Ground Gas Risk in Kensington & Chelsea: What Developers Need to Know
Basements, extensions, and refurbishments are common features of development across the Royal Borough of Kensington and Chelsea (RBKC). But below ground, risks from hazardous gases and vapours often trigger strict planning conditions.
Whether you're planning a rear extension, a new basement, or a significant refurbishment, RBKC planners expect careful consideration of ground gas and vapour risks, particularly where Made Ground, historical land uses, or potential contamination sources exist.
This guide explains when investigations are needed, what the local planning guidance (CLC1–CLC4) says, and how Clear Environmental can help you comply smoothly and commercially.
Why Ground Gas Matters in Kensington and Chelsea
Ground gases and vapours can migrate into buildings through cracks, service penetrations, or porous materials. In enclosed spaces, such as basements, they may accumulate and cause health risks or even explosion hazards. The main gases of concern include:
- Methane (CH₄) – from organic decay or hydrocarbons
- Carbon dioxide (CO₂) – from natural respiration or carbonate reactions
- Volatile Organic Compounds (VOCs) – from historic fuels or solvents
- Radon – naturally occurring, found in some geological formations
RBKC is underlain by a mix of Made Ground, London Clay, River Terrace Deposits and Alluvium. Made Ground, often 2–10m thick, is particularly important as it may contain demolition waste, domestic refuse, and industrial fill. It acts as both a source and potential pathway for ground gas and vapours.
Understanding RBKC's Planning Guidance (CLC1–CLC4)
RBKC has published detailed planning guidance to help developers address ground gas risks:
- CLC1: Applies to all development. Sets out the overall policy framework for contaminated land planning.
- CLC2: Applies to small refurbishments. Requires soil testing in gardens and soft landscaping.
- CLC3: Applies to basements. Involves risk assessment and ground gas surveys.
- CLC4: Applies to extensions. Focuses on vapour/gas protection and verification guidance.
If your site involves ground disturbance and is located near potential contamination, it’s likely your planning approval will reference one or more of these.
When Is a Ground Gas Investigation Required?
A Ground Gas and Vapour Strategy (GGVS) is typically required if:
- The development includes a basement or below-ground structure
- The site lies within 250m of a landfill or historical industrial activity
- The Phase 1 Desk Study identifies potential pollutant linkages
- The proposed formation level lies within permeable strata (e.g., River Terrace Deposits)
RBKC encourages applicants to submit a Phase 1 Ground Gas Risk Assessment early in the process. This screens the site based on historic use, Made Ground thickness, geology, and groundwater setting.
Ground Gas Risk Assessment – The Process
Phase 1 – Desk Study and Preliminary Risk Assessment
- Review of historical land uses and contamination records
- Assessment of geology, hydrogeology, and potential receptors
- Development of a Conceptual Site Model (CSM)
If suitable historic intrusive data exists, such as monitoring records, borehole logs or TOC results, these can be reused. Clear Environmental regularly reviews historic data to save clients time and avoid unnecessary fieldwork.
Phase 2 – Site Investigation and Monitoring (Only if Required)
Triggered when the Phase 1 identifies credible risks. Typical activities include:
- Installing monitoring wells in boreholes
- Measuring CO₂, CH₄, VOCs, flow rates, and pressure
- Completing 3–6 monitoring rounds under varied conditions
- Risk classification using CIRIA C665 and BS8485 standards
For small developments in RBKC, three rounds are generally considered acceptable.
Can Monitoring Be Avoided?
Yes, in many cases.
A Phase 1 may conclude that no gas risk exists if:
- The site is underlain by shallow Made Ground (typically <1.5m) and intact London Clay
- No credible sources or pathways are identified
Additionally, CLC4 permits developers to install gas/vapour protection by default on smaller extensions instead of undertaking monitoring. This pragmatic approach often applies to:
- Rear extensions over Made Ground
- Sites with low or uncertain gas risk
In these cases, protection measures can be installed upfront — but must be properly verified.
