Radon is a naturally occurring radioactive gas responsible for around 1,100 lung cancer deaths in the UK every year — more than road traffic accidents. It is colourless, odourless, and entirely undetectable without specialist equipment. Yet the vast majority of UK homeowners have never tested for it, and most sellers do not disclose radon levels unless specifically asked.
If you are buying, renovating, or already living in a home in a radon-affected area, this guide explains what radon is, where it is found, how to test reliably, and how much mitigation costs.
What Is Radon and Why Does It Matter?
Radon is produced by the natural radioactive decay of uranium-238, which is present in varying concentrations in rock and soil across the UK. The gas seeps up through the ground and, in a building without adequate ventilation or protective measures, can accumulate to levels that significantly increase the risk of lung cancer.
The UK Health Security Agency (UKHSA) has established two reference levels:
- Action Level: 200 Bq/m³ — at or above this concentration, remediation is strongly recommended
- Target Level: 100 Bq/m³ — the UKHSA recommends reducing levels to below this wherever practicable
The average UK home has a radon level of around 20 Bq/m³. Homes in high-radon areas without mitigation can measure 500–2,000 Bq/m³ or more.
The health risk is cumulative: the longer you are exposed at elevated levels, the higher your lifetime lung cancer risk. Smokers in high-radon homes face a substantially amplified risk compared to non-smokers.
Where Is Radon Found in the UK?
Radon levels depend primarily on the underlying geology. Areas with granite, limestone, and certain metamorphic rocks have the highest radon potential. According to UKHSA data, the following areas have the greatest proportion of homes exceeding the action level:
- South West England (particularly Devon, Cornwall, and Somerset) — the highest-risk region in the UK
- East Midlands (Derbyshire, Northamptonshire)
- Parts of Wales (especially Powys, Ceredigion, Pembrokeshire)
- Northern Ireland (particularly granite areas)
- Parts of Scotland (Grampian region, Caithness)
A high radon potential at postcode level does not guarantee your specific property is above the action level — geology varies house by house. The only way to know is to test. You can check your postcode's radon potential for free at ukhsa.gov.uk/radon.
Building Regulations: What New Builds Must Do
Approved Document C (Part C) of the Building Regulations requires radon protective measures in new builds and extensions in designated Affected Areas (where more than 1% of homes are estimated to exceed the action level). The requirements are tiered by risk:
- Basic protection (1–3% of homes above action level): a radon barrier (a continuous, sealed membrane below the ground floor slab) is required
- Full protection (more than 3% of homes above action level): both a radon barrier and a sump system (or provision for one to be added later) are required
If you are buying a post-2000 home in a radon-affected area, the Building Regulations completion documentation should confirm these measures were installed. If you are extending or undertaking a basement conversion in an affected area, your structural engineer and Building Control officer will advise on what Part C requires. For more on basement construction requirements, see the guide to underpinning — when it is needed and what it costs.
How to Test for Radon
Short-Term vs Long-Term Detectors
There are two types of passive radon detector used for residential testing:
| Detector Type | Duration | Best Use | Cost |
|---|---|---|---|
| Short-term detector | 3–7 days | Quick pre-purchase check, initial screening | £30–£50 per kit |
| Long-term detector | 3–12 months | Accurate annual average; recommended by UKHSA | £30–£60 per kit |
The UKHSA strongly recommends long-term testing (at least 3 months) because radon levels vary significantly with season, weather, and ventilation habits. A short-term test will give you a snapshot, but a long-term test gives the annual average — which is what the action level thresholds are based on.
Where to Get Test Kits
Test kits are available from:
- The UK Radon Association: ukradon.org
- Public Health England / UKHSA direct supply: ukhsa.gov.uk/radon
- NRPB-approved laboratories
Kits typically include two detectors (placed in a bedroom and a living room), with laboratory analysis included in the price.
