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.
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.
Planning a basement or extension in Kensington & Chelsea? Learn when ground gas risk assessments are needed, what CLC1–CLC4 guidance requires, and how Clear Environmental delivers fast, compliant solutions.
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