Designing Mitigation: Ground Gas Protection Options
If monitoring confirms a risk, or if default protection is chosen, measures may include:
- Gas-resistant membranes (must meet BS8485 and be suitable for vapours/radon if also posing a risk)
- Structural barriers, such as cast in-situ slabs or waterproofed basement walls (BS8102 compliant)
- Passive sub-floor ventilation systems
- Active ventilation, if higher risks are confirmed
Verification and Compliance Reporting
Verification is a non-negotiable requirement in RBKC. Evidence must be provided to discharge planning conditions. This includes:
- Photographs showing membrane installation
- Product datasheets
- Signed statements from contractors or site managers
- Evidence of sealed penetrations and overlaps
Where appropriate, additional testing such as tracer gas or smoke tests may be required, especially for high-risk sites.
A full Verification Report must be submitted on completion to confirm that the site is safe and suitable for its intended use.
What Clear Environmental Offers
We’ve supported clients across Kensington and Chelsea, delivering:
- Phase 1 Desk Studies
- Ground Gas & Vapour Strategies (GGVS)
- Remediation strategies and risk assessments
- Verification reporting and regulator liaison
Our in-house team understands the expectations of RBKC’s planners and environmental officers and we design commercially sensible solutions that keep projects moving.
Final Thoughts
Ground gas and vapour risks don’t have to slow your development but they must be addressed correctly.
Clear Environmental provides fast-turnaround, regulator-friendly assessments and protection strategies that are aligned to both RBKC planning policy and real-world construction priorities.
Need help with gas risk or planning conditions in RBKC? Contact Clear Environmental for advice, reports, or project-specific guidance.
When a Phase 1 Desk Study identifies potential environmental risks, the next step is a Phase 2 Site Investigation, also called an intrusive investigation. This critical stage gathers real-world data to confirm or rule out contamination, protecting your project from unforeseen costs, planning delays, or environmental liabilities.
In this guide, we break down:
- How Phase 2 builds on Phase 1.
- The different investigation techniques (and when to use each).
- Typical timescales.
- How laboratory testing refines the site’s risk profile.
- What to expect at the end of a Phase 2 investigation.
How Phase 2 Builds on Phase 1
While a Phase 1 is based on desk research and a site walkover, a Phase 2 collects physical evidence through soil, groundwater, and ground gas sampling.
In short:
Phase 1 = Identify Potential Risks.
Phase 2 = Confirm or Dismiss Risks with Hard Data.
The findings from Phase 2 enable:
- Quantitative Risk Assessments (QRA) based on actual contamination levels.
- Decisions about whether remediation is necessary.
- Planning condition discharge if no unacceptable risks are found.
Phase 2 Site Investigation Methods
Different methods are used depending on the site, the anticipated risks, and the required depth of investigation:
- Best For: Shallow contamination, made ground assessment, visual soil profiling, locating foundations.
- Typical Depths: Up to 4 metres.
- Equipment: Mechanical excavator or hand-dug.
- Best For: Shallow contamination, installation of ground gas and groundwater wells, restricted access sites, cost-effective sampling.
- Typical Depths: Generally 5–6 metres, but can extend up to 10 metres depending on ground conditions.
- Equipment: Window sampling rig (tracked or towed).
- Best For: Deeper soils, groundwater sampling, ground gas monitoring, and collecting geotechnical data.
- Typical Depths: Up to 30 metres or more.
- Equipment: Cable percussion drilling rig.
- Best For: Hard ground, bedrock investigations, and installation of deep groundwater wells.
- Typical Depths: Up to 100 metres or more.
- Equipment: Rotary drilling rig (open hole, cored, or augered).
Tip: On smaller development sites, a combination of trial pits and window sampling often provides excellent coverage at a cost-effective price.
