When Do You Need a Phase 1 Desk Study?

A Phase 1 desk study is the starting point for any contaminated land assessment. It is a desk-based review of a site’s history, geology, hydrogeology, and environmental setting, designed to identify potential sources of contamination and assess whether further investigation is needed. Despite being a relatively straightforward piece of work, it is one of the most frequently misunderstood requirements in the planning process — developers either commission one when it is not needed, or, more commonly, fail to anticipate the requirement and find themselves scrambling to produce one after the planning application has been submitted.

Understanding when a Phase 1 desk study is required, and why, saves time, money, and frustration. This article sets out the common triggers and explains what happens at each stage of the process.

What Is a Phase 1 Desk Study?

A Phase 1 desk study — sometimes referred to as a preliminary risk assessment, a Phase 1 contaminated land assessment, or a Phase 1 environmental site assessment — is a desk-based investigation that identifies potential contamination risks associated with a site. It does not involve any physical investigation of the ground; that comes later, in the Phase 2 site investigation, if the Phase 1 identifies risks that need to be explored further.

The Phase 1 report typically includes:

• Site history: A review of historical maps, aerial photographs, trade directories, and other records to establish what activities have taken place on the site and surrounding area over time. Former industrial uses, fuel storage, waste disposal, mining, and agricultural activities can all leave a legacy of contamination.
• Environmental setting: An assessment of the site’s geology, hydrogeology, and proximity to sensitive receptors such as watercourses, groundwater abstractions, and ecological designations. This establishes the pathways by which contamination could migrate and the receptors that could be affected.
• Regulatory data: A review of Environment Agency records, local authority data, and other regulatory databases to identify known contamination issues, landfill sites, pollution incidents, and controlled waters in the vicinity.
• Conceptual site model (CSM): The central output of the Phase 1 study is the conceptual site model, which identifies the potential sources of contamination, the pathways by which contamination could reach receptors, and the receptors themselves (human health, controlled waters, property, ecosystems). The CSM is the framework that determines whether further investigation is required.
• Recommendations: Based on the CSM, the report recommends whether a Phase 2 intrusive investigation is necessary, and if so, what it should target.

The Phase 1 desk study is carried out in accordance with BS 10175:2011+A2:2017 (Investigation of Potentially Contaminated Sites — Code of Practice) and the Environment Agency’s Land Contamination Risk Management (LCRM) guidance. It is the first stage of the four-stage LCRM process: risk assessment, options appraisal, remediation, and verification.

When Do Local Authorities Require a Phase 1?

The requirement for a Phase 1 desk study is driven by planning policy. In England, the National Planning Policy Framework (NPPF) requires that planning decisions ensure a site is suitable for its proposed use, taking account of ground conditions and any risks arising from land instability and contamination. Planning Practice Guidance (PPG) provides further detail on how this should be applied.

In practice, local planning authorities (LPAs) will require a Phase 1 desk study in the following circumstances:

Brownfield (Previously Developed) Land

This is the most common trigger. Any site that has been previously developed — whether for industrial, commercial, residential, or agricultural use — has the potential for contamination, and most LPAs will require a Phase 1 desk study as a matter of course. The definition of brownfield land is broad: it includes former factories, workshops, petrol stations, garages, farms, and even residential gardens in some cases.

The rationale is straightforward. Previous uses may have involved the storage, handling, or disposal of substances that could contaminate the soil, groundwater, or ground gas environment. A Phase 1 desk study establishes whether those risks exist and whether they need to be investigated further.

Known or Suspected Former Industrial Use

Even if a site does not look obviously industrial today, its historical use matters. A site that is currently a grassed field may have been a gasworks, a tannery, or a chemical works 100 years ago. Historical mapping and trade directory searches within the Phase 1 will identify these former uses.

Certain former uses are considered particularly high-risk for contamination:

• Gasworks and gas holders
• Chemical manufacturing and storage
• Metal working, smelting, and foundries
• Tanneries and leather works
• Petrol stations and fuel depots
• Railway land (including goods yards, engine sheds, and sidings)
• Scrapyards and vehicle dismantlers
• Dry cleaners and laundries (solvent use)
• Agricultural uses involving pesticide/herbicide storage

If any of these uses are identified in the site’s history, a Phase 2 intrusive investigation will almost certainly be required following the Phase 1.

Proximity to a Landfill Site

Sites located within 250 metres of a current or former landfill are routinely flagged for contaminated land assessment. Landfills can generate ground gas (methane and carbon dioxide) which can migrate laterally through permeable strata and accumulate in enclosed spaces — including buildings. This is a serious risk: methane is explosive at concentrations between 5% and 15% in air, and there have been fatalities in the UK caused by landfill gas migration.

The Phase 1 desk study will identify the proximity of landfill sites, their status (active, closed, historical), the types of waste accepted, and the geological conditions that might facilitate or inhibit gas migration. Where a risk is identified, the Phase 2 investigation will typically include ground gas monitoring over a period of several months.

