What Does Building Regulations Part C Cover for Site Contamination and Ground Conditions?

Summary

Part C of the Building Regulations addresses the preparatory and protective measures needed at ground level to ensure that buildings are not affected by moisture, contaminants, or gases present in the ground. It is an often-underestimated part of building regulations but covers some of the highest-risk issues in new construction and conversion — particularly contaminated land and radon gas.

The regulatory framework for Part C sits at the intersection of Building Regulations (construction standards), Environmental Protection legislation (contaminated land — Part IIA of the Environmental Protection Act 1990), and Health and Safety law (protecting workers and occupants). A builder or contractor encountering apparent contamination has obligations under all three frameworks.

Approved Document C is supported by a range of other documents, including the CIRIA guides to contaminated land, the British Geological Survey's radon maps, and British Standards for site investigation and soil classification. Professional guidance from a geotechnical or contamination specialist is recommended for all but the simplest sites.

Key Facts

• Approved Document C (2004, amended 2013) — the compliance document for Part C of the Building Regulations in England
• C1 — Preparation of the site — covers removal of turf and organic material, removal of unsuitable fill, and management of trees affecting foundations
• C2 — Dangerous and offensive substances — covers identification and management of contaminated land, harmful gases (radon, methane), and hazardous materials in the ground
• Radon zones — the UK is divided into radon risk zones by the UK Health Security Agency (UKHSA); high-radon areas (principally Cornwall, Devon, Somerset, Northamptonshire, and Derbyshire) require protective measures in new buildings
• Full radon protection — required where more than 10% of homes in the area are estimated to exceed the Action Level (200 Bq/m³); includes a gas-resistant membrane, positive pressure sub-floor ventilation sump, and provision for future installation of a radon pump
• Basic radon protection — required in areas with 3–10% estimated exceedance; includes a gas-resistant membrane and a passive sump capable of being activated later
• Gas membranes — a 300-micron minimum polyethylene DPM (or dedicated radon/gas barrier membrane) laid under the ground floor slab; joints taped and edges turned up to lap with wall DPC
• Methane risk — landfill sites, peaty ground, and areas with organic deposits may generate methane; requires specialist assessment; gas monitoring may be needed during site preparation
• Japanese knotweed — must be managed in accordance with the Environmental Protection Act (Duty of Care); waste classification is Special Waste; cannot be removed without appropriate waste carriers licence; treatment on site or specialist removal required
• Asbestos in ground — may be encountered on demolition sites, former industrial sites, or where asbestos-containing waste was fly-tipped; specialist removal under the Control of Asbestos Regulations 2012 required
• Contaminated land register — local authorities maintain a register of land formally identified as contaminated under Part IIA of the Environmental Protection Act 1990; searches available through solicitors in property transactions
• Ground investigation — BS EN ISO 14688 (soil classification) and BS 5930:2015 (Code of Practice for Ground Investigations) apply to site investigation works
• Made ground — previously disturbed or infilled ground; may contain debris, voids, hazardous materials; always requires assessment

Quick Reference Table

Detailed Guidance

Approved Document C: Structure and Scope

Approved Document C is divided into two main parts:

C1 (Preparation of site): Requires that the ground covered by the building shall be reasonably free of vegetable matter (turf, roots, organic topsoil) before construction of the ground floor. This prevents organic decay, differential settlement, and the generation of gases from decomposing organic matter. In practice, strip topsoil to a minimum depth of 150mm or until all organic material is removed, whichever is deeper.

C2 (Resistance to contaminants): Requires assessment of any hazardous substances in the ground and appropriate protection measures. This is the most complex part of Approved Document C and is where specialist input is most often needed. C2 covers radon, methane, contaminants from industrial use, and any other hazardous material that could affect occupants.

Radon: A Serious But Addressable Risk

Radon is a naturally occurring radioactive gas produced by the decay of uranium in granite and certain other rocks. It seeps through the ground and, if it accumulates in buildings, it is the second largest cause of lung cancer in the UK after smoking.

The UK Health Security Agency (UKHSA) maintains radon maps dividing the UK into zones based on the estimated percentage of homes likely to exceed the Action Level of 200 Becquerels per cubic metre (Bq/m³). Building Regulations require protective measures in new builds in higher-risk zones.

How radon protection works:

1. Radon-resistant membrane — a continuous low-permeability membrane (300-micron polythene or dedicated radon membrane) under the ground floor construction, with all joints lapped and taped. The membrane prevents radon from entering the building through the floor construction.
2. Passive sump — a perforated pipe or gravel-filled void beneath the membrane, connected to a vertical pipe that exits above the roof level. In passive mode, natural convection draws radon from beneath the membrane and vents it safely outside. In full protection zones, this pipe is connected to a low-power suction fan to ensure positive ventilation.
3. Sub-floor ventilation — in suspended timber floors, adequate cross-ventilation of the sub-floor space is required; ventilation bricks/airbricks at both sides of the building ensure radon cannot accumulate.

For existing homes where radon is measured above the Action Level, retrofitting a radon sump (drilling through the slab and installing a suction fan) is the most effective remedy, typically reducing radon levels by 80–90%.

Methane and Carbon Dioxide from Ground

Former landfill sites, peat deposits, and areas with high organic content can generate significant quantities of methane (CH4) and carbon dioxide (CO2). Methane is explosive at concentrations of 5–15% in air (lower explosive limit 5%). CO2 displaces oxygen and is an asphyxiation risk.