How to Place Detectors
- Place detectors in the ground floor or basement (where radon is highest)
- Keep at least one in a bedroom, given the time spent there
- Avoid placing directly over heating sources, in draughty positions, or near windows
- Leave in place for the full recommended duration before returning for analysis
Mitigation Options
If your test result is at or above 200 Bq/m³, remediation is recommended. Even between 100–200 Bq/m³, the UKHSA advises reducing levels as a reasonable precaution. The main mitigation options are:
Radon Sump Installation
A radon sump is a small void excavated beneath the floor slab (or within the existing sub-floor void), connected to a pipe that vents radon to the outside — either passively (no fan) or actively (with a low-energy fan). Active sumps are the most effective solution for most properties.
Effectiveness: A well-installed active sump reduces radon by 75–95%. Most homes achieve levels well below the action level after installation.
Positive Pressure Units (PPUs)
A PPU draws in air from the loft space — which typically has much lower radon than the ground floor — and gently pressurises the house. By creating a slight positive pressure inside, radon is prevented from entering through the floor and walls.
PPUs are effective in many homes, particularly where the loft is well-ventilated. They are usually quicker and cheaper to install than a sump. However, they are less effective in highly draughty homes or those with significant structural gaps.
Underfloor Ventilation
For suspended timber floors, improving underfloor ventilation (by clearing blocked airbricks, adding extra vents, or installing a sub-floor ventilation fan) can significantly reduce radon concentrations. This is often the first measure tried in older properties with original suspended floors.
Sealing
Sealing cracks and gaps in concrete floors and around service penetrations can reduce radon entry, though sealing alone rarely achieves sufficient reduction to move below the action level. It is best used as a complement to sump or PPU installation.
Radon Mitigation Costs
| Mitigation Option | Typical Cost | Notes |
|---|---|---|
| Passive sump (no fan) | £500–£800 | Effective in lower-risk areas; can be converted to active later |
| Active sump (with fan) | £800–£1,500 | Most effective solution; fan uses ~20W of electricity |
| Active sump (complex install, concrete slab) | £1,500–£3,000+ | Older solid concrete floors or limited access |
| Positive pressure unit | £500–£900 | Useful where sump access is limited |
| Sub-floor ventilation fan | £200–£500 | For suspended timber floors only |
| Post-installation re-test | £30–£60 | Essential to confirm mitigation has worked |
All remediation work should be carried out by a UKHSA-registered radon remediation contractor. A list is available at ukradon.org.
Radon at Point of Sale: What Sellers and Buyers Need to Know
There is no statutory obligation for sellers to test for or disclose radon levels. However:
- TA6 Property Information Form (Question 8) asks whether the seller is aware of any environmental issues affecting the property, including radon. If a seller has tested and found elevated levels, failure to disclose could constitute misrepresentation.
- Environmental search results often flag whether a property is in a radon-affected area. Your conveyancer's search pack should include this.
- Mortgage lenders do not routinely require radon testing, though some specialist lenders in high-risk areas may ask for evidence of a long-term test or installed mitigation.
If you are buying in an affected postcode, request that the seller provides any existing test results. If none exist, factor in the cost of a long-term test (£30–£60) and potential remediation (£500–£3,000+) when negotiating on price.
Radon and Damp: Addressing Both Together
Radon mitigation work often involves work beneath the ground floor slab — the same area where damp-proofing measures are installed. If you are renovating or extending and addressing ground-level damp, it is worth installing a radon membrane and sump provision at the same time, even if your current levels are not elevated. The marginal cost of adding a membrane during a floor replacement is minimal compared to retrofitting later. For more on below-ground moisture protection, see the guide to damp-proof course types, failures, and chemical injection.
Radon Diagnosis Checklist
- Check your postcode at ukhsa.gov.uk/radon — is it in an affected area?
- Order a long-term detector kit (3–12 months) for an accurate annual average
- Place detectors on the ground floor and in a bedroom
- If result is above 100 Bq/m³, consider mitigation; above 200 Bq/m³, act
- Use a UKHSA-registered contractor for all remediation work
- Re-test after remediation to confirm levels have reduced
- If buying: check TA6 Question 8, request any existing test results, check environmental search
- If renovating: install radon barrier and sump provision at the same time as any ground-floor work
Related Guides
- Underpinning: When It Is Needed and What It Costs — below-ground structural work that may coincide with radon protection installation
- Damp-Proof Course Types, Failures, and Chemical Injection — moisture protection that shares the same ground-level zone as radon barriers