Types of Laboratory Testing During Phase 2
After sampling, the materials collected are sent for laboratory analysis to determine contamination levels:
- Soil Testing:
- Heavy metals (e.g., lead, arsenic)
- Hydrocarbons (TPH, PAHs)
- Asbestos in soil
- Emerging contaminants (e.g., PFAS)
- Groundwater Testing:
- Soluble contaminants (e.g., hydrocarbons, BTEX, solvents)
- Metals in solution
- PFAS detection
- Ground Gas Monitoring:
- Methane, carbon dioxide, oxygen concentrations
- Flow rates (for CS classification)
The results are then compared against appropriate assessment criteria to determine whether remediation is needed.
What Happens After Phase 2 Sampling?
1. Data Analysis and Quantitative Risk Assessment (QRA):
Risk to human health, controlled waters, or property assessed.
2. Conceptual Site Model Update:
Confirmed pollutant linkages mapped and risk-ranked.
3. Decision Point:
No significant risks: Reporting and planning condition discharge.
Significant risks: Recommend remediation (Phase 3).
Phase 1 vs Phase 2 - Quick Comparison Table
FAQs About Phase 2 Site Investigations
What happens if contamination is found in a Phase 2?
If contaminants exceed safe levels, a Remediation Strategy (Phase 3) will be needed to outline how risks will be managed.
Can a Site Investigations rule out the need for remediation?
Yes — many sites require no further action after a properly scoped Phase 2 investigation.
Is ground gas monitoring always required during a Phase 2?
Not always. It depends on the site's history, geology, and desk study findings.
Why a Clear Phase 2 Strategy Matters
A well-designed Phase 2 Site Investigation provides confidence, clarity, and control.
It ensures you're fully informed about site risks, avoids unnecessary remediation, and moves your project efficiently through planning.
At Clear Environmental, we tailor Phase 2 investigations to your site. No over-scoping, no hidden costs, just clear advice.
If your Phase 2 results raise concerns, understanding how to move forward with proportional, commercially realistic remediation is crucial.
To learn what happens next from Phase 2 to Remediation Click Here.
Related Services
What is a Phase 1 Desk Study (Preliminary Risk Assessment)?
A Phase 1 Desk Study (Preliminary Risk Assessment) is an initial environmental assessment that researches a site’s history and conditions to identify potential contamination risks.
Whether you're planning to develop land, submit a planning application, or purchase a brownfield site, understanding environmental risks early is essential.
A Phase 1 Desk Study (also called a Preliminary Risk Assessment) is the first critical step in managing those risks efficiently, helping avoid costly surprises later in your project.
This guide explains exactly what a Phase 1 involves, why it’s required, and what you can expect from the process.
What is a Phase 1 Desk Study?
A Phase 1 Desk Study is a preliminary environmental assessment that evaluates the potential for land contamination based on historical and current land uses.
It is called a “desk study” because it mainly relies on reviewing existing records and data sources, alongside a site visit.
It does not involve any intrusive ground investigation, that's reserved for Phase 2 if risks are identified.
In simple terms:
A Phase 1 identifies if there’s a potential problem — before you commit to site purchase, design, or construction.
Why is a Phase 1 Desk Study Needed?
A Phase 1 is often required to:
- Support planning applications (especially brownfield or previously developed sites).
- Satisfy lender due diligence for property transactions.
- Manage liability and avoid unexpected abnormal costs.
- Confirm if further investigations (Phase 2) are needed.
Without a Phase 1, local authorities, insurers, and funders may refuse to progress development or financing.
Key Components of a Phase 1 Desk Study
A thorough Phase 1 typically includes:
What Does a Phase 1 Report Contain?
A typical Phase 1 Desk Study Report will include:
- Executive Summary (key findings and recommendations).
- Site location and description.
- Historical land use review (maps and aerial photography).
- Environmental setting (geology, hydrology, groundwater).
- Regulatory database review (permits, enforcement notices, pollution incidents).
- Walkover survey findings (including annotated site photos).
- Preliminary Conceptual Site Model (CSM).
- Risk assessment and clear conclusions.
- Next steps, if further investigation is needed.
At Clear Environmental, our Phase 1 reports are concise, commercially-focused, and designed to satisfy planning authorities and lenders without unnecessary complexity.
