Which parts of the UK have the highest radon risk?

The highest radon potential areas in England are Cornwall, Devon, parts of Somerset, Northamptonshire, Derbyshire (particularly around the Peak District), parts of Yorkshire, and areas of the West Midlands overlying granitic geology. In Scotland, Aberdeen and Aberdeenshire have elevated granite-related radon. In Wales, parts of Ceredigion and Powys. The UK Health Security Agency publishes a radon potential map by postcode.

What is the UK action level for radon?

The UK Health Security Agency (UKHSA) sets the action level at 200 becquerels per cubic metre (Bq/m³) — the level at which mitigation is recommended. The target level (indicative for new buildings and as a goal for existing properties) is 100 Bq/m³. Background levels in low-risk areas are typically 20–40 Bq/m³. Radon measurement is carried out using detectors placed in the property over 3 months.

How much does radon mitigation cost?

A positive pressure ventilation (PPV) system costs £600–£1,200 installed. A sump and fan extraction system (sub-slab depressurisation) — the most effective measure for high levels — costs £800–£2,500 installed by a UKAS-accredited contractor. Ongoing electricity cost for a fan is approximately £20–£40/year. Post-mitigation testing is required (£50–£80 for detector hire) to confirm the reduction.

Do mortgage lenders check radon risk?

Mortgage lenders do not typically refuse or restrict lending based on radon potential alone — radon risk is not a category that appears in most lenders' standard property checklist in the way flood risk does. However, a RICS surveyor who notes high radon potential may recommend a test in their report, and some lenders' solicitors will raise a radon question in the local authority search responses for high-risk areas.

Radon: the invisible risk

Radon is the second leading cause of lung cancer in the UK after smoking — responsible for around 1,100 deaths per year (UK Health Security Agency, 2023). It is colourless, odourless, and undetectable without a test. Most buyers in high-risk areas never commission one. This guide explains where the risk is highest, what the numbers mean, and how much it costs to fix.

Last reviewed June 2026.

1. What radon is and where it comes from

Radon is a naturally occurring radioactive gas formed by the decay of uranium in rocks and soils. It seeps through the ground and usually disperses harmlessly in the open air. The problem arises when it accumulates inside buildings — entering through gaps in floors, cracks in foundations, construction joints, and service entries.

The underlying geology drives the risk. Granite, limestone, and some sandstone formations release more radon than clay or chalk. The risk map therefore follows geological boundaries, not administrative ones — a high-risk postcode can be directly adjacent to a low-risk one.

Radon decays into short-lived radioactive particles that attach to dust and can be inhaled deep into lung tissue. Prolonged exposure at elevated levels increases the risk of lung cancer — the risk compounds with time, so a property in a high-risk area with elevated levels that has been lived in for 30 years presents much higher cumulative exposure than a recently-purchased one.

2. High-risk areas in the UK

The UKHSA classifies postcodes by radon potential — the estimated percentage of homes likely to exceed the action level of 200 Bq/m³ without mitigation. The categories are:

Affected area (≥3%): More than 3% of homes likely to exceed the action level. Testing recommended for all ground-floor and basement properties.

The highest-risk areas in England and Wales include: Cornwall (the highest radon county; some postcodes have 30%+ potential), Devon, parts of Somerset (Mendips area), Northamptonshire, Derbyshire (Peak District margins), Lincolnshire (some limestone areas), Yorkshire (upper limestone areas), and parts of the West Midlands. In Scotland, Aberdeenshire, Moray, and Highland carry elevated risk from granitic geology.

Radon potential is postcode-level data — not street-level and not property-level. A high potential percentage means more tests are recommended, not that every property in the area will exceed the action level. The only way to know the level in a specific property is to test it.

3. Becquerel levels and the action threshold

Radon is measured in becquerels per cubic metre (Bq/m³). A becquerel is one radioactive disintegration per second per cubic metre of air.

Background level (low-risk areas): 20–40 Bq/m³. Normal outdoor background in the UK averages around 5 Bq/m³.