Sensitive End Uses

The proposed end use of the site affects the level of assessment required. Sites being developed for residential use (particularly where gardens are proposed), schools, nurseries, and allotments are classified as more sensitive because the end users — especially children — are more likely to come into direct contact with contaminated soil and have greater vulnerability to health effects.

For sensitive end uses, LPAs are more likely to require a Phase 1 desk study even where the contamination risk appears low, as a precautionary measure.

Local Plan Policies

Many LPAs have adopted local plan policies that require contaminated land assessments for specific areas or site typologies. Some authorities have mapped areas of potential contamination risk based on historical land use data and require Phase 1 assessments for any development within those areas. Others take a blanket approach and require Phase 1 assessments for all developments above a certain size threshold.

Always check the LPA’s validation checklist and local plan policies before submitting a planning application. If a Phase 1 desk study is listed as a validation requirement, the application will not be registered without it.

What Happens If You Skip It?

Failing to submit a Phase 1 desk study when one is required leads to one of two outcomes, neither of which is desirable.

Scenario 1: The application is invalidated. If the LPA’s validation checklist requires a Phase 1 desk study and one is not submitted, the application will be returned as invalid. This is not a planning refusal — it simply means the application cannot be registered and the determination clock does not start. The applicant must submit the missing report before the application can proceed. This typically adds four to eight weeks to the programme, depending on how quickly the Phase 1 can be prepared.

Scenario 2: A planning condition is imposed. If the LPA accepts the application without a Phase 1, it will almost certainly impose a planning condition requiring a contaminated land investigation before development can commence. This is typically a pre-commencement condition, meaning no works of any kind — not even site clearance or enabling works — can begin until the condition is discharged. Discharging a contaminated land condition requires the Phase 1, and potentially a Phase 2 investigation and remediation strategy as well, which can take several months.

In both cases, the delay could have been avoided by commissioning the Phase 1 at the outset. A Phase 1 desk study typically takes two to three weeks to complete and costs a fraction of the overall project budget. The time and cost of dealing with the consequences of not having one are invariably greater.

What Happens After the Phase 1?

The Phase 1 desk study ends with one of three conclusions:

1. No significant contamination risk identified. The conceptual site model shows no plausible source-pathway-receptor linkages, and no further investigation is recommended. This is the best outcome and is common for greenfield sites or sites with a benign history. The LPA will typically accept the Phase 1 as sufficient to discharge the contaminated land condition.

2. Potential risks identified — Phase 2 investigation recommended. The CSM identifies plausible contamination risks that need to be confirmed or ruled out through intrusive investigation. This is the most common outcome for brownfield sites. The Phase 2 investigation involves drilling boreholes, excavating trial pits, and collecting soil, groundwater, and ground gas samples for laboratory analysis.

3. Significant risks identified — Phase 2 investigation required with specific focus. The Phase 1 identifies specific, high-risk contamination sources (such as a former underground fuel tank or a historical waste deposit) that require targeted investigation. The Phase 2 investigation will be designed to characterise these specific risks.

The Phase 1 desk study is the foundation of the entire contaminated land assessment process. A thorough Phase 1 ensures that the Phase 2 investigation is properly targeted, that no significant risks are overlooked, and that the overall assessment process is as efficient as possible.

Practical Advice for Developers

Commission the Phase 1 early. The ideal time to commission a Phase 1 desk study is before the planning application is submitted — or even earlier, at the land acquisition stage. A Phase 1 can identify deal-breaking contamination risks before you commit to purchasing the site. We have seen developers discover former landfills, underground fuel tanks, and significant asbestos contamination through Phase 1 studies that would have been far more expensive to deal with after exchange.

Do not assume greenfield means clean. Agricultural land can be contaminated by pesticides, herbicides, fuel storage, sheep dip, and asbestos in farm buildings. Former military land, mining areas, and sites near historical industry can also carry contamination risks that are not obvious from a site visit.

Provide the Phase 1 consultant with as much information as possible. If you have site investigation data from a geotechnical survey, previous environmental reports, or information about the site’s history that may not be captured in standard database searches, share it. The more information the Phase 1 consultant has, the more focused and useful the report will be.

Read the Phase 1 before submitting it. This may sound obvious, but we regularly see Phase 1 reports submitted to LPAs that contain errors, incomplete information, or recommendations that the developer has not read. The environmental health officer reviewing the report will read it carefully, and inconsistencies or gaps will result in queries that delay the determination.

How We Can Help

Aegaea provides Phase 1 desk studies as part of our environmental consultancy services. We prepare reports that comply with BS 10175 and the LCRM framework, and we design our assessments to give LPAs exactly what they need to discharge planning conditions efficiently. If a Phase 2 is required, we manage the full process from investigation design through to remediation and verification.

If you are unsure whether your site needs a Phase 1, get in touch. We can usually confirm the requirement within 24 hours based on a site address and a brief description of the proposed development.