Radon is responsible for around 1,100 lung cancer deaths in the UK every year — more than road traffic accidents. It is naturally occurring, colourless, odourless, and undetectable without specialist equipment. If you are buying or renovating in an affected area, or if you have never had your home tested, this guide covers everything you need to know: where radon is found, how to test, what the action levels mean, and what mitigation costs.
What Is Radon?
Radon is a radioactive gas produced by the natural decay of uranium-238, which is present in varying concentrations in rock and soil across the UK. As uranium decays, it produces radium, which in turn produces radon gas. This gas seeps up through soil and rock, and in a well-sealed building without adequate ventilation, it can accumulate to levels that significantly increase the risk of lung cancer over time.
Radon is measured in becquerels per cubic metre (Bq/m³). The UK Health Security Agency (UKHSA) sets two key reference levels:
- Action level: 200 Bq/m³ — at or above this level, householders are strongly advised to take remedial action
- Target level: 100 Bq/m³ — the level the UKHSA recommends as a long-term target after mitigation
To put this in context: the average UK home has a radon level of around 20 Bq/m³. Homes in high-radon areas can measure 500–2,000 Bq/m³ or more without any mitigation.
Where Is Radon a Problem in the UK?
Radon levels depend on the underlying geology. Areas with granite, limestone, and certain metamorphic rocks have the highest radon potential. The UKHSA publishes an interactive Radon Affected Areas map, and the following areas have the highest proportion of homes above the action level:
| Region | Notes |
|---|---|
| Cornwall and the Isles of Scilly | Highest concentration nationally — granite-rich geology |
| Devon | Particularly Dartmoor, the South Hams, and Exmoor fringe |
| Derbyshire | Peak District limestone, parts of north and east county |
| Northamptonshire | Ironstone and limestone geology |
| Parts of Wales | Ceredigion, Pembrokeshire, and Gwynedd have elevated areas |
| Parts of Scotland | Aberdeen and Grampian, parts of Argyll and Bute |
| Somerset | Parts of the Mendips and Quantocks |
It is important to note that high radon potential at a postcode level does not mean your specific property is above the action level — it means the probability is elevated. The only way to know is to test.
You can check your property's postcode against UKHSA data at ukhsa.gov.uk/radon. A basic postcode check is free.
Legal Disclosure Position
There is no mandatory requirement for sellers to test for radon or disclose the results before sale. However:
- Question 8 of the TA6 Property Information Form asks whether the seller is aware of any environmental issues affecting the property, including radon
- If the seller knows the property has a high radon level and does not disclose it, they could face a misrepresentation claim after completion
- Building Regulations (Approved Document C, Part C) require radon protective measures in new builds in affected areas — this should be in the building regulations completion documentation
In practice, most sellers in affected areas have never tested. The TA6 answer will typically be "not known." This means the risk transfers to the buyer. For more on what the TA6 covers and how to interpret "not known" answers, see our conveyancing searches explained guide.
How to Test for Radon
UKHSA Detectors
The most accessible and cost-effective testing method uses passive electret or solid-state nuclear track detectors supplied by the UKHSA (via their partner, Radon UK). These are small plastic devices that you place in the lowest habitable room and a bedroom.
| Test Type | Cost | Duration | Best For |
|---|---|---|---|
| Short-term UKHSA detector | £30–£50 | 3–10 days | Initial indication only |
| Long-term UKHSA detector | £50–£80 | 3 months minimum | Accurate annual average |
| Professional radon survey | £200–£400 | 1 day (plus lab analysis) | Pre-purchase, lender queries |
Long-term testing is strongly recommended for accurate results. Radon levels fluctuate significantly with weather, season, and ventilation behaviour. A short-term test taken in a well-ventilated property in summer can dramatically underestimate the winter average when windows are closed. The UKHSA recommends testing for at least three months for a reliable annual average estimate.
For pre-purchase purposes, a professional survey with immediate results may be appropriate if you are under time pressure — but discuss the limitations with the surveyor and commission a long-term follow-up test after moving in.