Target level: 100 Bq/m³ — recommended as a goal for new buildings and as a reduction target for existing properties above the action level. Achievable with good construction practice or modest mitigation.

Action level: 200 Bq/m³ — UKHSA recommends taking action to reduce radon when levels exceed this in a living room or bedroom. Risk at 200 Bq/m³ is broadly equivalent to smoking 8 cigarettes per day (based on BEIR VI lifetime risk model).

High levels (500+ Bq/m³): Uncommon but documented in Cornwall and Devon. The risk is substantial and mitigation is urgent. Cases above 1,000 Bq/m³ exist — building ventilation alone is insufficient and active depressurisation is required.

4. How radon testing works

Radon levels in a property fluctuate daily and seasonally — higher in winter (when windows are closed), higher at night, higher in ground-floor rooms. A reliable measurement requires a minimum of 3 months, using two passive detectors placed in a bedroom and a living room on the ground floor.

Alpha-track detectors (small plastic squares) are the standard measurement tool. Detectors are ordered from a UKAS-accredited laboratory (Radonsol, Radon Solutions, or the UKHSA's own service), placed in the property, then returned for laboratory analysis. Cost: £50–£80 for a two-detector kit. Results within 2–4 weeks of return.

For a conveyancing context, 3 months is often impractical. Short-term electronic radon monitors (48–72 hours) are available as an indicative measure — they do not meet the UKHSA standard but give a useful directional signal. If a short-term test shows elevated levels, a full 3-month test (which could be run during the ownership period after exchange) is the appropriate follow-up.

5. Mitigation options and costs

Radon mitigation is well-understood and generally effective. The appropriate measure depends on the radon level, the floor construction type, and the property's ventilation.

Natural ventilation improvements: For levels just above the target but below the action level — increasing underfloor ventilation by clearing airbricks or adding new ones. Cost: £200–£600. Limited effectiveness at higher levels.

Positive pressure ventilation (PPV): A fan unit in the loft draws filtered outside air into the house, pressurising it slightly and diluting radon. Cost: £600–£1,200 installed. Effective for levels up to around 400–500 Bq/m³ in houses with suspended timber floors or good natural ventilation pathways. Less effective in concrete slab constructions.

Sub-slab depressurisation (sump and fan): A sump is created in the floor (or uses an existing drainage sump) and a fan draws radon out from beneath the slab before it enters the building. The most effective measure for concrete slab construction and the recommended approach for levels above 500 Bq/m³. Cost: £800–£2,500 installed. Running cost: approximately £20–£40/year in electricity.

Post-mitigation testing is essential — commission a further 3-month detector test after installation to confirm the reduction achieved. Reputable contractors will guarantee a result below 200 Bq/m³ or revisit their installation.

6. Radon protection in new builds

Building Regulations Part C (2000, as amended) requires radon-protective measures in new construction in affected areas — typically a radon-resistant membrane under the floor slab, plus a passive sump that can be activated if a test later shows elevated levels. The required measures scale with the local radon potential class.

On a new build in an affected area, confirm that the building regulations completion certificate confirms the required radon-protective construction. The passive sump can be activated to active (fan-driven) at low cost if a post-occupation test shows levels above the target.

7. Mortgages, insurance, and disclosure

Unlike flood risk, radon potential does not appear in mainstream lender mortgage conditions and does not typically affect insurability. Buildings and contents insurers do not price radon risk explicitly.

The TA6 Property Information Form (Fourth Edition, 2022) does not directly ask about radon, though the general environmental question covers physical risks. Sellers in high-risk areas are not required to test before sale. A RICS Level 2 or Level 3 surveyor may note the radon potential and recommend testing — this is standard practice in Cornwall, Devon, and Derbyshire.

For buyers purchasing in UKHSA-affected postcodes, commissioning a test (or negotiating a short-term monitor run during the purchase process) is straightforward and low cost relative to the information it provides. A confirmed reading above the action level with a cost-effective mitigation route in hand is a reasonable basis for negotiating a modest price reduction — particularly on older properties in areas like Cornwall where high readings are common.