment using parameters for the actual scheme, or a decision to design the exceedance out with a cover system. The structure of that reporting is set out under sampling and risk sections in a Phase 2 report.
Ground gas and when monitoring is needed
Ground gas is the part of the process that adds the most programme time, because it cannot be measured in a single visit.
Monitoring is needed wherever the model identifies a gas source: made ground with organic content, infilled ground, peat or alluvium, a landfill within the influencing distance, or mine workings. Installations are put in during the intrusive phase and then visited repeatedly, with readings of methane, carbon dioxide, oxygen, flow rate and atmospheric pressure at each visit.
Industry guidance expects several rounds spread over a period of months rather than weeks, including visits during falling atmospheric pressure when gas emission is highest. A minimum of six visits over about three months is a common starting point for a low-risk site, and a landfill-influenced site needs considerably more.
The output is a gas screening value combining concentration and flow, which places the site in a characteristic situation and determines the protection measures the building needs, from nothing at all through to a fully designed and verified membrane and ventilation system. Build the monitoring period into the programme at the start. Discovering in month four that you need three more months of readings before foundations can be designed is the classic self-build delay.
Remediation strategy and verification
Where a risk needs managing, the remediation strategy sets out the options considered, the chosen approach, the design and the verification plan. On small residential sites the usual answers are a clean cover system in gardens and soft landscaping, hard cover in driveways and paths, removal of hotspots, gas protection measures in the building, and specification of sulfate-resisting concrete.
Verification is the part that people underestimate. It means evidence, collected as the work happens: chemical testing certificates for every batch of imported soil, photographs and levels showing the cover thickness and the marker layer, integrity testing of gas membranes before they are covered, and as-built records. A verification report assembled from memory after the event will not be accepted.
Keep the verification plan and the remediation strategy as separate documents. Councils approve the strategy before work starts and the verification report after it finishes, and the sequence in the condition usually requires exactly that.
Discharging the contamination condition
Most contamination conditions have four parts and they have to be discharged in order.
Part one requires the site characterisation, which is the Phase 1 and, where triggered, the Phase 2 with the risk assessment. It is normally a pre-commencement requirement, so it has to be approved before anything happens on site.
Part two requires the remediation scheme, based on the approved characterisation.
Part three requires implementation and a verification report, usually before occupation.
Part four requires unexpected contamination found during the works to be reported to the council and dealt with under an agreed scheme. That part stays live for the whole build.
Submit each part as a separate application to discharge conditions, in sequence, and do not try to bundle them. And read the condition wording rather than assuming the standard form, because authorities vary in whether the verification report is required before occupation or before the building is watertight.
Land contamination reports have to be prepared by suitably qualified and experienced persons, and the local planning authority will expect the author's competence to be stated. A contaminated land assessment drafted by PlanScope is a structured starting point for that professional rather than a substitute for them. The cost side of the exercise is covered under budgeting a contaminated land investigation.
Enter your postcode on PlanScope to see which reports your site is likely to need.
PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required.
Frequently asked questions
What is the difference between a Phase 1 and a Phase 2 contamination report?
Phase 1 is a desk study and walkover that builds a conceptual site model and tests whether a plausible pollutant linkage exists. Phase 2 is an intrusive investigation under BS 10175, with exploratory holes, soil and water sampling, laboratory analysis and, where needed, ground gas monitoring, to establish whether that linkage is real.
Can a Phase 1 report conclude that no further work is needed?
Yes, and councils accept it routinely where the desk study is thorough. The weakness is usually evidential: two map editions instead of the full sequence, no walkover, or a conclusion that does not follow from the recorded site history. Made ground of unknown origin under proposed gardens almost always triggers Phase 2.
What triggers intrusive investigation?
Made ground of unknown origin, a former industrial, commercial or agricultural use on or next to the site, infilled ground such as a quarry or pond, a residential end use with private gardens over any of those, or a site over a principal aquifer or in a source protection zone.
How long does ground gas monitoring take?
Months rather than weeks. Installations go in during the intrusive phase and are then visited repeatedly, including during falling atmospheric pressure. Around six visits over three months is a common starting point for a low-risk site, and a landfill-influenced site needs considerably more. Build it into the programme early.
How is a contamination condition discharged?
In sequence: site characterisation first, usually before development begins, then the remediation scheme, then implementation with a verification report before occupation, with a standing requirement to report unexpected contamination. Submit each part as a separate application to discharge conditions rather than bundling them.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.