The risk assessment process follows the CIRIA C735 guidance (Good Practice on the Testing and Verification of Protection Systems for Buildings Against Hazardous Ground Gases, 2019). Key steps:

1. Phase 1 desk study — historical maps, Ordnance Survey records, BGS borehole records, Environment Agency contamination data; identifies potential sources
2. Phase 2 intrusive investigation — ground gas monitoring boreholes installed and monitored over weeks to months; characterises gas concentrations and ground conditions
3. Classification — based on ground gas screening values and borehole assessment, sites are classified on a risk scale (typically GSI 1–5 or similar); each class has prescribed protection measures
4. Protection measures — range from passive ventilation (low risk) to full gas-resistant membrane and active sub-slab ventilation (high risk)

Radon membranes do not provide adequate protection against methane or CO2 — dedicated gas-resistant membranes (Serco or equivalent products) with higher chemical resistance and specific permeability values are required.

Contaminated Land: Desk Studies and Site Investigation

For any development on a site with potential contamination (former industrial use, petrol stations, dry cleaners, print works, gasworks, railway land, etc.), a Phase 1 contamination assessment is best practice and is increasingly required by local planning authorities as a planning condition.

Phase 1 (Desk Study): Reviews historical maps, aerial photographs, BGS records, Environment Agency data, and local authority records. Produces a conceptual site model identifying potential source-pathway-receptor linkages. Usually carried out by an environmental consultant; no intrusive investigation.

Phase 2 (Intrusive Investigation): Trial pits, boreholes, and laboratory analysis of soil and groundwater samples. Identifies actual contaminant types and concentrations. Compared against relevant screening criteria (Environment Agency Generic Assessment Criteria).

Remediation: Where contamination exceeds acceptable levels for the intended end use, remediation is required before construction. Methods include soil washing, bioremediation, encapsulation, and excavation and disposal to licensed landfill.

Japanese Knotweed

Japanese knotweed (Reynoutria japonica) is an invasive species with a legal status as a controlled waste under the Environmental Protection (Duty of Care) Regulations 1991. It can damage building foundations, drainage, and hard surfaces as its rhizomes (underground stems) can grow through asphalt, gaps in brickwork, and drainage pipes.

Key points for builders:

• All knotweed material (above and below ground) is classified as controlled waste and must be transported by a registered waste carrier
• Knotweed cannot be composted or disposed of in green waste
• On-site treatment using approved herbicides (typically glyphosate) is the preferred method for large infestations but takes 3–5 years for effective eradication
• A knotweed management plan must be prepared for any site where knotweed is present
• Lenders and mortgage companies are increasingly refusing to lend on properties where knotweed is within 7m of a habitable structure without a management plan in place

Made Ground and Ground Investigation

Made ground — previously disturbed soil or engineered fill — is commonly encountered on brownfield sites, urban gardens, and areas near former buildings. It may contain:

• Building rubble, ash, and clinker (common in Victorian and Edwardian areas)
• Industrial waste and demolition debris
• Historical contamination from previous uses
• Voids and poorly compacted zones leading to differential settlement

Any significant structure on made ground should be preceded by a ground investigation. The investigation methodology should follow BS 5930:2015. Soil classification uses BS EN ISO 14688. Foundation design on made ground typically requires either ground improvement, piling, or raft foundations — the choice depends on the made ground depth, compressibility, and the load requirements of the building.

Frequently Asked Questions

How do I know if a site is in a radon risk zone?

The UK Health Security Agency (UKHSA) maintains an online radon postcode checker at ukhsa.gov.uk. Enter the postcode and the tool indicates the radon risk class for that location. For planning applications in high-radon areas, Building Control may require evidence of appropriate radon protection measures before sign-off.

Does Part C apply to extensions and conversions?

Yes. Part C applies to any new ground floor construction, including extensions. A single-storey extension requires radon protection appropriate to its location, removal of organic topsoil beneath the floor, and appropriate protection against any contaminants in the ground. Conversions that involve a new ground floor slab also trigger Part C requirements.

Who is responsible for investigating contamination — the builder or the developer?

The developer (the person intending to carry out the development) is responsible for commissioning contamination assessments. If a planning condition requires Phase 1 and Phase 2 investigations, the developer must satisfy these before, during, or after construction (depending on the condition timing). The builder/contractor is responsible for complying with the planning conditions and not creating a statutory nuisance or risk during construction. If unexpected contamination is encountered during excavation, work should stop, the site should be secured, and the local Environmental Health Officer should be notified.

What is the difference between a radon sump and a radon barrier membrane?

A radon barrier membrane prevents radon from passing through the floor construction. A radon sump creates a low-pressure zone beneath the membrane that draws radon away and vents it outside. In full protection zones, both are used together. The sump is more effective because it actively removes radon rather than just blocking it. In existing buildings where radon is an issue, a sump can be installed by drilling through the floor slab.

Regulations & Standards

• Building Regulations Part C (Approved Document C, 2004 with 2013 amendments) — site preparation, contaminants, and moisture

• Environmental Protection Act 1990 Part IIA — contaminated land regime; local authority enforcement

• Control of Asbestos Regulations 2012 (SI 2012/632) — asbestos management and removal

• BS 5930:2015 — Code of Practice for Ground Investigations

• BS EN ISO 14688 — Geotechnical investigation and testing; identification and classification of soil

• CIRIA C735:2019 — Good Practice on Testing and Verification of Protection Systems for Buildings Against Hazardous Ground Gases

• UK Health Security Agency — Radon — radon maps, postcode checker, protection guidance

• Environment Agency — Land Contamination — risk assessment guidance

• CIRIA — Ground Gas Guidance — CIRIA C735 guidance on ground gas protection

• Planning Portal — Approved Document C — official text of Approved Document C

• building regs overview — overview of all Building Regulations parts

• self build regulations — how Part C fits into a self-build project

• part a structure — structural implications of poor ground conditions

• nhbc warranty — NHBC requirements for contaminated land