Mitigation Options
If your test result is at or above the action level (200 Bq/m³), you should act. Even between 100–200 Bq/m³, the UKHSA recommends improving radon levels as a reasonable precaution.
Positive Pressure Unit (PPU)
A positive pressure unit draws in filtered air from the loft space and gently pressurises the house, preventing radon from entering from below. They are:
- Best suited to: detached and semi-detached houses with accessible loft space
- Installation cost: £500–£1,000 installed by a specialist
- Running cost: approximately £15–£25 per year in electricity
- Effectiveness: typically reduces radon by 40–75%
- Disruption: low — one fan unit in the loft, one grille in the ceiling
PPUs are not suitable for all properties. They work less well in very leaky buildings or flats where loft access is shared.
Sub-Slab Depressurisation (Radon Sump)
A radon sump creates a void beneath the floor slab, with a pipe and fan that extracts radon from under the floor and expels it safely above the roofline. This is the most effective mitigation method for most property types.
- Best suited to: properties with solid concrete ground floors
- Installation cost: £800–£2,500 (depending on floor construction and fan system)
- Running cost: approximately £20–£40 per year
- Effectiveness: typically reduces radon by 75–99%
- Disruption: moderate — requires core drilling through the floor slab
Ventilation Improvements
Improved ventilation is the lowest-cost option for borderline results (100–200 Bq/m³). Opening trickle vents, fitting underfloor ventilation, or improving cross-ventilation can achieve meaningful reductions. However, it is rarely sufficient on its own for properties above the action level.
Radon Membranes in New Builds
Building Regulations Approved Document C requires radon-protective measures in all new buildings in areas where 1% or more of homes are estimated to be above the action level. In higher-risk areas (3%+ threshold), full radon barriers (membranes) are required under the floor slab, along with provision for a sump if future testing shows elevated levels. If you are buying a new build in an affected area, check the Building Regulations completion certificate confirms these measures were installed. See our building regulations completion certificate guide for how to find and verify this documentation.
Mortgage and Insurance Implications
Mainstream mortgage lenders do not routinely decline applications on radon grounds, and radon levels above the action level are not typically listed as a standard mortgage condition. However:
- Some specialist survey-triggered valuations may note elevated radon as a material defect
- Buildings insurance is rarely affected by radon once mitigation is in place
- If you are buying a property where radon is known to be elevated and mitigation is not in place, your solicitor should flag this and negotiate a price reduction or require the seller to fund mitigation
For a full picture of property conditions that can affect mortgage availability, see our mortgage on a property with issues guide.
What to Do If You're Buying in an Affected Area
- Check the UKHSA postcode data before making an offer — free at ukhsa.gov.uk/radon
- Commission a long-term UKHSA test as a condition of purchase, or ask the seller to provide test results if they have any
- Budget for mitigation — a radon sump (£800–£2,500) is a reasonable contingency in high-risk areas
- Check new build documentation for radon membrane installation if buying new
- Seek specialist advice from a UKHSA-registered radon measurement specialist for properties in the highest-risk postcodes
What to Do Next
- Order UKHSA long-term detectors at radon.ukhsa.gov.uk — £50–£80 for a two-detector set is the most cost-effective starting point
- If your result exceeds 200 Bq/m³, contact a specialist contractor registered with the UKHSA or UK Radon Association for a mitigation survey
- For a sub-slab sump, ensure your contractor issues a post-installation certificate confirming the result is below the action level
Further Reading
- Mortgage on a Property with Issues UK — property conditions that affect lending, including environmental issues
- EPC Rating UK: How to Improve Your Energy Performance Certificate — energy efficiency improvements that can be paired with radon mitigation
- Building Regulations Completion Certificate UK — how to verify that radon membranes were installed in new builds
- Conveyancing Searches Explained UK — how environmental searches flag radon risk areas
- Radon Gas in UK Homes: Testing, Regulations and Mitigation — deeper guide covering Part C Building Regulations, the TA6 disclosure obligation for sellers, and sub-membrane depressurisation mitigation options