How Long Does a Phase 1 Desk Study Take?
Most Phase 1 assessments can be completed within 10 Days.
We also offer fast-track services if you’re working to tight planning or transaction deadlines.
Phase 1 Desk Study vs. Environmental Search: What's the Difference?
Key point: An Environmental Search is not a substitute for a Phase 1 — it’s simply a basic desktop screening.
Why Clear Environmental for Your Phase 1?
A Phase 1 Desk Study isn’t just a box-ticking exercise, it’s your first opportunity to manage risks, reduce abnormal development costs, and protect your investment.
At Clear Environmental, we combine technical expertise with commercial insight to deliver Phase 1 Reports that meet planning, legal, and financial requirements quickly and clearly.
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FAQs About Phase 1 Environmental Desk Studies
When should I commission a Phase 1 Environmental Report?
Ideally before site acquisition or at the earliest planning stage, so risks and costs are identified early.
Will a Phase 1 Desk Study always recommend a Phase 2 Site Investigation?
No — many sites with clean histories and no risk linkages can be signed off at Phase 1.
Who needs a Phase 1 Environmental Desk Study?
Developers, property investors, planners, architects, solicitors, and lenders commonly require Phase 1 studies.
Will the council accept my Phase 1 report for planning?
Yes — provided it follows UK guidance (such as EA Land Contamination Risk Management (LCRM)) and is prepared by qualified specialists.
What happens if my Phase 1 shows contamination risks?
We’ll outline the recommended Phase 2 site investigation scope and cost early, so you remain in control.
FAQs About Contaminated Land Assessments
What is a contaminated land assessment?
A contaminated land assessment identifies whether land poses environmental or health risks, helping manage liabilities before development or purchase.
What is the difference between Phase 1 and Phase 2?
Phase 1 is a desktop and site walkover study identifies potential risks. Phase 2 collects soil, water, and gas samples to confirm actual contamination.
How long does a Phase 1 Environmental Report and Phase 2 Site Investigation take?
A Phase 1 takes 10 days. A Phase 2 takes 4-6 weeks depending on complexity.
What happens if contamination is found during a Site Investigation?
A remediation strategy is developed to remove or manage the risks to acceptable levels.
Do I always need a Phase 2 investigation?
Not always — if Phase 1 finds no significant risks, Phase 2 may not be necessary.
FAQs About Contaminated Land Assessments
What’s a Ground Gas and Vapour Strategy (GGVS)?
A GGVS is a targeted investigation to assess the presence and risk of gases such as methane, carbon dioxide, and vapours. It typically involves installing monitoring wells, conducting several monitoring visits, and using standards like CIRIA C665 and BS8485 to determine whether protection is needed.
Do I need gas monitoring for a small extension in RBKC?
Not necessarily. If the Phase 1 Ground Gas Risk Assessment confirms no credible risk — for example, if the site is underlain by intact London Clay with no Made Ground — monitoring may not be needed. In low-risk situations, RBKC may accept the installation of basic gas protection measures with a verification statement instead.
How long does a ground gas investigation take from start to finish?
The monitoring phase typically requires 3 - 6 visits spread over a few weeks, depending on site complexity and weather pressure conditions. Including report preparation, the whole process can take around 6 - 12 weeks.
Can I reuse historical ground gas data for planning?
Yes — provided the data is recent, reliable, and representative of current conditions. Clear Environmental reviews historic site investigation records early to help reduce costs and avoid unnecessary repeat monitoring.
What happens if gas risks are confirmed?
If gas concentrations exceed safe thresholds, you’ll need to install mitigation measures such as membranes, venting layers, or structural barriers. Clear Environmental will design a practical, proportionate solution and prepare the necessary Remediation and Verification reports.
What should be included in the Ground Gas Verification Report?
The Verification Report must include photographic evidence, product specifications, a signed statement of installation quality, and confirmation that gas protection was installed in accordance with RBKC expectations. For some projects, additional tracer gas testing may be required.
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