Radon Gas in UK Homes: What It Is and What to Do About It

Radon levels vary significantly by location, depending on the underlying geology. In England and Wales, the UK Health Security Agency (UKHSA) produces radon maps showing where elevated radon potential exists.

• **High-risk geological areas in England and Wales**:
• •Cornwall and Devon (granite bedrock — the highest radon area in England; over 50% of homes in some postcode sectors exceed the UK Action Level)
• •Parts of Northamptonshire, Derbyshire, and the Peak District
• •Parts of the Pennines and Yorkshire Dales
• •Pockets throughout the south and west of England on limestone or granite formations

**London and the South East**: London sits predominantly on London Clay — a geology that produces relatively low radon. However, pockets of higher radon potential exist in areas of the South East on chalk, Greensand, or Permian geology. The risk is lower in Greater London than in high-risk counties, but it cannot be assumed to be zero without checking.

**How to find your specific risk level**: 1. Use the UKHSA radon map tool at ukhsa.ac.uk — enter your postcode to see the percentage of homes in your area likely to be above the Action Level (200 Bq/m³) 2. For a definitive answer, commission a radon measurement — an activated charcoal or electret detector placed in your property for 3 months, then sent for laboratory analysis. Cost: £30–£60 per detector (two recommended — one in the living space, one in a bedroom). UKHSA-approved suppliers include Public Health England and RadonSystems.co.uk

• **The UK Radon Action Level**:
• •Action Level: 200 Bq/m³ (Becquerels per cubic metre) — above this, mitigation is strongly recommended
• •Target Level: 100 Bq/m³ — UKHSA recommends reducing to below this target in all mitigation work

Part C of the Building Regulations ('Site Preparation and Resistance to Contaminants and Moisture') requires that new buildings and extensions in Radon Affected Areas are constructed with radon protective measures.

Part C defines a 'Radon Affected Area' for Building Regulations purposes as any area where 3% or more of homes are estimated to exceed the Action Level.

**What Part C requires**:

• *Basic protection* (for areas where 3–10% of homes exceed the Action Level):
• •A radon-resistant membrane (a continuous, lapped, gas-resistant membrane under the ground floor) — the minimum requirement for extensions in affected areas
• •The membrane must be continuous and all penetrations (pipes, cables) sealed with appropriate radon-resistant sealant

• *Full protection* (for areas where more than 10% of homes exceed the Action Level):
• •Radon-resistant membrane
• •A void or underfloor ventilation system designed to allow the installation of a sump or fan later if monitoring shows levels remain elevated
• •Alternatively, an active sump system installed at construction stage

• **Practical implication for extensions and loft conversions in London**:
• •Most of Greater London falls below the 3% threshold — radon protection measures are not typically required by Building Control in these areas
• •However, in pockets of higher risk (parts of the South East, borders of London's commuter zone), Building Control may require confirmation of radon risk before waiving Part C requirements
• •Always check with your Building Control officer or approved inspector at pre-submission stage — they have access to UKHSA radon potential maps and will confirm whether radon protection is required for your specific project address

If you test your existing home and the result exceeds 200 Bq/m³, UKHSA recommends taking action to reduce levels. Effective mitigation options:

• **Active sump system (positive pressure underfloor depressurisation)** — the most effective and most commonly installed mitigation:
• •A small electric fan draws air from below the floor slab through a purpose-built sump (a pit in the hardcore layer under the floor)
• •The fan creates negative pressure under the floor, drawing radon-laden air away from the building before it can enter
• •The extracted air is vented harmlessly to the outside (typically via a pipe run up through the house and exiting above eaves level or through an external wall)
• •Cost: £800–£1,500 installed (single sump; may need multiple for larger properties)
• •Effectiveness: typically reduces radon by 80–90%+; average post-mitigation measurement: 30–50 Bq/m³ in most cases
• •Running cost: the fan draws approximately 20–40W — very low electricity use

• **Passive sump** (no fan — relies on natural stack effect):
• •Less effective than active sump, particularly in modern well-sealed homes
• •Can be converted to active system later if results remain high

• **Positive pressure ventilation (PIV)**:
• •A unit mounted in the loft draws fresh filtered air into the property, slightly pressurising the interior and reducing radon ingress
• •Less targeted than sump systems — appropriate when a sump is not possible (e.g., solid floor with no void access)
• •Cost: £400–£800 installed

• **Natural ventilation improvement**:
• •Increasing sub-floor ventilation in suspended timber floor properties — opening vents, adding additional airbricks — can reduce radon in some cases but is rarely sufficient alone above 400–500 Bq/m³

**Re-test after mitigation**: Always re-measure radon levels 3–6 months after mitigation to confirm the system is working. Any reputable radon mitigation contractor will include a re-test in their service.

**Approved radon mitigation contractors**: The UKHSA maintains a list of approved radon mitigation contractors — always use a contractor on this list for assured quality and methodology.